Manpage for TXR (version 99)

Oct 05, 2014

 

Index

1 NAME
2 SYNOPSIS
3 DESCRIPTION
4 ARGUMENTS AND OPTIONS
5 STATUS AND ERROR REPORTING
6 BASIC QUERY SYNTAX AND SEMANTICS
6.1 Comments
6.2 Hash Bang Support
6.3 Whitespace
6.4 Text
6.5 Special Characters in Text
6.6 Character Handling and International Characters
6.7 Regular Expression Directives
6.8 Variables
6.9 Negative Match
6.9.1 Variable Followed by Nothing
6.9.2 Variable Followed by Text
6.9.3 Variable Followed by a Function Call or Directive
6.9.4 Consecutive Variables
6.9.5 Consecutive Variables Via Directive
6.9.6 Longest Match
6.10 Positive Match
6.11 Special Symbols nil and t
6.12 Keyword Symbols
6.13 Regular Expressions
6.14 Directives
6.15 Character Literals
6.16 String Literals
6.17 Word List Literals
6.18 String Quasiliterals
6.19 Quasiword List Literals
6.20 Numbers
6.21 Comments
6.22 Directives-driven Syntax
7 INPUT SCANNING AND DATA MANIPULATION
7.1 The next directive
7.2 The skip directive
7.3 Reducing Backtracking with Blocks
7.4 The trailer directive
7.5 The freeform directive
7.6 The fuzz directive
7.7 The some, all, none, maybe, cases and choose directives
7.8 The require directive
7.9 The if directive
7.10 The gather directive
7.10.1 The until / last clause in gather
7.10.2 Keyword parameters in gather
7.11 The collect directive
7.11.1 The until / last clause in collect
7.11.2 Keyword Parameters in collect
7.11.3 Specifying Variables in collect
7.12 The coll directive
7.12.1 Keyword parameters in coll
7.13 The flatten directive
7.14 The merge directive
7.15 The cat directive
7.16 The bind directive
7.16.1 Keywords in the bind directive
7.17 The set directive
7.18 The rebind directive
7.19 The forget directive
7.20 The do directive
8 BLOCKS
8.1 Overview
8.2 The block directive
8.3 Block Scope
8.4 Block Nesting
8.5 Block Semantics
8.6 Data Extent of Terminated Blocks
8.7 Interaction Between the trailer and accept Directives
9 FUNCTIONS
9.1 Overview
9.2 Definition Syntax
9.3 Two Forms of The Horizontal Function
9.4 Vertical-Horizontal Overloading
9.5 Call Syntax
9.6 Vertical Versus Horizontal Calls
9.7 Local Variables
9.8 Nested Functions
10 MODULARIZATION
10.1 The load directive
11 OUTPUT
11.1 Introduction
11.2 The output directive
11.3 Output Text
11.4 Output Variables
11.5 Output Variables: Indexing
11.6 Output Substitutions
11.7 The repeat directive
11.8 Nested repeat directives
11.9 The rep directive
11.10 repeat and rep Examples
11.11 The close directive
11.12 Output Filtering
11.13 Function Filters
11.14 The deffilter directive
11.15 The filter directive
12 EXCEPTIONS
12.1 Introduction
12.2 The try directive
12.3 The finally clause
12.4 catch clauses
12.5 catch Clauses with Parameters
12.6 The throw directive
12.7 The defex directive
12.8 The assert directive
13 TXR LISP
13.1 Overview
13.2 Additional Syntax
13.3 Symbol Tokens
13.4 Consing Dot
13.5 Quote and Quasiquote
13.6 Quasiquoting non-List Objects
13.7 Quasiquoting combined with Quasiliterals
13.8 Vectors
13.9 Hashes
13.10 The .. notation
13.11 The DWIM Brackets
13.12 Compound Forms
13.13 Regular Expressions
13.14 Generalization of List Accessors
13.15 Callable Objects
13.16 Special Variables
14 TXR LISP OPERATOR AND FUNCTION LIBRARY
14.1 Control Flow and Sequencing
14.1.1 Operators progn and prog1
14.1.2 Operator cond
14.1.3 Macros caseq, caseql and casequal
14.1.4 Macros when and unless
14.1.5 Macros while and until
14.1.6 Operator/function if
14.1.7 Operator/function and
14.1.8 Operator/function or
14.1.9 Operator unwind-protect
14.1.10 Operator block
14.1.11 Operators return and return-from
14.2 Evaluation
14.2.1 Operator dwim
14.2.2 Function identity
14.2.3 Function eval
14.2.4 Function make-env
14.2.5 Functions env-vbind and env-fbind
14.3 Mutation
14.3.1 Operators inc dec set push pop flip and del
14.4 Binding and Iteration
14.4.1 Operator defvar
14.4.2 Operators let and let*
14.4.3 Operators for and for*
14.4.4 Operators each, each*, collect-each, collect-each*, append-each and append-each*
14.5 Function Objects and Named Functions
14.5.1 Operator defun
14.5.2 Operator lambda
14.5.3 Function call
14.5.4 Operator fun
14.5.5 Functions symbol-function and symbol-value
14.5.6 Functions boundp and fboundp
14.5.7 Function func-get-form
14.5.8 Function func-get-env
14.5.9 Function functionp
14.5.10 Function interp-fun-p
14.5.11 Macros flet and labels
14.6 Object Type And Equivalence
14.6.1 Function typeof
14.6.2 Functions null, not, and false
14.6.3 Function true
14.6.4 Functions eq, eql and equal
14.7 Basic List Library
14.7.1 Function cons
14.7.2 Function atom
14.7.3 Function consp
14.7.4 Functions car and first
14.7.5 Functions cdr and rest
14.7.6 Functions rplaca and rplacd
14.7.7 Functions second, third, fourth, fifth and sixth
14.7.8 Functions append, nconc and append*
14.7.9 Function list
14.7.10 Function list*
14.7.11 Function sub-list
14.7.12 Function replace-list
14.7.13 Functions listp and proper-listp
14.7.14 Function length-list
14.7.15 Function copy-list
14.7.16 Function copy-cons
14.7.17 Functions reverse and nreverse
14.7.18 Function ldiff
14.7.19 Function last
14.7.20 Functions flatten and flatten*
14.7.21 Functions memq, memql and memqual
14.7.22 Functions member and member-if
14.7.23 Functions remq, remql and remqual
14.7.24 Functions remq, remql* and remqual*
14.7.25 Functions countqual, countql and countq
14.8 Applicative List Processing
14.8.1 Functions remove-if, keep-if, remove-if* and keep-if*
14.8.2 Function count-if
14.8.3 Functions posqual, posql and posq
14.8.4 Functions posqual, posql and posq
14.8.5 Functions pos and pos-if
14.8.6 Functions pos-max and pos-min
14.8.7 Function where
14.8.8 Function select
14.8.9 Funtion partition
14.8.10 Function partition*
14.8.11 Function tree-find
14.8.12 Functions find and find-if
14.8.13 Functions find-max and find-min
14.8.14 Function set-diff
14.8.15 Functions mapcar, mappend mapcar* and mappend*
14.8.16 Function mapdo
14.8.17 Functions transpose and zip
14.8.18 Function interpose
14.8.19 Functions conses and conses*
14.8.20 Functions apply and iapply
14.8.21 Functions reduce-left and reduce-right
14.8.22 Function some, all and none
14.8.23 Function multi
14.9 Association Lists
14.9.1 Function assoc
14.9.2 Function assql
14.9.3 Function acons
14.9.4 Function acons-new
14.9.5 Function aconsql-new
14.9.6 Function alist-remove
14.9.7 Function alist-nremove
14.9.8 Function copy-alist
14.10 Property Lists
14.10.1 Function prop
14.11 List Sorting
14.11.1 Function merge
14.11.2 Function multi-sort
14.12 Lazy Lists and Lazy Evaluation
14.12.1 Function make-lazy-cons
14.12.2 Function lconsp
14.12.3 Function lcons-fun
14.12.4 Functions lazy-stream-cons and get-lines
14.12.5 Macro delay
14.12.6 Function force
14.13 Lazy Sequences, Ranges, Permutations and Combinations
14.13.1 Functions generate and giterate
14.13.2 Function repeat
14.13.3 Macros gen and gun
14.13.4 Functions range and range*
14.13.5 Function perm
14.13.6 Function rperm
14.13.7 Function comb
14.13.8 Function rcomb
14.14 Characters and Strings
14.14.1 Function mkstring
14.14.2 Function copy-str
14.14.3 Function upcase-str
14.14.4 Function downcase-str
14.14.5 Function string-extend
14.14.6 Function stringp
14.14.7 Function length-str
14.14.8 Function search-str
14.14.9 Function search-str-tree
14.14.10 Function match-str
14.14.11 Function match-str-tree
14.14.12 Function sub-str
14.14.13 Function replace-str
14.14.14 Function cat-str
14.14.15 Function split-str
14.14.16 Function split-str-set
14.14.17 Functions tok-str and tok-where
14.14.18 Function list-str
14.14.19 Function trim-str
14.14.20 Function chrp
14.14.21 Function chr-isalnum
14.14.22 Function chr-isalpha
14.14.23 Function chr-isascii
14.14.24 Function chr-iscntrl
14.14.25 Function chr-isdigit
14.14.26 Function chr-isgraph
14.14.27 Function chr-islower
14.14.28 Function chr-isprint
14.14.29 Function chr-ispunct
14.14.30 Function chr-isspace
14.14.31 Function chr-isupper
14.14.32 Function chr-isxdigit
14.14.33 Function chr-toupper
14.14.34 Function chr-tolower
14.14.35 Functions num-chr and chr-num
14.14.36 Function chr-str
14.14.37 Function chr-str-set
14.14.38 Function span-str
14.14.39 Function compl-span-str
14.14.40 Function break-str
14.15 Lazy Strings
14.15.1 Function lazy-str
14.15.2 Function lazy-stringp
14.15.3 Function lazy-str-force-upto
14.15.4 Function lazy-str-force
14.15.5 Function lazy-str-get-trailing-list
14.15.6 Functions length-str->, length-str->=, length-str-< and length-str-<=
14.15.7 Function cmp-str
14.15.8 Functions str=, str<, str>, str>= and str<=
14.15.9 Function string-lt
14.16 Vectors
14.16.1 Function vector
14.16.2 Function vec
14.16.3 Function vectorp
14.16.4 Function vec-set-length
14.16.5 Function vecref
14.16.6 Function vec-push
14.16.7 Function length-vec
14.16.8 Function size-vec
14.16.9 Function vector-list
14.16.10 Function list-vector
14.16.11 Function copy-vec
14.16.12 Function sub-vec
14.16.13 Function replace-vec
14.16.14 Function cat-vec
14.17 Sequence Manipulation
14.17.1 Function seqp
14.17.2 Function length
14.17.3 Function empty
14.17.4 Function copy
14.17.5 Function sub
14.17.6 Function replace
14.17.7 Function search
14.17.8 Functions ref and refset
14.17.9 Function update
14.17.10 Function sort
14.17.11 Function uniq
14.17.12 Function tuples
14.17.13 Function partition-by
14.17.14 Function make-like
14.17.15 Function nullify
14.18 Math Library
14.18.1 Arithmetic functions + and -
14.18.2 Functions /, trunc and mod
14.18.3 Function gcd
14.18.4 Function abs
14.18.5 Functions floor and ceil
14.18.6 Functions sin, cos, tan, asin, acos, atan and atan2
14.18.7 Functions exp, log, log10 and log2
14.18.8 Functions expt, sqrt and isqrt
14.18.9 Function exptmod
14.18.10 Function cum-norm-dist
14.18.11 Functions n-choose-k and n-perm-k
14.18.12 Functions fixnump, bignump, integerp, floatp and numberp
14.18.13 Function zerop
14.18.14 Functions evenp and oddp
14.18.15 Functions >, <, >=, <= and =
14.18.16 Function /=
14.18.17 Functions max and min
14.18.18 Functions int-str, flo-str and num-str
14.18.19 Functions int-flo and flo-int
14.18.20 Functions tofloat and toint
14.18.21 Variables *flo-min*, *flo-max*, and *flo-epsilon*
14.18.22 Variable *flo-dig*
14.18.23 Variables *pi* and *e*
14.19 Bit Operations
14.19.1 Functions logand, logior, and logxor
14.19.2 Function logtest
14.19.3 Functions lognot and logtrunc
14.19.4 Function ash
14.19.5 Function bit
14.19.6 Function mask
14.20 Exceptions
14.20.1 Functions throw, throwf and error
14.20.2 Operator catch
14.21 Regular Expression Library
14.21.1 Functions search-regex and range-regex
14.21.2 Function match-regex
14.21.3 Function match-regex-right
14.21.4 Function regsub
14.21.5 Function regexp
14.21.6 Function regex-compile
14.21.7 Function regex-parse
14.22 Hashing Library
14.22.1 Functions make-hash, and hash
14.22.2 Function hash-construct
14.22.3 Function hash-update
14.22.4 Function hash-update-1
14.22.5 Function group-by
14.22.6 Functions make-similar-hash and copy-hash
14.22.7 Function inhash
14.22.8 Function gethash
14.22.9 Function sethash
14.22.10 Function pushhash
14.22.11 Function remhash
14.22.12 Function hash-count
14.22.13 Function get-hash-userdata
14.22.14 Function set-hash-userdata
14.22.15 Function hashp
14.22.16 Function maphash
14.22.17 Functions hash-eql and hash-equal
14.22.18 Functions hash_keys, hash_values, hash_pairs and hash_alist
14.22.19 Operator dohash
14.22.20 Functions hash-uni, hash-diff and hash-isec
14.23 Partial Evaluation and Combinators
14.23.1 Macros op and do
14.23.2 Macros ap, ip, ado and ido.
14.23.3 Macro ret
14.23.4 Function chain
14.23.5 Function juxt
14.23.6 Functions andf and orf
14.23.7 Functions iff and iffi
14.23.8 Functions tf and nilf
14.23.9 Function retf
14.23.10 Functions apf and ipf
14.24 Input and Output (Streams)
14.24.1 Special variables *stdout*, *stddebug*, *stdin*, *stderr* and *stdnull*
14.24.2 Function format
14.24.3 Functions print, pprint, prinl, pprinl, tostring and tostringp
14.24.4 Function streamp
14.24.5 Function real-time-stream-p
14.24.6 Function make-string-input-stream
14.24.7 Function make-string-byte-input-stream
14.24.8 Function make-string-output-stream
14.24.9 Function get-string-from-stream
14.24.10 Function make-strlist-output-stream
14.24.11 Function get-list-from-stream
14.24.12 Function close-stream
14.24.13 Functions get-line, get-char and get-byte
14.24.14 Function get-string
14.24.15 Functions unget-char and unget-byte
14.24.16 Functions put-string, put-line, put-char and put-byte
14.24.17 Functions put-strings and put-lines
14.24.18 Function flush-stream
14.24.19 Function seek-stream
14.24.20 Functions stream-get-prop and stream-set-prop
14.24.21 Function make-catenated-stream
14.24.22 Functions open-files and open-files*
14.24.23 Function abs-path-p
14.24.24 Function read
14.25 Filesystem Access
14.25.1 Function stat
14.25.2 Special variables s-ifmt, s-iflnk, s-ifreg, s-ifblk, ... , s-ixoth
14.25.3 Function open-directory
14.25.4 Function open-file
14.25.5 Function open-tail
14.25.6 Function remove-path
14.25.7 Function rename-path
14.26 Coprocesses
14.26.1 Functions open-command and open-process
14.27 Symbols and Packages
14.27.1 Special variables *user-package*, *keyword-package*, and *system-package*
14.27.2 Function make-sym
14.27.3 Function gensym
14.27.4 Special variable *gensym-counter*
14.27.5 Function make-package
14.27.6 Function packagep
14.27.7 Function find-package
14.27.8 Function intern
14.27.9 Function rehome-sym
14.27.10 Function symbolp
14.27.11 Function symbol-name
14.27.12 Function symbol-package
14.27.13 Function packagep
14.27.14 Function keywordp
14.28 Pseudo-random Numbers
14.28.1 Special variable *random-state*
14.28.2 Function make-random-state
14.28.3 Function random-state-p
14.28.4 Functions random-fixnum, random and rand
14.29 Time
14.29.1 Functions time and time-usec
14.29.2 Functions time-string-local and time-string-utc
14.29.3 Functions time-fields-local and time-fields-utc
14.29.4 Functions make-time and make-time-utc
14.30 Environment Variables and Command Line
14.30.1 Special variables *args* and *args-full*
14.30.2 Function env
14.30.3 Function env-hash
14.31 Unix Programming
14.31.1 Function errno
14.31.2 Function exit
14.31.3 Function usleep
14.31.4 Function mkdir
14.31.5 Function chdir
14.31.6 Function pwd
14.31.7 Functions getpid and getppid
14.31.8 Function daemon
14.31.9 Functions makedev, minor and major
14.31.10 Function mknod
14.31.11 Functions symlink and link
14.31.12 Function readlink
14.31.13 Functions sh and run
14.32 Unix Signal Handling
14.32.1 Special variables sig-hup, sig-int, sig-quit, sig-ill, sig-trap, sig-abrt, sig-bus, sig-fpe, sig-kill, sig-usr1, sig-segv, sig-usr2, sig-pipe, sig-alrm, sig-term, sig-chld, sig-cont, sig-stop, sig-tstp, sig-ttin, sig-ttou, sig-urg, sig-xcpu, sig-xfsz, sig-vtalrm, sig-prof, sig-poll, sig-sys, sig-winch, sig-iot, sig-stkflt, sig-io, sig-lost and sig-pwr
14.32.2 Functions set-sig-handler and get-sig-handler
14.32.3 The sig-check function
14.32.4 The kill function
14.33 Unix Itimers
14.33.1 Variables itimer-real, itimer-virtual, and itimer-prof
14.33.2 Functions getitimer and setitimer
14.34 Unix Syslog
14.34.1 Special variables log-pid, log-cons, log-ndelay, log-odelay, log-nowait and log-perror
14.34.2 Special variables log-user, log-daemon, log-auth and log-authpriv
14.34.3 Special variables log-emerg, log-alert, log-crit, log-err, log-warning, log-notice, log-info and log-debug
14.34.4 The *stdlog* special variable
14.34.5 The openlog function
14.34.6 The closelog function
14.34.7 The setlogmask function
14.34.8 The syslog function
14.35 Web Programming Support
14.35.1 Functions url-encode and url-decode
14.35.2 Functions html-encode and html-decode
14.36 Filter Module
14.36.1 Function make-trie
14.36.2 Function trie-add
14.36.3 Function trie-compress
14.36.4 Function trie-lookup-begin
14.36.5 Function trie-lookup-feed-char
14.36.6 Function tree-value-at
14.36.7 Function filter-string-tree
14.36.8 Function filter-equal
14.37 Access To TXR Pattern Language From Lisp
14.37.1 Function match-fun
14.38 Quote/Quasiquote Operator Syntax
14.38.1 Operator quote
14.38.2 Macro qquote
14.38.3 Operator unquote
14.38.4 Operator splice
14.39 Macros
14.39.1 Macro parameter lists
14.39.2 Operator macro-time
14.39.3 Operator defmacro
14.39.4 Operator macrolet
14.39.5 Function macro-form-p
14.39.6 Functions macroexpand-1 and macroexpand
14.39.7 Operator defsymacro
14.39.8 Operator symacrolet
14.39.9 Operator tree-bind
14.39.10 Operator tree-case
14.40 Debugging Functions
14.40.1 Functions source-loc and source-loc-str
14.40.2 Function rlcp
14.41 Profiling
14.41.1 Operator prof
14.41.2 Macro pprof
14.42 Garbage Collection
14.42.1 Function sys:gc
14.42.2 Function sys:gc-set-delta
14.43 Modularization
14.43.1 Special variable *self-path*
14.44 Debugger
14.44.1 Sample Session
14.45 Compatibility
15 Appendix
15.1 A. NOTES ON EXOTIC REGULAR EXPRESSIONS

 

1 NAME

TXR - text processing language (version 99)  

2 SYNOPSIS

txr [options ] query-file data-files ..

 

3 DESCRIPTION

TXR is a language oriented toward processing text from files or streams, using multiple programming paradigms.

A TXR script is called a query, and it specifies a pattern which matches (a prefix of) an entire file, or multiple files. Patterns can consists of large chunks of multi-line free-form text, which is matched literally against material in the input sources. Free variables occurring in the pattern (denoted by the @ symbol) are bound to the pieces of text occurring in the corresponding positions. If the overall match is successful, then TXR can do one of two things: it can report the list of variables which were bound, in the form of a set of variable assignments which can be evaluated by the eval command of the POSIX shell language, or generate a custom report according to special directives in the query. Patterns can be arbitrarily complex, and can be broken down into named pattern functions, which may be mutually recursive. TXR patterns can work horizontally (characters within a line) or vertically (spanning multiple lines). Multiple lines can be treated as a single line.

In addition to embedded variables which implicitly match text, the TXR query language supports a number of directives, for matching text using regular expressions, for continuing a match in another file, for searching through a file for the place where an entire sub-query matches, for collecting lists, and for combining sub-queries using logical conjunction, disjunction and negation, and numerous others.

Furthermore, embedded within TXR is a powerful Lisp dialect. TXR Lisp supports functional and imperative programming, and provides data types such as symbols, strings, vectors, hash tables with weak reference support, lazy lists, and arbitrary-precision (bignum integers).

 

4 ARGUMENTS AND OPTIONS

Options which don't take an argument may be combined together. The -v and -q options are mutually exclusive. Of these two, the one which occurs in the rightmost position in the argument list dominates. The -c and -f options are also mutually exclusive; if both are specified, it is a fatal error.

-Dvar=value
Bind the variable var to the value value prior to processing the query. The name is in scope over the entire query, so that all occurrence of the variable are substituted and match the equivalent text. If the value contains commas, these are interpreted as separators, which give rise to a list value. For instance -Da,b,c creates a list of the strings "a", "b" and "c". (See Collect Directive bellow). List variables provide a multiple match. That is to say, if a list variable occurs in a query, a successful match occurs if any of its values matches the text. If more than one value matches the text, the first one is taken.

-Dvar
Binds the variable var to an empty string value prior to processing the query.

-q
Quiet operation during matching. Certain error messages are not reported on the standard error device (but the if the situations occur, they still fail the query). This option does not suppress error generation during the parsing of the query, only during its execution.

-d
--debugger

Invoke the interactive TXR debugger. See the DEBUGGER section.

-v
Verbose operation. Detailed logging is enabled.

-b
This is a deprecated option, which is silently ignored. In TXR versions prior to 90, the printing of variable bindings (see -B option) was implicit behavior which was automatically suppressed in certain situations. The -b option suppressed it unconditionally.

-B
If the query is successful, print the variable bindings as a sequence of assignments in shell syntax that can be eval-ed by a POSIX shell. II the query fails, print the word "false". Evaluation of this word by the shell has the effect of producing an unsuccessful termination status from the shell's eval command.

-l or --lisp-bindings
This option implies -B. Print the variable bindings in Lisp syntax instead of shell syntax.

-a num
This option implies -B. The decimal integer argument num specifies the maximum number of array dimensions to use for list-valued variable bindings. The default is 1. Additional dimensions are expressed using numeric suffixes in the generated variable names. For instance, consider the three-dimensional list arising out of a triply nested collect: ((("a" "b") ("c" "d")) (("e" "f") ("g" "h"))). Suppose this is bound to a variable V. With -a 1, this will be reported as:


  V_0_0[0]="a"
  V_0_1[0]="b"
  V_1_0[0]="c"
  V_1_1[0]="d"
  V_0_0[1]="e"
  V_0_1[1]="f"
  V_1_0[1]="g"
  V_1_1[1]="h"

With -a 2, it comes out as:


  V_0[0][0]="a"
  V_1[0][0]="b"
  V_0[0][1]="c"
  V_1[0][1]="d"
  V_0[1][0]="e"
  V_1[1][0]="f"
  V_0[1][1]="g"
  V_1[1][1]="h"

The leftmost bracketed index is the most major index. That is to say, the dimension order is: NAME_m_m+1_..._n[1][2]...[m-1].

-c query
Specifies the query in the form of a command line argument. If this option is used, the query-file argument is omitted. The first non-option argument, if there is one, now specifies the first input source rather than a query. Unlike queries read from a file, (non-empty) queries specified as arguments using -c do not have to properly end in a newline. Internally, TXR adds the missing newline before parsing the query. Thus -c "@a" is a valid query which matches a line.

Example:

Shell script which uses TXR to read two lines "1" and "2" from standard input, binding them to variables a and b. Standard input is specified as - and the data comes from shell "here document" redirection:

code:
 #!/bin/sh


 txr -B -c "@a
 @b" - <<!
 1
 2
 !

output:
 a=1
 b=2

The @; comment syntax can be used for better formatting:


  txr -B -c "@;
  @a
  @b"

-f query-file
Specifies the file from which the query is to be read, instead of the query-file argument. This is useful in #! ("hash bang") scripts. (See Hash Bang Support below).

-e expression
Evaluates a TXR Lisp expression for its side effects, without printing its value. Can be specified more than once. The query-file argument becomes optional if -e is used at least once.

-p expression
Evaluates a TXR Lisp expression and prints its value. Can be specified more than once. The query-file argument becomes optional if -p is used at least once.

-C number
--compat=number

Requests TXR to behave in a manner that is compatible with the specified version of TXR. This makes a difference in situations when a release of TXR breaks backward compatibility. If some version N+1 deliberately introduces a change which is backward incompatible, then -C N can be used to request the old behavior.

The requested value of N can be too low, in which case TXR will complain and exit with an unsuccessful termination status. This indicates that TXR refuses to be compatible with such an old version. Users requiring the behavior of that version will have to install an older version of TXR which supports that behavior, or even that exact version.

If the option is specified more than once, the behavior is not specified.

For more information, see the COMPATIBILITY section.

--gc-delta=number

The number argument to this option must be a decimal integer. It represents a megabyte value, the "GC delta": one megabyte is 1048576 bytes. The "GC delta" controls an aspect of the garbage collector behavior. See the gc-set-delta function for a description.

--help
Prints usage summary on standard output, and terminates successfully.

--license
Prints the software license. This depends on the software being installed such that the LICENSE file is in the data directory. Use of TXR implies agreement with the liability disclaimer in the license.

--version
Prints program version standard output, and terminates successfully.

--args
The --args option provides a way to encode multiple arguments as a single argument, which is useful on some systems which have limitations in their implementation of the "hash bang" mechanism. For details about its special syntax, See Hash Bang Support below.

--
Signifies the end of the option list.

-
This argument is not interpreted as an option, but treated as a filename argument. After the first such argument, no more options are recognized. Even if another argument looks like an option, it is treated as a name. This special argument - means "read from standard input" instead of a file. The query file, or any of the data files, may be specified using this option. If two or more files are specified as -, the behavior is system-dependent. It may be possible to indicate EOF from the interactive terminal, and then specify more input which is interpreted as the second file, and so forth.

After the options, the remaining arguments are files. The first file argument specifies the query, and is mandatory. A file argument consisting of a single - means to read the standard input instead of opening a file. A file argument which begins with an exclamation symbol means that the rest of the argument is a shell command which is to be run as a coprocess, and its output read like a file.

TXR begins by reading the query. The entire query is scanned, internalized and then begins executing, if it is free of syntax errors. The reading of data, on the other hand, is lazy. A file isn't opened until the query demands material from that file, and then the contents are read on demand, not all at once.

If no files arguments are specified on the command line, it is up to the query to open a file, pipe or standard input via the @(next) directive prior to attempting to make a match. If a query attempts to match text, but has run out of files to process, the match fails.

 

5 STATUS AND ERROR REPORTING

TXR sends errors and verbose logs to the standard error device. The following paragraphs apply when TXR is run without enabling verbose mode with -v, or the printing of variable bindings with -B or -a.

If the command line arguments are incorrect, or the query has a malformed syntax, TXR issues an error diagnostic and terminates with a failed status.

If the query fails due to a mismatch, TXR terminates with a failed status. No diagnostics are issued.

If the query is well-formed, and matches, then TXR issues no diagnostics, and terminates with a successful status.

In verbose mode (option -v), TXR issues diagnostics on the standard error device even in situations which are not erroneous.

In bindings-printing mode (options -B or -a), TXR prints the word false if the query fails, and exits with a failed termination status. If the query succeeds, the variable bindings, if any, are output on standard output.

 

6 BASIC QUERY SYNTAX AND SEMANTICS

 

6.1 Comments

A query may contain comments which are delimited by the sequence @; and extend to the end of the line. No whitespace can occur between the @ and ;. A comment which begins on a line swallows that entire line, as well as the newline which terminates it. In essence, the entire comment line disappears. If the comment follows some material in a line, then it does not consume the newline. Thus, the following two queries are equivalent:

1.
 @a@; comment: match whole line against variable @a
 @; this comment disappears entirely
 @b

2.
 @a
 @b

The comment after the @a does not consume the newline, but the comment which follows does. Without this intuitive behavior, line comment would give rise to empty lines that must match empty lines in the data, leading to spurious mismatches.

Instead of the ; character, the # character can be used. This is an obsolescent feature.

 

6.2 Hash Bang Support

TXR has several features which support use of the "hash bang" convention for creating apparently stand-alone executable programs.

If the first line of a query begins with the characters #!, that entire line is deleted from the query. This allows for TXR queries to be turned into standalone executable programs in the POSIX environment.

Shell example: create a simple executable program called "twoline.txr" and run it. This assumes TXR is installed in /usr/bin.


  $ cat > hello.txr
  #!/usr/bin/txr
  @(bind a "Hey")
  @(output)
  Hello, world!
  @(end)
  $ chmod a+x hello.txr
  $ ./hello.txr
  Hello, world!

When this plain hash bang line is used, TXR receives the name of the script as an argument. Therefore, it is not possible to pass additional options to TXR. For instance, if the above script is invoked like this


  $ ./hello.txr -B

the -B option isn't processed by TXR, but treated as an additional argument, just as if txr scriptname -B had been executed directly.

This behavior is useful if the script author wants not to expose the TXR options to the user of the script.

However, the hash bang line can use the -f option:


  #!/usr/bin/txr -f

Now, the name of the script is passed as an argument to the -f option, and TXR will look for more options after that, so that the resulting program appears to accept TXR options. Now we can run


  $ ./hello.txr -B
  Hello, world!
  a="Hey"

The -B option is honored.

On some operating systems, it is not possible to pass more than one argument through the hash bang mechanism. That is to say, this will not work.


  #!/usr/bin/txr -B -f

To support systems like this, TXR supports the special argument --args. With --args, it is possible to encode multiple arguments into one argument. The --args option must be followed by a separator character, chosen by the programmer. The characters after that are split into multiple arguments on the separator character. The --args option is then removed from the argument list and replaced with these arguments, which are processed in its place.

Example:


  #!/usr/bin/txr --args:-B:-f

The above has the same behavior as


  #!/usr/bin/txr -B -f

on a system which supports multiple arguments in hash bang. The separator character is the colon, and so the remainder of that argument, -B:-f, is split into the two arguments -B -f.

 

6.3 Whitespace

Outside of directives, whitespace is significant in TXR queries, and represents a pattern match for whitespace in the input. An extent of text consisting of an undivided mixture of tabs and spaces is a whitespace token.

Whitespace tokens match a precisely identical piece of whitespace in the input, with one exception: a whitespace token consisting of precisely one space has a special meaning. It is equivalent to the regular expression @/[ ]+/: match an extent of one or more spaces (but not tabs!). Multiple consecutive spaces do not have this meaning.

Thus, the query line "a b" (one space between a and b) matches "a b" with any number of spaces between the two letters.

For matching a single space, the syntax @\ can be used (backslash-escaped space).

It is more often necessary to match multiple spaces than to exactly match one space, so this rule simplifies many queries and adds inconvenience to only few.

In output clauses, string and character literals and quasiliterals, a space token denotes a space.

 

6.4 Text

Query material which is not escaped by the special character @ is literal text, which matches input character for character. Text which occurs at the beginning of a line matches the beginning of a line. Text which starts in the middle of a line, other than following a variable, must match exactly at the current position, where the previous match left off. Moreover, if the text is the last element in the line, its match is anchored to the end of the line.

An empty query line matches an empty line in the input. Note that an empty input stream does not contain any lines, and therefore is not matched by an empty line. An empty line in the input is represented by a newline character which is either the first character of the file, or follows a previous newline-terminated line.

Input streams which end without terminating their last line with a newline are tolerated, and are treated as if they had the terminator.

Text which follows a variable has special semantics, discussed in the section Variables below.

A query may not leave a line of input partially matched. If any portion of a line of input is matched, it must be entirely matched, otherwise a matching failure results. However, a query may leave unmatched lines. Matching only four lines of a ten line file is not a matching failure. The eof directive can be used to explicitly match the end of a file.

In the following example, the query matches the text, even though the text has an extra line.

code:
 Four score and seven
 years ago our

data:
 Four score and seven
 years ago our
 forefathers

In the following example, the query fails to match the text, because the text has extra material on one line that is not matched:

code:
 I can carry nearly eighty gigs
 in my head

data:
 I can carry nearly eighty gigs of data
 in my head

Needless to say, if the text has insufficient material relative to the query, that is a failure also.

To match arbitrary material from the current position to the end of a line, the "match any sequence of characters, including empty" regular expression @/.*/ can be used. Example:

code:
 I can carry nearly eighty gigs@/.*/

data:
 I can carry nearly eighty gigs of data

In this example, the query matches, since the regular expression matches the string "of data". (See Regular Expressions section below).

Another way to do this is:

code:
 I can carry nearly eighty gigs@(skip)

 

6.5 Special Characters in Text

Control characters may be embedded directly in a query (with the exception of newline characters). An alternative to embedding is to use escape syntax. The following escapes are supported:

@\ newline
A backslash immediately followed by a newline introduces a physical line break without breaking up the logical line. Material following this sequence continues to be interpreted as a continuation of the previous line, so that indentation can be introduced to show the continuation without appearing in the data.
@\ space
A backslash followed by a space encodes a space. This is useful in line continuations when it is necessary for some or all of the leading spaces to be preserved. For instance the two line sequence


  abcd@\
    @\  efg

is equivalent to the line


  abcd  efg

The two spaces before the @\ in the second line are consumed. The spaces after are preserved.

@\a
Alert character (ASCII 7, BEL).
@\b
Backspace (ASCII 8, BS).
@\t
Horizontal tab (ASCII 9, HT).
@\n
Line feed (ASCII 10, LF). Serves as abstract newline on POSIX systems.
@\v
Vertical tab (ASCII 11, VT).
@\f
Form feed (ASCII 12, FF). This character clears the screen on many kinds of terminals, or ejects a page of text from a line printer.
@\r
Carriage return (ASCII 13, CR).
@\e
Escape (ASCII 27, ESC)
@\x hex-digits
A @\x immediately followed by a sequence of hex digits is interpreted as a hexadecimal numeric character code. For instance @\x41 is the ASCII character A.
@\ octal-digits

A @\ immediately followed by a sequence of octal digits (0 through 7) is interpreted as an octal character code. For instance @\010 is character 8, same as @\b.

Note that if a newline is embedded into a query line with @\n, this does not split the line into two; it's embedded into the line and thus cannot match anything. However, @\n may be useful in the @(cat) directive and in @(output).

 

6.6 Character Handling and International Characters

TXR represents text internally using wide characters, which are used to represent Unicode code points. The query language, as well as all data sources, are assumed to be in the UTF-8 encoding. In the query language, extended characters can be used directly in comments, literal text, string literals, quasiliterals and regular expressions. Extended characters can also be expressed indirectly using hexadecimal or octal escapes. On some platforms, wide characters may be restricted to 16 bits, so that TXR can only work with characters in the BMP (Basic Multilingual Plane) subset of Unicode.

TXR does not use the localization features of the system library; its handling of extended characters is not affected by environment variables like LANG and L_CTYPE. The program reads and writes only the UTF-8 encoding.

If TXR encounters an invalid bytes in the UTF-8 input, what happens depends on the context in which this occurs. In a query, comments are read without regard for encoding, so invalid encoding bytes in comments are not detected. A comment is simply a sequence of bytes terminated by a newline. In lexical elements which represent text, such as string literals, invalid or unexpected encoding bytes are treated as syntax errors. The scanner issues an error message, then discards a byte and resumes scanning. Certain sequences pass through the scanner without triggering an error, namely some UTF-8 overlong sequences. These are caught when when the lexeme is subject to UTF-8 decoding, and treated in the same manner as other UTF-8 data, described in the following paragraph.

Invalid bytes in data are treated as follows. When an invalid byte is encountered in the middle of a multibyte character, or if the input ends in the middle of a multibyte character, or if a character is extracted which is encoded as an overlong form, the UTF-8 decoder returns to the starting byte of the ill-formed multibyte character, and extracts just that byte, mapping it to the Unicode character range U+DC00 through U+DCFF. The decoding resumes afresh at the following byte, expecting that byte to be the start of a UTF-8 code.

Furthermore, because TXR internally uses a null-terminated character representation of strings which easily interoperates with C language interfaces, when a null character is read from a stream, TXR converts it to the code U+DC00. On output, this code converts back to a null byte, as explained in the previous paragraph. By means of this representational trick, TXR can handle textual data containing null bytes.

 

6.7 Regular Expression Directives

In place of a piece of text (see section Text above), a regular expression directive may be used, which has the following syntax:


  @/RE/

where the RE part enclosed in slashes represents regular expression syntax (described in the section Regular Expressions below).

Long regular expressions can be broken into multiple lines using a backslash-newline sequence. Whitespace before the sequence or after the sequence is not significant, so the following two are equivalent:


  @/reg \
    ular/


  @/regular/

There may not be whitespace between the backslash and newline.

Whereas literal text simply represents itself, regular expression denotes a (potentially infinite) set of texts. The regular expression directive matches the longest piece of text (possibly empty) which belongs to the set denoted by the regular expression. The match is anchored to the current position; thus if the directive is the first element of a line, the match is anchored to the start of a line. If the regular expression directive is the last element of a line, it is anchored to the end of the line also: the regular expression must match the text from the current position to the end of the line.

Even if the regular expression matches the empty string, the match will fail if the input is empty, or has run out of data. For instance suppose the third line of the query is the regular expression @/.*/, but the input is a file which has only two lines. This will fail: the data has no line for the regular expression to match. A line containing no characters is not the same thing as the absence of a line, even though both abstractions imply an absence of characters.

Like text which follows a variable, a regular expression directive which follows a variable has special semantics, discussed in the section Variables below.

 

6.8 Variables

Much of the query syntax consists of arbitrary text, which matches file data character for character. Embedded within the query may be variables and directives which are introduced by a @ character. Two consecutive @@ characters encode a literal @.

A variable matching or substitution directive is written in one of several ways:


  @
sident
  @{
bident}
  @*
sident
  @*{
bident}
  @{
bident /regex/}
  @{
bident (fun [arg ... ])}
  @{
bident number}

The forms with an * indicate a long match, see Longest Match below. The last two three forms with the embedded regexp /regex/ or number or function have special semantics; see Positive Match below.

The identifier t cannot be used as a name; it is a reserved symbol which denotes the value true. An attempt to use the variable @t will result in an exception. The symbol nil can be used where a variable name is required syntacticaly, but it has special semantics, described in a section below.

A sident is a "simple identifier" form which is not delimited by braces.

A sident consists of any combination of one or more letters, numbers, and underscores. It may not look like a number, so that for instance 123 is not a valid sident, but 12A is valid. Case is sensitive, so that FOO is different from foo, which is different from Foo.

The braces around an identifier can be used when material which follows would otherwise be interpreted as being part of the identifer. When a name is enclosed in braces it is a bident.

The following additional characters may be used as part of bident which are not allowed in a sident:


 ! $ % & * + - < = > ? \ _ ~

The rule still holds that a name cannot look like a number so +123 is not a valid bident but these are valid: a->b, *xyz*, foo-bar.

The syntax @FOO_bar introduces the name FOO_bar, whereas @{FOO}_bar means the variable named "FOO" followed by the text "_bar". There may be whitespace between the @ and the name, or opening brace. Whitespace is also allowed in the interior of the braces. It is not significant.

If a variable has no prior binding, then it specifies a match. The match is determined from some current position in the data: the character which immediately follows all that has been matched previously. If a variable occurs at the start of a line, it matches some text at the start of the line. If it occurs at the end of a line, it matches everything from the current position to the end of the line.

 

6.9 Negative Match

If a variable is one of the plain forms


  @
sident
  @{
bident}
  @*
sident
  @*{
bident}

then this is a "negative match". The extent of the matched text (the text bound to the variable) is determined by looking at what follows the variable, and ranges from the current position to some position where the following material finds a match. This is why this is called a "negative match": the spanned text which ends up bound to the variable is that in which the match for the trailing material did not occur.

A variable may be followed by a piece of text, a regular expression directive, a function call, a directive, another variable, or nothing (i.e. occurs at the end of a line). These cases are discussed in detail below.

 

6.9.1 Variable Followed by Nothing

If the variable is followed by nothing, the negative match extends from the current position in the data, to the end of the line. Example:
code:
 a b c @FOO
data:
 a b c defghijk
result:
 FOO="defghijk"

 

6.9.2 Variable Followed by Text

For the purposes of determining the negative match, text is defined as a sequence of literal text and regular expressions, not divided by a directive. So for instance in this example:


  @a:@/foo/bcd e@(maybe)f@(end)

the variable @a is considered to be followed by ":@/foo/bcd e".

If a variable is followed by text, then the extent of the negative match is determined by searching for the first occurrence of that text within the line, starting at the current position.

The variable matches everything between the current position and the matching position (not including the matching position). Any whitespace which follows the variable (and is not enclosed inside braces that surround the variable name) is part of the text. For example:

code:
 a b @FOO e f
data:
 a b c d e f
result:
 FOO="c d"

In the above example, the pattern text "a b " matches the data "a b ". So when the @FOO variable is processed, the data being matched is the remaining "c d e f". The text which follows @FOO is " e f". This is found within the data "c d e f" at position 3 (counting from 0). So positions 0-2 ("c d") constitute the matching text which is bound to FOO.

 

6.9.3 Variable Followed by a Function Call or Directive

If the variable is followed by a function call, or a directive, the extent is determined by scanning the text for the first position where a match occurs for the entire remainder of the line. (For a description of functions, see FUNCTIONS.)

For example:


  @foo@(bind a "abc")xyz

Here, foo will match the text from the current position to where "xyz" occurs, even though there is a @(bind) directive. Furthermore, if more material is added after the xyz, it is part of the search. Note the difference between the following two:


  @foo@/abc/@(func)
  @foo@(func)@/abc/

In the first example, the variable foo matches the text from the current position until the match for the regular expression abc. @(func) is not considered when processing @foo. In the second example, the variable foo matches the text from the current position until the position which matches the function call, followed by a match for the regular expression. The entire sequence @(func)@/abc/ is considered.

 

6.9.4 Consecutive Variables

If an unbound variable specified a fixed-width match or a regular expression, then the issue of consecutive variables does not arise. Such a variable consumes text regardless of any context which follows it.

However, what if an unbound variable with no modifier is followed by another variable? The behavior depends on the nature of the other variable.

If the other variable also has no modifier, this is a semantic error which will cause the query to fail. A diagnostic message will be issued, unless operating in quiet mode via -q. The reason is that there is no way to bind two consecutive variables to an extent of text; this is an ambiguous situation, since there is no matching criterion for dividing the text between two variables. (In theory, a repetition of the same variable, like @FOO@FOO, could find a solution by dividing the match extent in half, which would work only in the case when it contains an even number of characters. This behavior seems to have dubious value).

An unbound variable may be followed by one which is bound. The bound variable is replaced by the text which it denotes, and the logic proceeds accordingly.

It is possible for a variable to be bound to a regular expression. If x is an unbound variable and y is bound to a regular expression RE, then @x@y means @x@/RE/. A variable v can be bound to a regular expression using, for example, @(bind v #/RE/).

The @* syntax for longest match is available. Example:

code:
 @FOO:@BAR@FOO
data:
 xyz:defxyz
result:
 FOO=xyz, BAR=def

Here, FOO is matched with "xyz", based on the delimiting around the colon. The colon in the pattern then matches the colon in the data, so that BAR is considered for matching against "defxyz". BAR is followed by FOO, which is already bound to "xyz". Thus "xyz" is located in the "defxyz" data following "def", and so BAR is bound to "def".

If an unbound variable is followed by a variable which is bound to a list, or nested list, then each character string in the list is tried in turn to produce a match. The first match is taken.

An unbound variable may be followed by another unbound variable which specifies a regular expression or function call match. This is a special case called a "double variable match". What happens is that the text is searched using the regular expression or function. If the search fails, than neither variable is bound: it is a matching failure. If the search succeeds, than the first variable is bound to the text which is skipped by the search. The second variable is bound to the text matched by the regular expression or function. Examples:

code:
 @foo@{bar /abc/}
data:
 xyz@#abc
result:
 foo="xyz@#", BAR="abc"

 

6.9.5 Consecutive Variables Via Directive

Two variables can be de facto consecutive in a manner shown in the following example:


  @var1@(all)@var2@(end)

This is treated just like the variable followed by directive. No semantic error is identified, even if both variables are unbound. Here, @var2 matches everything at the current position, and so @var1 ends up bound to the empty string.

Example 1: b matches at position 0 and a binds the empty string:

code:
 @a@(all)@b@(end)
data:
 abc
result:
 a=""
 b="abc"

Example 2: *a specifies longest match (see Longest Match below), and so it takes everything:

code:
 @*a@(all)@b@(end)
data:
 abc
result:
 a="abc"
 b=""

 

6.9.6 Longest Match

The closest-match behavior for the negative match can be overridden to longest match behavior. A special syntax is provided for this: an asterisk between the @ and the variable, e.g:
code:
 a @*{FOO}cd
data:
 a b cdcdcdcd
result:
 FOO="b cdcdcd"

code:
 a @{FOO}cd
data:
 a b cdcdcd
result:
 FOO="b "
 b=""

In the former example, the match extends to the rightmost occurrence of "cd", and so FOO receives "b cdcdcd". In the latter example, the * syntax isn't used, and so a leftmost match takes place. The extent covers only the "b ", stopping at the first "cd" occurrence.

 

6.10 Positive Match

There are syntactic variants of variable syntax which have an embedded expression enclosed with the variable in braces:


  @{
bident /regex/}
  @{
bident (fun [args...])}
  @{
bident number}

These specify a variable binding that is driven by a positive match derived from a regular expression, function or character count, rather than from trailing material (which is regarded as a "negative" match, since the variable is bound to material which is skipped in order to match the trailing material). In the /regex/ form, the match extends over all characters from the current position which match the regular expression regex. (see Regular Expressions section below). In the (fun [args ...]) form, the match extends over characters which are matched by the call to the function, if the call succeeds. Thus @{x (y z w)} is just like @(y z w), except that the region of text skipped over by @(y z w) is also bound to the variable x. See FUNCTIONS below.

In the number form, the match processes a field of text which consists of the specified number of characters, which must be non-negative number. If the data line doesn't have that many characters starting at the current position, the match fails. A match for zero characters produces an empty string. The text which is actually bound to the variable is all text within the specified field, but excluding leading and trailing whitespace. If the field contains only spaces, then an empty string is extracted.

This syntax is processed without consideration of what other syntax follows. A positive match may be directly followed by an unbound variable.

 

6.11 Special Symbols nil and t

Just like in the Common Lisp language, the names nil and t are special.

nil symbol stands for the empty list object, an object which marks the end of a list, and boolean false. It is synonymous with the syntax () which may be used interchangeably with nil in most constructs.

In TXR Lisp, nil and t cannot be used as variables. When evaluated, they evaluate to themselves.

In the TXR pattern language, nil can be used in the variable binding syntax, but does not create a binding; it has a special meaning. It allows the variable matching syntax to be used to skip material, in ways similar to the skip directive.

The nil symbol is also used as a block name, both in the TXR pattern language and in TXR Lisp. A block named nil is considered to be anonymous.

 

6.12 Keyword Symbols

Names whose names begin with the : character are keyword symbols. These also may not be used as variables either and stand for themselves. Keywords are useful for labeling information and situations.

 

6.13 Regular Expressions

Regular expressions are a language for specifying sets of character strings. Through the use of pattern matching elements, regular expression is able to denote an infinite set of texts. TXR contains an original implementation of regular expressions, which supports the following syntax:

.
(period) is a "wildcard" that matches any character.
[]
Character class: matches a single character, from the set specified by special syntax written between the square brackets. This supports basic regexp character class syntax. POSIX notation like [:digit:] is not supported. The regex tokens \s, \d and \w are permitted in character classes, but not their complementing counterparts. These tokens simply contribute their characters to the class. The class [a-zA-Z] means match an uppercase or lowercase letter; the class [0-9a-f] means match a digit or a lowercase letter; the class [^0-9] means match a non-digit, and so forth. There are no locale-specific behaviors in TXR regular expressions; [A-Z] denotes an ASCII/Unicode range of characters. The class [\d.] means match a digit or the period character. A ] or - can be used within a character class, but must be escaped with a backslash. A ^ in the first position denotes a complemented class, unless it is escaped by backslash. In any other position, it denotes itself. Two backslashes code for one backslash. So for instance [\[\-] means match a [ or - character, [^^] means match any character other than ^, and [\^\\] means match either a ^ or a backslash. Regex operators such as *, + and & appearing in a character class represent ordinary characters. The characters -, ] and ^ occurring outside of a character class are ordinary. Unescaped / characters can appear within a character class. The empty character class [] matches no character at all, and its complement [^] matches any character, and is treated as a synonym for the . (period) wildcard operator.
\s, \w and \d
These regex tokens each match a single character. The \s regex token matches a wide variety of ASCII whitespace characters and Unicode spaces. The \w token matches alphabetic word characters; it is equivalent to the character class [A-Za-z_]. The \d token matches a digit, and is equivalent to [0-9].
\S, \W and \D
These regex tokens are the complemented counterparts of \s, \w and \d. The \S token matches all those characters which \s does not match, \W matches all characters that \w does not match and \D matches nondigits.
empty
An empty expression is a regular expression. It represents the set of strings consisting of the empty string; i.e. it matches just the empty string. The empty regex can appear alone as a full regular expression (for instance the TXR syntax @// with nothing between the slashes) and can also be passed as a subexpression to operators, though this may require the use of parentheses to make the empty regex explicit. For example, the expression a| means: match either a, or nothing. The forms * and (*) are syntax errors; though not useful, the correct way to match the empty expression zero or more times is the syntax ()*.
nomatch
The nomatch regular expression represents the empty set: it matches no strings at all, not even the empty string. There is no dedicated syntax to directly express nomatch in the regex language. However, the empty character class [] is equivalent to nomatch, and may be considered to be a notation for it. Other representations of nomatch are possible: for instance, the regex ~.* which is the complement of the regex that denotes the set of all possible strings, and thus denotes the empty set. A nomatch has uses; for instance, it can be used to temporarily "comment out" regular expressions. The regex ([]abc|xyz) is equivalent to (xyz), since the []abc branch cannot match anything. Using [] to "block" a subexpression allows you to leave it in place, then enable it later by removing the "block".
(R)
If R is a regular expression, then so is (R). The contents of parentheses denote one regular expression unit, so that for instance in (RE)*, the * operator applies to the entire parenthesized group. The syntax () is valid and equivalent to the empty regular expression.
R?
optionally match the preceding regular expression R.
R*
match the expression R zero or more times. This operator is sometimes called the "Kleene star", or "Kleene closure". The Kleene closure favors the longest match. Roughly speaking, if there are two or more ways in which R1*R2 can match, than that match occurs in which R1* matches the longest possible text.
R+
match the preceding expression R one or more times. Like R*, this favors the longest possible match: R+ is equivalent to RR*.
R1%R2
match R1 zero or more times, then match R2. If this match can occur in more than one way, then it occurs such that R1 is matched the fewest number of times, which is opposite from the behavior of R1*R2. Repetitions of R1 terminate at the earliest point in the text where a non-empty match for R2 occurs. Because it favors shorter matches, % is termed a non-greedy operator. If R2 is the empty expression, or equivalent to it, then R1%R2 reduces to R1*. So for instance (R%) is equivalent to (R*), since the missing right operand is interpreted as the empty regex. Note that whereas the expression (R1*R2) is equivalent to (R1*)R2, the expression (R1%R2) is not equivalent to (R1%)R2.
~R
match the opposite of the following expression R; that is, match exactly those texts that R does not match. This operator is called complement, or logical not.
R1R2
Two consecutive regular expressions denote catenation: the left expression must match, and then the right.
R1|R2
match either the expression R1 or R2. This operator is known by a number of names: union, logical or, disjunction, branch, or alternative.
R1&R2
match both the expression R1 and R2 simultaneously; i.e. the matching text must be one of the texts which are in the intersection of the set of texts matched by R1 and the set matched by R2. This operator is called intersection, logical and, or conjunction.

Any escaped character which does not fall into the above escaping conventions, or any unescaped character which is not a regular expression operator, denotes one-position match of that character itself.

Any of the special characters, including the delimiting /, can be escaped with a backslash to suppress its meaning and denote the character itself.

Furthermore, all of the same escapes as are described in the section Special Characters in Text above are supported - the difference is that in regular expressions, the @ character is not required, so for example a tab is coded as \t rather than @\t. Octal and hex character escapes can be optionally terminated by a semicolon, which is useful if the following characters are octal or hex digits not intended to be part of the escape.

Precedence table, highest to lowest:
OperatorsClassAssociativity
(R) []primary
R? R+ R* R%...postfixleft-to-right
R1R2catenationleft-to-right
~R ...%Runaryright-to-left
R1&R2intersectionleft-to-right
R1|R2unionleft-to-right

The % operator is like a postfix operator with respect to its left operand, but like a unary operator with respect to its right operand. Thus a~b%c~d is a(~(b%(c(~d)))) , demonstrating right-to-left associativity, where all of b% may be regarded as a unary operator being applied to c~d. Similarly, a?*+%b means (((a?)*)+)%b, where the trailing %b behaves like a postfix operator.

In TXR, regular expression matches do not span multiple lines. The regex language has no feature for multi-line matching. However, the @(freeform) directive allows the remaining portion of the input to be treated as one string in which line terminators appear as explicit characters. Regular expressions may freely match through this sequence.

It's possible for a regular expression to match an empty string. For instance, if the next input character is z, facing a the regular expression /a?/, there is a zero-character match: the regular expression's state machine can reach an acceptance state without consuming any characters. Examples:

code:
 @A@/a?/@/.*/
data:
 zzzzz
result:
 A=""

code:
 @{A /a?/}@B
data:
 zzzzz
result:
 A="", B="zzzz"

code:
 @*A@/a?/
data:
 zzzzz
result:
 A="zzzzz"

In the first example, variable @A is followed by a regular expression which can match an empty string. The expression faces the letter z at position 0 in the data line. A zero-character match occurs there, therefore the variable A takes on the empty string. The @/.*/ regular expression then consumes the line.

Similarly, in the second example, the /a?/ regular expression faces a z, and thus yields an empty string which is bound to A. Variable @B consumes the entire line.

The third example requests the longest match for the variable binding. Thus, a search takes place for the rightmost position where the regular expression matches. The regular expression matches anywhere, including the empty string after the last character, which is the rightmost place. Thus variable A fetches the entire line.

For additional information about the advanced regular expression operators, NOTES ON EXOTIC REGULAR EXPRESSIONS below.

 

6.14 Directives

The general syntax of a directive is:


  @
expr

where expr stands for a parenthesized list of subexpressions. A subexpression is a symbol, number, string literal, character literal, quasiliteral, regular expression, or a parenthesized expression. So, examples of syntactically valid directives are:


  @(banana)


  @(a b c (d e f))


  @(  a (b (c d) (e  ) ))


  @("apple" #\b #\space 3)


  @(a #/[a-z]*/ b)


  @(_ `@file.txt`)

A symbol has a slight more permissive lexical than the bident in the syntax @{bident} introduced earlier. The / (slash) character may be part of an identifier, or even constitute an entire identifier. In fact a symbol inside a directive is a lident. This is discussed in the Symbol Tokens section under TXR LISP. A symbolmust not be a number; tokens that look like numbers are treated as numbers and not symbols.

 

6.15 Character Literals

Character literals are introduced by the #\ syntax, which is either followed by a character name, the letter x followed by hex digits, the letter o followed by octal digits, or a single character. Valid character names are:


  nul                 linefeed            return
  alarm               newline             esc
  backspace           vtab                space
  tab                 page                pnul

For instance #\esc denotes the escape character.

This convention for character literals is similar to that of the Scheme language. Note that #\linefeed and #\newline are the same character. The #\pnul character is specific to TXR and denotes the U+DC00 code in Unicode; the name stands for "pseudo-null", which is related to its special function. For more information about this, see the section "Character Handling and International Characters".

 

6.16 String Literals

String literals are delimited by double quote respectively. A double quote within a string literal is encoded using \" and a backslash is encoded as \\. Backslash escapes like \n and \t are recognized, as are hexadecimal escapes like \xFF or \xxabc and octal escapes like \123. Ambiguity between an escape and subsequent text can be resolved by using trailing semicolon delimiter: "\xabc;d" is a string consisting of the character U+0ABC followed by "d". The semicolon delimiter disappears. To write a literal semicolon immediately after a hex or octal escape, write two semicolons, the first of which will be interpreted as a delimiter. Thus, "\x21;;" represents "!;".

If the line ends in the middle of a literal, it is an error, unless the last character is a backslash. This backslash is a special escape which does not denote a character; rather, it indicates that the string literal continues on the next line. The backslash is deleted, along with whitespace which immediately precedes it, as well as leading whitespace in the following line. The escape sequence "\ " (backslash space) can be used to encode a significant space.

Example:


  "foo   \
   bar"


  "foo   \
   bar"


  "foo  \
   bar"

The first string literal is the string "foobar". The second two are "foo bar".

 

6.17 Word List Literals

A word list literal (WLL) provides a convenient way to write a list of strings when such a list can be given as whitespace-delimited words.

There are two flavors of the WLL: the regular WLL which begins with #" (hash, double-quote) and the splicing list literal which begins with #*" (hash, star, double-quote).

Both types are terminated by a double quote, which may be escaped as \" in order to include it as a character. All the escaping conventions used in string literals can be used in word literals.

Unlike in string literals, whitespace (tabs, spaces and newlines) is not significant in word literals: it separates words. Whitespace may be escaped with a backslash in order to include it as a literal character.

Example:


  #"abc def ghi"  --> notates ("abc" "def" "ghi")


  #"abc   def
  ghi"            --> notates ("abc" "def" "ghi")


  #"abc def ghi" --> notates ("abc def" "ghi")

A splicing word literal differs from a word literal in that it does not produce a list of string literals, but rather it produces a sequence of string literals that is merged into the surrounding syntax.

Example:


  (1 2 3 #*"abc def" 4 5 #"abc def")


  --> (1 2 3 "abc" "def" 4 5 ("abc" "def"))

The regular WLL produced a single list object, but the splicing WLL expanded into multiple string literal objects.

 

6.18 String Quasiliterals

Quasiliterals are similar to string literals, except that they may contain variable references denoted by the usual @ syntax. The quasiliteral represents a string formed by substituting the values of those variables into the literal template. If a is bound to "apple" and b to "banana", the quasiliteral `one @a and two @{b}s` represents the string "one apple and two bananas". A backquote escaped by a backslash represents itself, and two consecutive @ characters code for a literal @. There is no \@ escape. Quasiliterals support the full output variable syntax. Expressions within variables substitutions follow the evaluation rules of TXR Lisp when the quasiliteral occurs in TXR Lisp, and the rules of the TXR pattern language when the quasiliteral occurs in the pattern language.

Quasiliterals can be split into multiple lines in the same way as ordinary string literals.

 

6.19 Quasiword List Literals

The quasiword list literals (QLL-s) are to quasiliterals what WLL-s are to ordinary literals. (See the above section Word List Literals.)

A QLL combines the convenience of the WLL with the power of quasistrings.

Just as in the case of WLL-s, there are two flavors of the QLL: the regular QLL which begins with #`  (hash, backquote) and the splicing list literal which begins with #*`  (hash, star, backquote).

Both types are terminated by a backquote, which may be escaped as \`  in order to include it as a character. All the escaping conventions used in quasiliterals can be used in QLL.

Unlike in quasiliterals, whitespace (tabs, spaces and newlines) is not significant in QLL: it separates words. Whitespace may be escaped with a backslash in order to include it as a literal character.

Note that the delimiting into words is done before the variable substitution. If the variable a contains spaces, then #`@a` nevertheless expands into a list of one item: the string derived from a.

Examples:


  #`abc @a ghi`  --> notates (`abc` `@a` `ghi`)


  #`abc   @d@e@f
  ghi`            --> notates (`abc` `@d@e@f` `ghi`)


  #`@a @b @c` --> notates (`@a @b` `@c`)

A splicing QLL differs from an ordinary QLL in that it does not produce a list of quasiliterals, but rather it produces a sequence of quasiliterals that is merged into the surrounding syntax.

 

6.20 Numbers

TXR supports integers and floating-point numbers.

An integer constant is made up of digits 0 through 9, optionally preceded by a + or - sign.

Examples:


  123
  -34
  +0
  -0
  +234483527304983792384729384723234

An integer constant can also be specified in hexadecimal using the prefix #x followed by an optional sign, followed by hexadecimal digits: 0 through 9 and the upper or lower case letters A through F:


  #xFF    ;; 255
  #x-ABC  ;; -2748

Similarly, octal numbers are supported with the prefix #o followed by octal digits:


  #o777   ;; 511

and binary numbers can be written with a #b prefix:


  #b1110  ;; 14

A floating-point constant is marked by the inclusion of a decimal point, the exponential "e notation", or both. It is an optional sign, followed by a mantissa consisting of digits, a decimal point, more digits, and then an optional exponential notation consisting of the letter e or E, an optional + or - sign, and then digits indicating the exponent value. In the mantissa, the digits are not optional. At least one digit must either precede the decimal point or follow. That is to say, a decimal point by itself is not a floating-point constant.

Examples:


  .123
  123.
  1E-3
  20E40
  .9E1
  9.E19
  -.5
  +3E+3

Examples which are not floating-point constant tokens:


  .      ;; consing dot
  123E   ;; the symbol 123E
  1.0E-  ;; syntax error: invalid floating point constant
  1.0E   ;; syntax error: invalid floating point constant
  1.E    ;; 1; consing dot; symbol E
  .e     ;; consing dot followed by symbol e

In TXR there is a special "dotdot" token consisting of two consecutive periods. An integer constant followed immediately by dotdot is recognized as such; it is not treated as a floating constant followed by a dot. That is to say, 123.. does not mean 123. . (floating point 123.0 value followed by dot token). It means 123 .. (integer 123 followed by .. token).

Dialect note: unlike in Common Lisp, 123. is not an integer, but the floating-point number 123.0.

 

6.21 Comments

Comments of the form @; were already covered. Inside directives, comments are introduced just by a ; character.

Example:


  @(foo  ; this is a comment
    bar  ; this is another comment 
    )

This is equivalent to @(foo bar).

 

6.22 Directives-driven Syntax

Some directives not only denote an expression, but are also involved in surrounding syntax. For instance, the directive


  @(collect)

not only denotes an expression, but it also introduces a syntactic phrase which requires a matching @(end) directive. In other words, @(collect) is not only an expression, but serves as a kind of token in a higher level phrase structure grammar.

Usually if this type of "syntactic directive" occurs alone in a line, not preceded or followed by other material, it is involved in a "vertical" (or line oriented) syntax.

If such a directive is embedded in a line (has preceding or trailing material) then it is in a horizontal syntactic and semantic context (character-oriented).

There is an exception: the definition of a horizontal function looks like this:


  @(define name (arg))body material@(end)

Yet, this is considered one vertical item, which means that it does not match a line of data. (This is necessary because all horizontal syntax matches something within a line of data.)

Many directives have a horizontal and vertical syntax, with different but closely related semantics. A few are still "vertical only", and some are horizontal only but in future releases, these exceptions will be minimized.

A summary of the available directives follows:

@(eof)
Explicitly match the end of file. Fails if unmatched data remains in the input stream.

@(eol)
Explicitly match the end of line. Fails if the current position is not the end of a line. Also fails if no data remains (there is no current line).

@(next)
Continue matching in another file or other data source.

@(block)
Groups together a sequence of directives into a logical name block, which can be explicitly terminated from within using the @(accept) and @(fail) directives. Blocks are discussed in the section BLOCKS below.

@(skip)
Treat the remaining query as a subquery unit, and search the lines (or characters) of the input file until that subquery matches somewhere. A skip is also an anonymous block.

@(trailer)
Treat the remaining query or subquery as a match for a trailing context. That is to say, if the remainder matches, the data position is not advanced.

@(freeform)
Treat the remainder of the input as one big string, and apply the following query line to that string. The newline characters (or custom separators) appear explicitly in that string.

@(fuzz)
The fuzz directive, inspired by the patch utility, specifies a partial match for some lines.

@(some)
Multiple clauses are each applied to the same input. Succeeds if at least one of the clauses matches the input. The bindings established by earlier successful clauses are visible to the later clauses.

@(all)
Multiple clauses are applied to the same input. Succeeds if and only if each one of the clauses matches. The clauses are applied in sequence, and evaluation stops on the first failure. The bindings established by earlier successful clauses are visible to the later clauses.

@(none)
Multiple clauses are applied to the same input. Succeeds if and only if none of them match. The clauses are applied in sequence, and evaluation stops on the first success. No bindings are ever produced by this construct.

@(maybe)
Multiple clauses are applied to the same input. No failure occurs if none of them match. The bindings established by earlier successful clauses are visible to the later clauses.

@(cases)
Multiple clauses are applied to the same input. Evaluation stops on the first successful clause.

@(require)
The require directive is similar to the do directive: it evaluates one or more TXR Lisp expressions. If the result of the rightmost expression is nil, then require triggers a match failure. See the TXR LISP section far below.

@(if), @(elif), and @(else)
The if directive with optional elif and else clauses is a syntactic sugar which translates to a combination of @(cases) and @(require).

@(choose)
Multiple clauses are applied to the same input. The one whose effect persists is the one which maximizes or minimizes the length of a particular variable.

@(empty)
The @(empty) directive matches the empty string. It is useful in certain situations, such as expressing an empty match in a directive that doesn't accept an empty clause. The @(empty) syntax has another meaning in @(output) clauses, in conjunction with @(repeat).

@(define name (args ...))
Introduces a function. Functions are discussed in the FUNCTIONS section below.

@(gather)
Searches text for matches for multiple clauses which may occur in arbitrary order. For convenience, lines of the first clause are treated as separate clauses.

@(collect)
Search the data for multiple matches of a clause. Collect the bindings in the clause into lists, which are output as array variables. The @(collect) directive is line oriented. It works with a multi-line pattern and scans line by line. A similar directive called @(coll) works within one line.

A collect is an anonymous block.

@(and)
Separator of clauses for @(some), @(all), @(none), @(maybe) and @(cases). Equivalent to @(or). The choice is stylistic.

@(or)
Separator of clauses for @(some), @(all), @(none), @(maybe) and @(cases). Equivalent to @(and). The choice is stylistic.

@(end)
Required terminator for @(some), @(all), @(none), @(maybe), @(cases), @(if), @(collect), @(coll), @(output), @(repeat), @(rep), @(try), @(block) and @(define).

@(fail)
Terminate the processing of a block, as if it were a failed match. Blocks are discussed in the section BLOCKS below.

@(accept)
Terminate the processing of a block, as if it were a successful match. What bindings emerge may depend on the kind of block: collect has special semantics. Blocks are discussed in the section BLOCKS below.

@(try)
Indicates the start of a try block, which is related to exception handling, discussed in the EXCEPTIONS section below.

@(catch) and @(finally)
Special clauses within @(try). See EXCEPTIONS below.

@(defex) and @(throw)
Define custom exception types; throw an exception. See EXCEPTIONS below.

@(assert)
The assert directive requires the following material to match, otherwise it throws an exception. It is useful for catching mistakes or omissions in parts of a query that are sure-fire matches.

@(flatten)
Normalizes a set of specified variables to one-dimensional lists. Those variables which have scalar value are reduced to lists of that value. Those which are lists of lists (to an arbitrary level of nesting) are converted to flat lists of their leaf values.

@(merge)
Binds a new variable which is the result of merging two or more other variables. Merging has somewhat complicated semantics.

@(cat)
Decimates a list (any number of dimensions) to a string, by catenating its constituent strings, with an optional separator string between all of the values.

@(bind)
Binds one or more variables against a value using a structural pattern match. A limited form of unification takes place which can cause a match to fail.

@(set)
Destructively assigns one or more existing variables using a structural pattern, using syntax similar to bind. Assignment to unbound variables triggers an error.

@(rebind)
Evaluates an expression in the current binding environment, and then creates new bindings for the variables in the structural pattern. Useful for temporarily overriding variable values in a scope.

@(forget)
Removes variable bindings.

@(local)
Synonym of @(forget).

@(output)
A directive which encloses an output clause in the query. An output section does not match text, but produces text. The directives above are not understood in an output clause.

@(repeat)
A directive understood within an @(output) section, for repeating multi-line text, with successive substitutions pulled from lists. The directive @(rep) produces iteration over lists horizontally within one line.

@(deffilter)
This directive is used for defining named filters, which are useful for filtering variable substitutions in output blocks. Filters are useful when data must be translated between different representations that have different special characters or other syntax, requiring escaping or similar treatment. Note that it is also possible to use a function as a filter. See Function Filters below.

@(filter)
The filter directive passes one or more variables through a given filter or chain or filters, updating them with the filtered values.

@(load)
The load directive loads another TXR file and interprets its contents.

@(do)
The do directive is used to evaluate TXR Lisp expressions, discarding their result values. See the TXR LISP section far below.

 

7 INPUT SCANNING AND DATA MANIPULATION

 

7.1 The next directive

The next directive indicates that the remainder of the query is to be applied to a new input source.

It can only occur by itself as the only element in a query line, and takes various arguments, according to these possibilities:


  @(next)
  @(next
source)
  @(next
source :nothrow)
  @(next :args)
  @(next :env)
  @(next :list
expr)
  @(next :string
expr)

The lone @(next) without arguments switches to the next file in the argument list which was passed to the TXR utility. However, "switch to the next file" means in a pattern matching way, not in an imperative way. It is possible for the pattern matching logic to implicitly backtrack to the previous file.

If source is given, it must be text-valued expression which denotes an input source; it may be a string literal, quasiliteral or a variable. For instance, if variable A contains the text "data", then @(next A) means switch to the file called "data", and @(next `@A.txt`) means to switch to the file "data.txt".

If the input source cannot be opened for whatever reason, TXR throws an exception (see EXCEPTIONS below). An unhandled exception will terminate the program. Often, such a drastic measure is inconvenient; if @(next) is invoked with the :nothrow keyword, then if the input source cannot be opened, the situation is treated as a simple match failure.

The variant @(next :args) means that the remaining command line arguments are to be treated as a data source. For this purpose, each argument is considered to be a line of text. If an argument is currently being processed as an input source, that argument is included at the front of the list. As the arguments are matched, they are consumed. This means that if a @(next) directive without arguments is executed in the scope of @(next :args), it opens the file named by the first unconsumed argument.

To process arguments, and then continue with the original file and argument list, wrap the argument processing in a @(block).

The variant @(next :env) means that the list of process environment variables is treated as a source of data. It looks like a text file stream consisting of lines of the form "name=value". If this feature is not available on a given platform, an exception is thrown.

The syntax @(next :list expr) treats the expression as a source of text. The value of the expression is flattened to a list in a way similar to the @(flatten) directive. The resulting list is treated as if it were the lines of a text file: each element of the list is a line. If the lines happen contain embedded newline characters, they are a visible constituent of the line, and do not act as line separators.

The syntax @(next :string expr) treats the expression as a source of text. The value of the expression must be a string. Newlines in the string are interpreted as line terminators.

A string which is not terminated by a newline is tolerated, so that:


  @(next :string "abc")
  @a

binds a to "abc". Likewise, this is also the case with input files and other streams whose last line is not terminated by a newline.

However, watch out for empty strings, which are analogous to a correctly formed empty file which contains no lines:


  @(next :string "")
  @a

This will not bind a to ""; it is a matching failure. The behavior of :list is different. The query


  @(next :list "")
  @a

binds a to "". The reason is that under :list the string "" is flattened to the list ("") which is not an empty input stream, but a stream consisting of one empty line.

Note that "remainder of the query" which is applied to the stream opened by @(next) refers to the subquery in which the next directive appears, not necessarily the entire query. For example, the following query looks for the line starting with "xyz" at the top of the file "foo.txt", within a some directive. After the @(end) which terminates the @(some), the "abc" is matched in the previous file again.


  @(some)
  @(next "foo.txt")
  xyz@suffix
  @(end) abc

However, if the @(some) subquery successfully matched "xyz@suffix" within the file foo.text, there is now a binding for the suffix variable, which is visible to the remainder of the entire query. The variable bindings survive beyond the clause, but the data stream does not.

The @(next) directive supports the file name conventions as the command line. The name - means standard input. Text which starts with a ! is interpreted as a shell command whose output is read like a file. These interpretations are applied after variable substitution. If the file is specified as @a, but the variable a expands to "!echo foo", then the output of the "echo foo" command will be processed.

 

7.2 The skip directive

The skip directive considers the remainder of the query as a search pattern. The remainder is no longer required to strictly match at the current line in the current file. Rather, the current file is searched, starting with the current line, for the first line where the entire remainder of the query will successfully match. If no such line is found, the skip directive fails. If a matching position is found, the remainder of the query is understood to be processed there.

Of course, the remainder of the query can itself contain skip directives. Each such directive performs a recursive subsearch.

Skip comes in vertical and horizontal flavors. For instance, skip and match the last line:


  @(skip)
  @last
  @(eof)

Skip and match the last character of the line:


  @(skip)@{last 1}@(eol)

The skip directive has two optional arguments. If the first argument is a number, its value limits the range of lines scanned for a match. Judicious use of this feature can improve the performance of queries.

Example: scan until "size: @SIZE" matches, which must happen within the next 15 lines:


  @(skip 15)
  size: @SIZE

Without the range limitation skip will keep searching until it consumes the entire input source. While sometimes this is what is intended, often it is not. Sometimes a skip is nested within a collect, or following another skip. For instance, consider:


  @(collect)
  begin @BEG_SYMBOL
  @(skip)
  end @BEG_SYMBOL
  @(end)

The collect iterates over the entire input. But, potentially, so does the skip. Suppose that "begin x" is matched, but the data has no matching "end x". The skip will search in vain all the way to the end of the data, and then the collect will try another iteration back at the beginning, just one line down from the original starting point. If it is a reasonable expectation that an end x occurs 15 lines of a "begin x", this can be written instead:


  @(collect)
  begin @BEG_SYMBOL
  @(skip 15)
  end @BEG_SYMBOL
  @(end)

If the symbol nil is used in place of a number, it means to scan an unlimited range of lines; thus, @(skip nil) is equivalent to @(skip).

If the symbol :greedy is used, it changes the semantics of the skip to longest match semantics. For instance, match the last three space-separated tokens of the line:


  @(skip :greedy) @a @b @c

Without :greedy, the variable @c will can match multiple tokens, and end up with spaces in it, because nothing follows @c and so it matches from any position which follows a space to the end of the line. Also note the space in front of @a. Without this space, @a will get an empty string.

A line oriented example of greedy skip: match the last line without using @eof:


  @(skip :greedy)
  @last_line

There may be a second numeric argument. This specifies a minimum number of lines to skip before looking for a match. For instance, skip 15 lines and then search indefinitely for begin ...:


  @(skip nil 15)
  begin @BEG_SYMBOL

The two arguments may be used together. For instance, the following matches if, and only if, the 15th line of input starts with begin :


  @(skip 1 15)
  begin @BEG_SYMBOL

Essentially, @(skip 1 n) means "hard skip by n lines". @(skip 1 0) is the same as @(skip 1), which is a noop, because it means: "the remainder of the query must match starting on the very next line", or, more briefly, "skip exactly zero lines", which is the behavior if the skip directive is omitted altogether.

Here is one trick for grabbing the fourth line from the bottom of the input:


  @(skip)
  @fourth_from_bottom
  @(skip 1 3)
  @(eof)

Or using greedy skip:


  @(skip :greedy)
  @fourth_from_bottom
  @(skip 1 3)

Nongreedy skip with the @(eof) has a slight advantage because the greedy skip will keep scanning even though it has found the correct match, then backtrack to the last good match once it runs out of data. The regular skip with explicit @(eof) will stop when the @(eof) matches.

 

7.3 Reducing Backtracking with Blocks

skip can consume considerable CPU time when multiple skips are nested. Consider:


  @(skip)
  A
  @(skip)
  B
  @(skip)
  C

This is actually nesting: the second a third skips occur within the body of the first one, and thus this creates nested iteration. TXR is searching for the combination of skips which find match the pattern of lines A, B and C, with backtracking behavior. The outermost skip marches through the data until it finds A, followed by a pattern match for the second skip. The second skip iterates within to find B, followed by the third skip, and the third skip iterates to find C. If there is only one line A, and one B, then this is reasonably fast. But suppose there are many lines matching A and B, giving rise to a large number combinations of skips which match A and B, and yet no match for C, triggering backtracking. The nested stepping which tries the combinations of A and B can give rise to a considerable running time.

One way to deal with the problem is to unravel the nesting with the help of blocks. For example:


  @(block)
  @  (skip)
  A
  @(end)
  @(block)
  @  (skip)
  B
  @(end)
  @(skip)
  C

Now the scope of each skip is just the remainder of the block in which it occurs. The first skip finds A, and then the block ends. Control passes to the next block, and backtracking will not take place to a block which completed (unless all these blocks are enclosed in some larger construct which backtracks, causing the blocks to be re-executed.

Of course, this rewrite is not equivalent, and cannot be used for instance in backreferencing situations such as:


  @;
  @; Find some three lines which are the same.
  @;
  @(skip)
  @line
  @(skip)
  @line
  @(skip)
  @line

This example depends on the nested search-within-search semantics.

 

7.4 The trailer directive

The trailer directive introduces a trailing portion of a query or subquery which matches input material normally, but in the event of a successful match, does not advance the current position. This can be used, for instance, to cause @(collect) to match partially overlapping regions.

Example:


  @(collect)
  @line
  @(trailer)
  @(skip)
  @line
  @(end)

This script collects each line which has a duplicate somewhere later in the input. Without the @(trailer) directive, this does not work properly for inputs like:


  111
  222
  111
  222

Without @(trailer), the first duplicate pair constitutes a match which spans over the 222. After that pair is found, the matching continues after the second 111.

With the @(trailer) directive in place, the collect body, on each iteration, only consumes the lines matched prior to @(trailer).

 

7.5 The freeform directive

The freeform directive provides a useful alternative to TXR's line-oriented matching discipline. The freeform directive treats all remaining input from the current input source as one big line. The query line which immediately follows freeform is applied to that line.

The syntax variations are:


  @(freeform)
  ... query line ..


  @(freeform
number)
  ... query line ..


  @(freeform
string)
  ... query line ..


  @(freeform
number string)
  ... query line ..

If number and string are both present, they may be given in either order.

If a numeric argument is given, it limits the range of lines which are combined together. For instance @(freeform 5) means to only consider the next five lines to to be one big line. Without a numeric argument, freeform is "bottomless". It can match the entire file, which creates the risk of allocating a large amount of memory.

If a string argument is given, it specifies a custom line terminator. The default terminator is "\n". The terminator does not have to be one character long.

Freeform does not convert the entire remainder of the input into one big line all at once, but does so in a dynamic, lazy fashion, which takes place as the data is accessed. So at any time, only some prefix of the data exists as a flat line in which newlines are replaced by the terminator string, and the remainder of the data still remains as a list of lines.

After the subquery is applied to the virtual line, the unmatched remainder of that line is broken up into multiple lines again, by looking for and removing all occurrences of the terminator string within the flattened portion.

Care must be taken if the terminator is other than the default "\n". All occurrences of the terminator string are treated as line terminators in the flattened portion of the data, so extra line breaks may be introduced. Likewise, in the yet unflattened portion, no breaking takes place, even if the text contains occurrences of the terminator string. The extent of data which is flattened, and the amount of it which remains, depends entirely on the query line underneath @(flatten).

In the following example, lines of data are flattened using $ as the line terminator.

code:
 @(freeform "$")
 @a$@b:
 @c
 @d

data:
 1
 2:3
 4

output (-B):
 a="1"
 b="2"
 c="3"
 d="4"

The data is turned into the virtual line 1$2:3$4$. The @a$@b: subquery matches the 1$2: portion, binding a to "1", and b to "2". The remaining portion 3$4$ is then split into separate lines again according to the line terminator $i:


  3
  4

Thus the remainder of the query


  @c
  @d

faces these lines, binding c to 3 and d to 4. Note that since the data does not contain dollar signs, there is no ambiguity; the meaning may be understood in terms of the entire data being flattened and split again.

In the following example, freeform is used to solve a tokenizing problem. The Unix password file has fields separated by colons. Some fields may be empty. Using freeform, we can join the password file using ":" as a terminator. By restricting freeform to one line, we can obtain each line of the password file with a terminating ":", allowing for a simple tokenization, because now the fields are colon-terminated rather than colon-separated.

Example:


  @(next "/etc/passwd")
  @(collect)
  @(freeform 1 ":")
  @(coll)@{token /[^:]*/}:@(end)
  @(end)

 

7.6 The fuzz directive

The fuzz directive allows for an imperfect match spanning a set number of lines. It takes two arguments, both expressions that should evaluate to integers:


  @(fuzz m n)
  ...

This expresses that over the next n query lines, the matching strictness is relaxed a little bit. Only m out of those n lines have to match. Afterward, the rest of the query follows normal, strict processing.

In the degenerate situation that there are fewer than n query lines following the fuzz directive, then m of them must succeed nevertheless. (If there are fewer than m, then this is impossible.)

 

7.7 The some, all, none, maybe, cases and choose directives

These directives, called the parallel directives, combine multiple subqueries, which are applied at the same input position, rather than to consecutive input.

They come in vertical (line mode) and horizontal (character mode) flavors.

In horizontal mode, the current position is understood to be a character position in the line being processed. The clauses advance this character position by moving it to the right. In vertical mode, the current position is understood to be a line of text within the stream. A clause advances the position by some whole number of lines.

The syntax of these parallel directives follows this example:


  @(some)
  subquery1
  .
  .
  .
  @(and)
  subquery2
  .
  .
  .
  @(and)
  subquery3
  .
  .
  .
  @(end)

And in horizontal mode:


  @(some)subquery1...@(and)subquery2...@(and)subquery3...@(end)

Long horizontal lines can be broken up with line continuations, allowing the above example to be written like this, which is considered a single logical line:


  @(some)@\
     subquery1...@\
  @(and)@\
     subquery2...@\
  @(and)@\
     subquery3...@\
  @(end)

The @(some), @(all), @(none), @(maybe), @(cases) or @(choose) must be followed by at least one subquery clause, and be terminated by @(end). If there are two or more subqueries, these additional clauses are indicated by @(and) or @(or), which are interchangeable. The separator and terminator directives also must appear as the only element in a query line.

The choose directive requires keyword arguments. See below.

The syntax supports arbitrary nesting. For example:


  QUERY:            SYNTAX TREE:


  @(all)            all -+
  @  (skip)              +- skip -+
  @  (some)              |        +- some -+
  it                     |        |        +- TEXT
  @  (and)               |        |        +- and
  @    (none)            |        |        +- none -+
  was                    |        |        |        +- TEXT
  @    (end)             |        |        |        +- end
  @  (end)               |        |        +- end
  a dark                 |        +- TEXT
  @(end)                 *- end

nesting can be indicated using whitespace between @ and the directive expression. Thus, the above is an @(all) query containing a @(skip) clause which applies to a @(some) that is followed by the text line "a dark". The @(some) clause combines the text line "it", and a @(none) clause which contains just one clause consisting of the line "was".

The semantics of the parallel directives is:

@(all)
Each of the clauses is matched at the current position. If any of the clauses fails to match, the directive fails (and thus does not produce any variable bindings). Clauses following the failed directive are not evaluated. Bindings extracted by a successful clause are visible to the clauses which follow, and if the directive succeeds, all of the combined bindings emerge.

@(some [ :resolve (var ...) ])
Each of the clauses is matched at the current position. If any of the clauses succeed, the directive succeeds, retaining the bindings accumulated by the successfully matching clauses. Evaluation does not stop on the first successful clause. Bindings extracted by a successful clause are visible to the clauses which follow.

The :resolve parameter is for situations when the @(some) directive has multiple clauses that need to bind some common variables to different values: for instance, output parameters in functions. Resolve takes a list of variable name symbols as an argument. This is called the resolve set. If the clauses of @(some) bind variables in the resolve set, those bindings are not visible to later clauses. However, those bindings do emerge out of the @(some) directive as a whole. This creates a conflict: what if two or more clauses introduce different bindings for a variable in the resolve set? This is why it is called the resolve set: conflicts for variables in the resolve set are automatically resolved in favor of later directives.

Example:


  @(some :resolve (x))
  @  (bind a "a")
  @  (bind x "x1")
  @(or)
  @  (bind b "b")
  @  (bind x "x2")
  @(end)

Here, the two clauses both introduce a binding for x. Without the :resolve parameter, this would mean that the second clause fails, because x comes in with the value "x1", which does not bind with "x2". But because x is placed into the resolve set, the second clause does not see the "x1" binding. Both clauses establish their bindings independently creating a conflict over x. The conflict is resolved in favor of the second clause, and so the bindings which emerge from the directive are:


  a="a"
  b="b"
  x="x2"

@(none)
Each of the clauses is matched at the current position. The directive succeeds only if all of the clauses fail. If any clause succeeds, the directive fails, and subsequent clauses are not evaluated. Thus, this directive never produces variable bindings, only matching success or failure.

@(maybe)
Each of the clauses is matched at the current position. The directive always succeeds, even if all of the clauses fail. Whatever bindings are found in any of the clauses are retained. Bindings extracted by any successful clause are visible to the clauses which follow.

@(cases)
Each of the clauses is matched at the current position. The clauses are matched, in order, at the current position. If any clause matches, the matching stops and the bindings collected from that clause are retained. Any remaining clauses after that one are not processed. If no clause matches, the directive fails, and produces no bindings.

@(choose [ :longest var | :shortest var ])
Each of the clauses is matched at the current position in order. In this construct, bindings established by an earlier clause are not visible to later clauses. Although any or all of the clauses can potentially match, the clause which succeeds is the one which maximizes or minimizes the length of the text bound to the specified variable. The other clauses have no effect.

For all of the parallel directives other than @(none) and @(choose), the query advances the input position by the greatest number of lines that match in any of the successfully matching subclauses that are evaluated. The @(none) directive does not advance the input position.

For instance if there are two subclauses, and one of them matches three lines, but the other one matches five lines, then the overall clause is considered to have made a five line match at its position. If more directives follow, they begin matching five lines down from that position.

 

7.8 The require directive

The syntax of @(require) is:


  @(require
lisp-expression)

The require directive evaluates a TXR Lisp expression. (See TXR LISP far below.) If the expression yields a true value, then it succeeds, and matching continues with the directives which follow. Otherwise the directive fails.

In the context of the require directive, the expression should not be introduced by the @ symbol; it is expected to be a Lisp expression.

Example:


  @; require that 4 is greater than 3
  @; This succeeds; therefore, @a is processed
  @(require (> (+ 2 2) 3))
  @a

 

7.9 The if directive

The syntax of the if directive can be exemplified as follows:


  @(if
lisp-expr)
  .
  .
  .
  @(elif
lisp-expr)
  .
  .
  .
  @(elif
lisp-expr)
  .
  .
  .
  @(else)
  .
  .
  .
  @(end)

The @(elif) and @(else) clauses are all optional. If @(else) is present, it must be last, before @(end), after any @(elif) clauses. Any of the clauses may be empty.

See the TXR Lisp section about TXR Lisp expressions. In this directive, TXR Lisp expressions are not introduced by the @ symbol, just like in the require directive.

For example:


  @(if (> (length str) 42))
  foo: @a @b
  @(else)
  {@c}
  @(end)

In this example, if the length of the variable str is greater than 42, then matching continues with "foo: @a b", otherwise it proceeds with {@c}.

The if directive is actually a syntactic sugar which is translated to @(cases) and @(require). That is to say, the following pattern:


  @(cases)
  @(require
lisp-expr-1)
  A
  @(or)
  @(require
lisp-expr-2)
  B
  @(or)
  C
  @(end)

corresponds to the somewhat shorter and clearer:


  @(if
lisp-expr-1)
  A
  @(elsif
lisp-expr-2)
  B
  @(else)
  C
  @(end)

 

7.10 The gather directive

Sometimes text is structured as items that can appear in an arbitrary order. When multiple matches need to be extracted, there is a combinatorial explosion of possible orders, making it impractical to write pattern matches for all the possible orders.

The gather directive is for these situations. It specifies multiple clauses which all have to match somewhere in the data, but in any order.

For further convenience, the lines of the first clause of the gather directive are implicitly treated as separate clauses.

The syntax follows this pattern


  @(gather)
  one-line-query1
  one-line-query2
  .
  .
  .
  one-line-queryN
  @(and)
  multi
  line
  query1
  .
  .
  .
  @(and)
  multi
  line
  query2
  .
  .
  .
  @(end)

Of course the multi-line clauses are optional. The gather directive takes keyword parameters, see below.

 

7.10.1 The until / last clause in gather

Similarly to collect, gather has an optional until/last clause:


  @(gather)
  ...
  @(until)
  ...
  @(end)

How gather works is that the text is searched for matches for the single line and multi-line queries. The clauses are applied in the order in which they appear. Whenever one of the clauses matches, any bindings it produces are retained and it is removed from further consideration. Multiple clauses can match at the same text position. The position advances by the longest match from among the clauses which matched. If no clauses match, the position advances by one line. The search stops when all clauses are eliminated, and then the cumulative bindings are produced. If the data runs out, but unmatched clauses remain, the directive fails.

Example: extract several environment variables, which do not appear in a particular order:


  @(next :env)
  @(gather)
  USER=@USER
  HOME=@HOME
  SHELL=@SHELL
  @(end)

If the until or last clause is present and a match occurs, then the matches from the other clauses are discarded and the gather terminates. The difference between until/last is that any bindings bindings established in last are retained, and the input position is advanced past the matched material. The until/last clause has visibility to bindings established in the previous clauses in that same iteration, even though those bindings end up thrown away.

 

7.10.2 Keyword parameters in gather

The gather directive accepts the keyword parameter :vars. The argument to vars is a list of required and optional variables. Optional variables are denoted by the specification of a default value. Example:


  @(gather :vars (a b c (d "foo")))
  ...
  @(end)

Here, a, b, c and e are required variables, and d is optional. Variable e is required because its default value is the empty list (), same as the symbol nil.

The presence of :vars changes the behavior in three ways.

Firstly, even if all the clauses in the gather match successfully and are eliminated, the directive will fail if the required variables do not have bindings. It doesn't matter whether the bindings are existing, or whether they are established by the gather.

Secondly, if some of the clauses of the gather did not match, but all of the required variables have bindings, then the directive succeeds. Without the presence of :vars, it would fail in this situation.

Thirdly, if gather succeeds (all required variables have bindings), then all of the optional variables which do not have bindings are given bindings to their default values.

 

7.11 The collect directive

The syntax of the collect directive is:


  @(collect)
  ... lines of subquery
  @(end)

or with an until or last clause:


  @(collect)
  ... lines of subquery: main clause
  @(until)
  ... lines of subquery: until clause
  @(end)


  @(collect)
  ... lines of subquery: main clause
  @(last)
  ... lines of subquery: last clause
  @(end)

The repeat symbol may be specified instead of collect, which changes the meaning, see below:


  @(repeat)
  ... lines of subquery
  @(end)

The subquery is matched repeatedly, starting at the current line. If it fails to match, it is tried starting at the subsequent line. If it matches successfully, it is tried at the line following the entire extent of matched data, if there is one. Thus, the collected regions do not overlap. (Overlapping behavior can be obtained: see the @(trailer) directive).

Unless certain keywords are specified, or unless the collection is explicitly failed with @(fail), it always succeeds, even if it collects nothing, and even if the until/last clause never finds a match.

If no until/last last clause is specified, and the collect is not limited using parameters, the collection is unbounded: it consumes the entire data file. If any query material follows such the collect clause, it will fail if it tries to match anything in the current file; but of course, it is possible to continue matching in another file by means of @(next).

 

7.11.1 The until / last clause in collect

If an until/last last clause is specified, the collection stops when that clause matches at the current position.

If an until clause terminates collect, no bindings are collected at that position, even if the main clause matches at that position also. Moreover, the position is not advanced. The remainder of the query begins matching at that position.

If a last clause terminates collect, the behavior is different. Any bindings captured by the main clause are thrown away, just like with the until clause. However, the bindings in the last clause itself survive, and the position is advanced to skip over that material.

Example:

code:
 @(collect)
 @a
 @(until)
 42
 @b
 @(end)
 @c
data:
 1
 2
 3
 42
 5
 6
result:
 a[0]="1"
 a[1]="2"
 a[2]="3"
 c="42"

The line 42 is not collected, even though it matches Furthermore, the @(until) does not advance the position, so variable c takes 42.

If the @(until) is changed to @(last) the output will be different:

result:
 a[0]="1"
 a[1]="2"
 a[2]="3"
 b="5"
 c="6"

The 42 is not collected into the a list, just like before. But now the binding captured by @b emerges. Furthermore, the position advances so variable now takes 6.

The binding variables within the clause of a collect are treated specially. The multiple matches for each variable are collected into lists, which then appear as array variables in the final output.

Example:

code:
 @(collect)
 @a:@b:@c
 @(end)
data:
 John:Doe:101
 Mary:Jane:202
 Bob:Coder:313
result:
 a[0]="John"
 a[1]="Mary"
 a[2]="Bob"
 b[0]="Doe"
 b[1]="Jane"
 b[2]="Coder"
 c[0]="101"
 c[1]="202"
 c[2]="313"

The query matches the data in three places, so each variable becomes a list of three elements, reported as an array.

Variables with list bindings may be referenced in a query. They denote a multiple match. The -D command line option can establish a one-dimensional list binding.

The clauses of collect may be nested. Variable matches collated into lists in an inner collect, are again collated into nested lists in the outer collect. Thus an unbound variable wrapped in N nestings of @(collect) will be an N-dimensional list. A one dimensional list is a list of strings; a two dimensional list is a list of lists of strings, etc.

It is important to note that the variables which are bound within the main clause of a collect. That is, the variables which are subject to collection appear, within the collect, as normal one-value bindings. The collation into lists happens outside of the collect. So for instance in the query:


 @(collect)
 @x=@x
 @(end)

The left @x establishes a binding for some material preceding an equal sign. The right @x refers to that binding. The value of is different in each iteration, and these values are collected. What finally comes out of the collect clause is a single variable called x which holds a list containing each value that was ever instantiated under that name within the collect clause.

Also note that the until clause has visibility over the bindings established in the main clause. This is true even in the terminating case when the until clause matches, and the bindings of the main clause are discarded.

 

7.11.2 Keyword Parameters in collect

By default, collect searches the rest of the input indefinitely, or until the until/last clause matches. It skips arbitrary amounts of nonmatching material before the first match, and between matches.

Within the @(collect) syntax, it is possible to specify some useful keyword parameters for additional control of the behavior. For instance


  @(collect :maxgap 5)

means that the collect will terminate if it does not find a match within five lines of the starting position, or if more than five lines are skipped since any successful match. A :maxgap of 0 means that the collected regions must be adjacent. For instance:


  @(collect :maxgap 0)
  M @a
  @(end)

means: from here, collect consecutive lines of the form "M ...". This will not search for the first such line, nor will it skip lines which do not match this form.

Other keywords are :mingap, and :gap. The :mingap keyword specifies a minimum gap between matches, but has no effect on the distance to the first match. The :gap keyword effectively specifies :mingap and :maxgap at the same time, and can only be used if these other two are not used. Thus:


  @(collect :gap 1)
  @a
  @(end)

means collect every other line starting with the current line. Several other supported keywords are :times, :mintimes, :maxtimes and :lines. The shorthand :times N means the same thing as :mintimes N :maxtimes N. These specify how many matches should be collected. If there are fewer than :mintimes matches, the collect fails. If :maxtimes matches are collected, collect stops collecting immediately. Example:


  @(collect :times 3)
  @a @b
  @(end)

This will collect a match for "@a @b" exactly three times. If three matches are not found, it will fail.

The :lines parameter specifies the upper bound on how many lines should be scanned by collect, measuring from the starting position. The extent of the collect body is not counted. Example:


  @(collect :lines 2)
  foo: @a
  bar: @b
  baz: @c
  @(end)

The above collect will look for a match only twice: at the current position, and one line down.

There is one more keyword, :vars, discussed in the following section.

 

7.11.3 Specifying Variables in collect

Normally, any variable for which a new binding occurs in a collect block is collected. A collect clause may be "sloppy": it can neglect to collect some variables on some iterations, or bind some variables which are intended to behave like local temporaries, but end up collated into lists. Another issue is that the collect clause might not match anything at all, and then none of the variables are bound.

The :vars keyword allows the query writer to add discipline the collect body.

The argument to :vars is a list of variable specs. A variable spec is either a symbol, or a (symbol expression) pair, where the expression specifies a default value.

When a :vars list is specified, it means that only the given variables can emerge from the successful collect. Any newly introduced bindings for other variables do not propagate.

Furthermore, for any variable which is not specified with a default value, the collect body, whenever it matches successfully, must bind that variable. If it neglects to bind the variable, an exception of type query-error is thrown. (If a collect body matches successfully, but produces no new bindings, then this error is suppressed.)

For any variable which does have a default value, if the collect body neglects to bind that variable, the behavior is as if collect did bind that variable to that default value.

The default values are expressions, and so can be quasiliterals.

Lastly, if in the event that collect does not match anything, the variables specified in vars (whether or not they have a default value) are all bound to empty lists. (These bindings are established after the processing of the until/last last clause, if present.)

Example:


  @(collect :vars (a b (c "foo")))
  @a @c
  @(end)

Here, if the body "@a @c" matches, an error will be thrown because one of the mandatory variables is b, and the body neglects to produce a binding for b.

Example:


  @(collect :vars (a (c "foo")))
  @a @b
  @(end)

Here, if "@a @b" matches, only a will be collected, but not b, because b is not in the variable list. Furthermore, because there is no binding for c in the body, a binding is created with the value "foo", exactly as if c matched such a piece of text.

In the following example, the assumption is that THIS NEVER MATCHES is not found anywhere in the input but the line THIS DOES MATCH is found and has a successor which is bound to a. Because the body did not match, the :vars a and should be bound to empty lists. But a is bound by the last clause to some text, so this takes precedence. Only b is bound to an empty list.


  @(collect :vars (a b))
  THIS NEVER MATCHES
  @(last)
  THIS DOES MATCH
  @a
  @(end)

The following means: do not allow any variables to propagate out of any iteration of the collect and therefore collect nothing:


  @(collect :vars nil)
  ...
  @(end)

Instead of writing @(collect :vars nil), it is possible to write @(repeat). @(repeat) takes all collect keywords, except for :vars. There is a @(repeat) directive used in @(output) clauses; that is a different directive.

 

7.12 The coll directive

The coll directive is the horizontal version of collect. Whereas collect works with multi-line clauses on line-oriented material, coll works within a single line. With coll, it is possible to recognize repeating regularities within a line and collect lists.

Regular-expression based Positive Match variables work well with coll.

Example: collect a comma-separated list, terminated by a space.

code:
 @(coll)@{A /[^, ]+/}@(until) @(end)@B
data:
 foo,bar,xyzzy blorch
result:
 A[0]="foo"
 A[1]="bar"
 A[2]="xyzzy"
 B=blorch

Here, the variable A is bound to tokens which match the regular expression /[^, ]+/: non-empty sequence of characters other than commas or spaces.

Like collect, coll searches for matches. If no match occurs at the current character position, it tries at the next character position. Whenever a match occurs, it continues at the character position which follows the last character of the match, if such a position exists.

If not bounded by an until clause, it will exhaust the entire line. If the until clause matches, then the collection stops at that position, and any bindings from that iteration are discarded. Like collect, coll also supports an until/last clause, which propagates variable bindings and advances the position.

coll clauses nest, and variables bound within a coll are available to clauses within the rest of the coll clause, including the until/last clause, and appear as single values. The final list aggregation is only visible after the coll clause.

The behavior of coll leads to difficulties when a delimited variable are used to match material which is delimiter separated rather than terminated. For instance, entries in a comma-separated files usually do not appear as "a,b,c," but rather "a,b,c".

So for instance, the following result is not satisfactory:

code:
 @(coll)@a @(end)
data:
 1 2 3 4 5
result:
 a[0]="1"
 a[1]="2"
 a[2]="3"
 a[3]="4"

The 5 is missing because it isn't followed by a space, which the text-delimited variable match "@a " looks for. After matching "4 ", coll continues to look for matches, and doesn't find any. It is tempting to try to fix it like this:

code:
 @(coll)@a@/ ?/@(end)
data:
 1 2 3 4 5
result:
 a[0]=""
 a[1]=""
 a[2]=""
 a[3]=""
 a[4]=""
 a[5]=""
 a[6]=""
 a[7]=""
 a[8]=""

The problem now is that the regular expression / ?/ (match either a space or nothing), matches at any position. So when it is used as a variable delimiter, it matches at the current position, which binds the empty string to the variable, the extent of the match being zero. In this situation, the coll directive proceeds character by character. The solution is to use positive matching: specify the regular expression which matches the item, rather than a trying to match whatever follows. The collect directive will recognize all items which match the regular expression:

code:
 @(coll)@{a /[^ ]+/}@(end)
data:
 1 2 3 4 5
result:
 a[0]="1"
 a[1]="2"
 a[2]="3"
 a[3]="4"
 a[4]="5"

The until clause can specify a pattern which, when recognized, terminates the collection. So for instance, suppose that the list of items may or may not be terminated by a semicolon. We must exclude the semicolon from being a valid character inside an item, and add an until clause which recognizes a semicolon:

code:
 @(coll)@{a /[^ ;]+/}@(until);@(end);
data:
 1 2 3 4 5;
result:
 a[0]="1"
 a[1]="2"
 a[2]="3"
 a[3]="4"
 a[4]="5"

Whether followed by the semicolon or not, the items are collected properly.

Note that the @(end) is followed by a semicolon. That's because when the @(until) clause meets a match, the matching material is not consumed.

This repetition can, of course, be avoided by using @(last) instad of @(until) since @(last) consumes the terminating material.

Instead of the above regular-expresion-based approach, this extraction problem can also be solved with cases:

code:
 @(coll)@(cases)@a @(or)@a@(end)@(end)
data:
 1 2 3 4 5
result:
 a[0]="1"
 a[1]="2"
 a[2]="3"
 a[3]="4"
 a[4]="5"

 

7.12.1 Keyword parameters in coll

The @(coll) directive takes most of the same parameters as @(collect). See the section Collect Keyword Parameters above. So for instance @(coll :gap 0) means that the collects must be consecutive, and @(coll :maxtimes 2) means that at most two matches will be collected. The :lines keyword does not exist, but there is an analogous :chars keyword.

 

7.13 The flatten directive

The flatten directive can be used to convert variables to one dimensional lists. Variables which have a scalar value are converted to lists containing that value. Variables which are multidimensional lists are flattened to one-dimensional lists.

Example (without @(flatten))

code:
 @b
 @(collect)
 @(collect)
 @a
 @(end)
 @(end)
data:
 0
 1
 2
 3
 4
 5
result:
 b="0"
 a_0[0]="1"
 a_1[0]="2"
 a_2[0]="3"
 a_3[0]="4"
 a_4[0]="5"

Example (with @(flatten)):

code:
 @b
 @(collect)
 @(collect)
 @a
 @(end)
 @(end)
 @(flatten a b)
data:
 0
 1
 2
 3
 4
 5
result:
 b="0"
 a[0]="1"
 a[1]="2"
 a[2]="3"
 a[3]="4"
 a[4]="5"

 

7.14 The merge directive

The merge directive provides a way of combining two or more variables in a somewhat complicated but very useful way.

To understand what merge does we first have to define a property called depth. The depth of an atom such as a string is defined as 1. The depth of an empty list is 0. The depth of a nonempty list is one plus the depth of its deepest element. So for instance "foo" has depth 1, ("foo") has depth 2, and ("foo" ("bar")) has depth three.

We can now define the binary (two argument) merge operation as follows.

1
(merge A B) first normalizes the values A and B such that they have equal depth.
2
A value which has depth zero is put into a one element list.
3
If either value has a smaller depth than the other, it is wrapped in a list as many times as needed to give it equal depth. Finally, the values are appended together.

Merge takes more than two arguments. These are merged by a left reduction. The leftmost two values are merged, and then this result is merged with the third value, and so on.

Merge is useful for combining the results from collects at different levels of nesting such that elements are at the appropriate depth.

 

7.15 The cat directive

The cat directive converts a list variable into a single piece of text. The syntax is:


  @(cat
var [sep])

The sep argument specifies a separating piece of text. If no separator is specified, then a single space is used.

Example:

code:
 @(coll)@{a /[^ ]+/}@(end)
 @(cat a ":")
data:
 1 2 3 4 5
result:
 a="1:2:3:4:5"

 

7.16 The bind directive

The syntax of the bind directive is:


  @(bind
pattern expression {keyword value}*)

The bind directive is a kind of pattern match, which matches one or more variables on the left hand side pattern to the value of a variable on the right hand side. The right hand side variable must have a binding, or else the directive fails. Any variables on the left hand side which are unbound receive a matching piece of the right hand side value. Any variables on the left which are already bound must match their corresponding value, or the bind fails. Any variables which are already bound and which do match their corresponding value remain unchanged (the match can be inexact).

The simplest bind is of one variable against itself, for instance bind A against A:


  @(bind A A)

This will fail if A is not bound, (and complain loudly). If A is bound, it succeeds, since A matches A.

The next simplest bind binds one variable to another:


  @(bind A B)

Here, if A is unbound, it takes on the same value as B. If A is bound, it has to match B, or the bind fails. Matching means that either

-
A and B are the same text
-
A is text, B is a list, and A occurs within B.
-
vice versa: B is text, A is a list, and B occurs within A.
-
A and B are lists and are either identical, or one is found as substructure within the other.

The right hand side does not have to be a variable. It may be some other object, like a string, quasiliteral, regexp, or list of strings, et cetera. For instance


  @(bind A "ab\tc")

will bind the string "ab\tc" to the variable A if A is unbound. If A is bound, this will fail unless A already contains an identical string. However, the right hand side of a bind cannot be an unbound variable, nor a complex expression that contains unbound variables.

The left hand side of bind can be a nested list pattern containing variables. The last item of a list at any nesting level can be preceded by a . (dot), which means that the variable matches the rest of the list from that position.

Example: suppose that the list A contains ("now" "now" "brown" "cow"). Then the directive @(bind (H N . C) A), assuming that H, N and C are unbound variables, will bind H to "how", code N to "now", and C to the remainder of the list ("brown" "cow").

Example: suppose that the list A is nested to two dimensions and contains (("how" "now") ("brown" "cow")). Then @(bind ((H N) (B C)) A) binds H to "how", N to "now", B to "brown" and C to "cow".

The dot notation may be used at any nesting level. it must be followed by an item. The forms (.) and (X .) are invalid, but (. X) is valid and equivalent to X.

The number of items in a left pattern match must match the number of items in the corresponding right side object. So the pattern () only matches an empty list. The notations () and nil mean exactly the same thing.

The symbols nil, t and keyword symbols may be used on either side. They represent themselves. For example @(bind :foo :bar) fails, but @(bind :foo :foo) succeeds since the two sides denote the same keyword symbol object.

 

7.16.1 Keywords in the bind directive

The bind directive accepts these keywords:

:lfilt
The argument to :lfilt is a filter specification. When the left side pattern contains a binding which is therefore matched against its counterpart from the right side expression, the left side is filtered through the filter specified by :lfilt for the purposes of the comparison. For example:


  @(bind "a" "A" :lfilt :upcase)

produces a match, since the left side is the same as the right after filtering through the :upcase filter.

:rfilt
The argument to :rfilt is a filter specification. The specified filter is applied to the right hand side material prior to matching it against the left side. The filter is not applied if the left side is a variable with no binding. It is only applied to determine a match. Binding takes place the unmodified right hand side object.

For example, the following produces a match:


  @(bind "A" "a" :rfilt :upcase)

:filter
This keyword is a shorthand to specify both filters to the same value. For instance :filter :upcase is equivalent to :lfilt :upcase :rfilt :upcase.

For a description of filters, see Output Filtering below.

Of course, compound filters like (:from_html :upcase) are supported with all these keywords. The filters apply across arbitrary patterns and nested data.

Example:


  @(bind (a b c) ("A" "B" "C"))
  @(bind (a b c) (("z" "a") "b" "c") :rfilt :upcase)

Here, the first bind establishes the values for a, b and c, and the second bind succeeds, because the value of a matches the second element of the list (z a) if it is upcased, and likewise b matches "b" and c matches "c" if these are upcased.

 

7.17 The set directive

The set directive syntactically resembles bind, but is not a pattern match. It overwrites the previous values of variables with new values from the right hand side. Each variable that is assigned must have an existing binding: set will not induce binding.

Examples follow.

Store the value of A back into A, an operation with no effect:


  @(set A A)

Exchange the values of A and B:


  @(set (A B) (B A))

Store a string into A:


  @(set A "text")

Store a list into A:


  @(set A ("line1" "line2"))

Destructuring assignment. A ends up with "A", B ends up with ("B1" "B2") and C binds to ("C1" "C2").


  @(bind D ("A" ("B1" "B2") "C1" "C2"))
  @(bind (A B C) (() () ()))
  @(set (A B . C) D)

 

7.18 The rebind directive

The rebind directive resembles set but it is not an assignment. It combines the semantics of local, bind and set. The expression on the right hand side is evaluated in the current environment. Then the variables in the pattern on the left are introduced as new bindings, whose values come from the pattern.

rebind makes it easy to create temporary bindings based on existing bindings.


  @(define pattern-function (arg))
  @;; inside a pattern function:
  @(rebind recursion-level @(+ recursion-level 1))
  @;; ...
  @(end)

When the function terminates, the previous value of recursion-level is restored. The effect is like the following, but much easier to write and faster to execute:


  @(define pattern-function (arg))
  @;; inside a pattern function:
  @(local temp)
  @(set temp recursion-level)
  @(local recursion-level)
  @(set recursion-level @(+ temp 1))
  @;; ...
  @(end)

 

7.19 The forget directive

The forget has two spellings: @(forget) and @(local).

The arguments are one or more symbols, for example:


  @(forget a)
  @(local a b c)

this can be written


  @(local a)
  @(local a b c)

Directives which follow the forget or local directive no longer see any bindings for the symbols mentioned in that directive, and can establish new bindings.

It is not an error if the bindings do not exist.

It is strongly recommended to use the @(local) spelling in functions, because the forgetting action simulates local variables: for the given symbols, the machine forgets any earlier variables from outside of the function, and consequently, any new bindings for those variables belong to the function. (Furthermore, functions suppress the propagation of variables that are not in their parameter list, so these locals will be automatically forgotten when the function terminates.)

 

7.20 The do directive

The syntax of @(do) is:


  @(do
lisp-expression)

The do directive evaluates a TXR Lisp expression. (See TXR LISP far below.) The value of the expression is ignored, and matching continues continues with the directives which follow the do directive, if any.

In the context of the do directive, the expression should not be introduced by the @ symbol; it is expected to be a Lisp expression.

Example:


  @; match text into variables a and b, then insert into hash table h
  @(bind h (hash :equal-based))
  @a:@b
  @(do (set [h a] b))

 

8 BLOCKS

 

8.1 Overview

Blocks are sections of a query which are either denoted by a name, or are anonymous. They may nest: blocks can occur within blocks and other constructs.

Blocks are useful for terminating parts of a pattern matching search prematurely, and escaping to a higher level. This makes blocks not only useful for simplifying the semantics of certain pattern matches, but also an optimization tool.

Judicious use of blocks and escapes can reduce or eliminate the amount of backtracking that TXR performs.

 

8.2 The block directive

The @(block name) directive introduces a named block, except when name is the symbol nil. The @(block) directive introduces an unnamed block, equivalent to @(block nil).

The @(skip) and @(collect) directives introduce implicit anonymous blocks, as do function bodies.

 

8.3 Block Scope

The names of blocks are in a distinct namespace from the variable binding space. So @(block foo) is unrelated to the variable @foo.

A block extends from the @(block ...) directive which introduces it, until the matching @(end), and may be empty. For instance:


  @(some)
  abc
  @(block foo)
  xyz
  @(end)
  @(end)

Here, the block foo occurs in a @(some) clause, and so it extends to the @(end) which terminates the block. After that @(end), the name foo is not associated with a block (is not "in scope"). The second @(end) terminates the @(some) block.

The implicit anonymous block introduced by @(skip) has the same scope as the @(skip): it extends over all of the material which follows the skip, to the end of the containing subquery.

 

8.4 Block Nesting

Blocks may nest, and nested blocks may have the same names as blocks in which they are nested. For instance:


  @(block)
  @(block)
  ...
  @(end)
  @(end)

is a nesting of two anonymous blocks, and


  @(block foo)
  @(block foo)
  @(end)
  @(end)

is a nesting of two named blocks which happen to have the same name. When a nested block has the same name as an outer block, it creates a block scope in which the outer block is "shadowed"; that is to say, directives which refer to that block name within the nested block refer to the inner block, and not to the outer one.

 

8.5 Block Semantics

A block normally does nothing. The query material in the block is evaluated normally. However, a block serves as a termination point for @(fail) and @(accept) directives which are in scope of that block and refer to it.

The precise meaning of these directives is:

@(fail name)
Immediately terminate the enclosing query block called name, as if that block failed to match anything. If more than one block by that name encloses the directive, the inner-most block is terminated. No bindings emerge from a failed block.

@(fail)
Immediately terminate the innermost enclosing anonymous block, as if that block failed to match.

If the implicit block introduced by @(skip) is terminated in this manner, this has the effect of causing skip itself to fail. I.e. the behavior is as if skip search did not find a match for the trailing material, except that it takes place prematurely (before the end of the available data source is reached).

If the implicit block associated with a @(collect) is terminated this way, then the entire collect fails. This is a special behavior, because a collect normally does not fail, even if it matches nothing and collects nothing!

To prematurely terminate a collect by means of its anonymous block, without failing it, use @(accept).

@(accept name)
Immediately terminate the enclosing query block called name, as if that block successfully matched. If more than one block by that name encloses the directive, the inner-most block is terminated. Any bindings established within that block until this point emerge from that block.

@(accept)
Immediately terminate the innermost enclosing anonymous block, as if that block successfully matched. Any bindings established within that block until this point emerge from that block.

If the implicit block introduced by @(skip) is terminated in this manner, this has the effect of causing the skip itself to succeed, as if all of the trailing material had successfully matched.

If the implicit block associated with a @(collect) is terminated this way, then the collection stops. All bindings collected in the current iteration of the collect are discarded. Bindings collected in previous iterations are retained, and collated into lists in accordance with the semantics of collect.

Example: alternative way to achieve @(until) termination:


  @(collect)
  @  (maybe)
  ---
  @  (accept)
  @  (end)
  @LINE
  @(end)

This query will collect entire lines into a list called LINE. However, if the line --- is matched (by the embedded @(maybe)), the collection is terminated. Only the lines up to, and not including the --- line, are collected. The effect is identical to:


  @(collect)
  @LINE
  @(until)
  ---
  @(end)

The difference (not relevant in these examples) is that the until clause has visibility into the bindings set up by the main clause.

However, the following example has a different meaning:


  @(collect)
  @LINE
  @  (maybe)
  ---
  @  (accept)
  @  (end)
  @(end)

Now, lines are collected until the end of the data source, or until a line is found which is followed by a --- line. If such a line is found, the collection stops, and that line is not included in the collection! The @(accept) terminates the process of the collect body, and so the action of collecting the last @LINE binding into the list is not performed.

 

8.6 Data Extent of Terminated Blocks

A query block may have matched some material prior to being terminated by accept. In that case, it is deemed to have only matched that material, and not any material which follows. This may matter, depending on the context in which the block occurs.

Example:

code:
 @(some)
 @(block foo)
 @first
 @(accept foo)
 @ignored
 @(end)
 @second
data:
 1
 2
 3
result:
 first="1"
 second="2"

At the point where the accept occurs, the foo block has matched the first line, bound the text "1" to the variable @first. The block is then terminated. Not only does the @first binding emerge from this terminated block, but what also emerges is that the block advanced the data past the first line to the second line. Next, the @(some) directive ends, and propagates the bindings and position. Thus the @second which follows then matches the second line and takes the text "2".

In the following query, the foo block occurs inside a maybe clause. Inside the foo block there is a @(some) clause. Its first subclause matches variable @first and then terminates block foo. Since block foo is outside of the @(some) directive, this has the effect of terminating the @(some) clause:

code:
 @(maybe)
 @(block foo)
 @  (some)
 @first
 @  (accept foo)
 @  (or)
 @one
 @two
 @three
 @four
 @  (end)
 @(end)
 @second
data:
 1
 2
 3
 4
 5
result:
 first="1"
 second="2"

The second clause of the @(some) directive, namely:


  @one
  @two
  @three
  @four

is never processed. The reason is that subclauses are processed in top to bottom order, but the processing was aborted within the first clause the @(accept foo). The @(some) construct never gets the opportunity to match four lines.

If the @(accept foo) line is removed from the above query, the output is different:

code:
 @(maybe)
 @(block foo)
 @  (some)
 @first
 @#          <--  @(accept foo) removed from here!!!
 @  (or)
 @one
 @two
 @three
 @four
 @  (end)
 @(end)
 @second
data:
 1
 2
 3
 4
 5
result:
 first="1"
 one="1"
 two="2"
 three="3"
 four="4"
 second="5"

Now, all clauses of the @(some) directive have the opportunity to match. The second clause grabs four lines, which is the longest match. And so, the next line of input available for matching is 5, which goes to the @second variable.

 

8.7 Interaction Between the trailer and accept Directives

If one of the clauses which follow a @(trailer) requests a successful termination to an outer block via @(accept), then @(trailer) intercepts the escape and adjusts the data extent to the position that it was given.

Example:

code:
 @(block)
 @(trailer)
 @line1
 @line2
 @(accept)
 @(end)
 @line3
data:
 1
 2
 3
result:
 line1="1"
 line2="2"
 line3="1"

The variable line3 is bound to "1" because although @(accept) yields a data position which has advanced to the third line, this is intercepted by @(trailer) and adjusted back to the first line. Neglecting to do this adjustment would violate the semantics of trailer.

Directives other than @(trailer) have no such special interaction with accept.

 

9 FUNCTIONS

 

9.1 Overview

TXR functions allow a query to be structured to avoid repetition. On a theoretical note, because TXR functions support recursion, functions enable TXR to match some kinds of patterns which exhibit self-embedding, or nesting, and thus cannot be matched by a regular language.

Functions in TXR are not exactly like functions in mathematics or functional languages, and are not like procedures in imperative programming languages. They are not exactly like macros either. What it means for a TXR function to take arguments and produce a result is different from the conventional notion of a function.

A TXR function may have one or more parameters. When such a function is invoked, an argument must be specified for each parameter. However, a special behavior is at play here. Namely, some or all of the argument expressions may be unbound variables. In that case, the corresponding parameters behave like unbound variables also. Thus TXR function calls can transmit the "unbound" state from argument to parameter.

It should be mentioned that functions have access to all bindings that are visible in the caller; functions may refer to variables which are not mentioned in their parameter list.

With regard to returning, TXR functions are also unconventional. If the function fails, then the function call is considered to have failed. The function call behaves like a kind of match; if the function fails, then the call is like a failed match.

When a function call succeeds, then the bindings emanating from that function are processed specially. Firstly, any bindings for variables which do not correspond to one of the function's parameters are thrown away. Functions may internally bind arbitrary variables in order to get their job done, but only those variables which are named in the function argument list may propagate out of the function call. Thus, a function with no arguments can only indicate matching success or failure, but not produce any bindings. Secondly, variables do not propagate out of the function directly, but undergo a renaming. For each parameter which went into the function as an unbound variable (because its corresponding argument was an unbound variable), if that parameter now has a value, that value is bound onto the corresponding argument.

Example:


  @(define collect-words (list))
  @(coll)@{list /[^ \t]+/}@(end)
  @(end)

The above function collect-words contains a query which collects words from a line (sequences of characters other than space or tab), into the list variable called list. This variable is named in the parameter list of the function, therefore, its value, if it has one, is permitted to escape from the function call.

Suppose the input data is:


  Fine summer day

and the function is called like this:


  @(collect-words wordlist)

The result (with txr -B) is:


  wordlist[0]=Fine
  wordlist[1]=summer
  wordlist[1]=day

How it works is that in the function call @(collect-words wordlist), wordlist is an unbound variable. The parameter corresponding to that unbound variable is the parameter list. Therefore, that parameter is unbound over the body of the function. The function body collects the words of "Fine summer day" into the variable list, and then yields the that binding. Then the function call completes by noticing that the function parameter list now has a binding, and that the corresponding argument wordlist has no binding. The binding is thus transferred to the wordlist variable. After that, the bindings produced by the function are thrown away. The only enduring effects are:

-
the function matched and consumed some input; and
-
the function succeeded; and
-
the wordlist variable now has a binding.

Another way to understand the parameter behavior is that function parameters behave like proxies which represent their arguments. If an argument is an established value, such as a character string or bound variable, the parameter is a proxy for that value and behaves just like that value. If an argument is an unbound variable, the function parameter acts as a proxy representing that unbound variable. The effect of binding the proxy is that the variable becomes bound, an effect which is settled when the function goes out of scope.

Within the function, both the original variable and the proxy are visible simultaneously, and are independent. What if a function binds both of them? Suppose a function has a parameter called P, which is called with an argument A, which is an unbound variable, and then, in the function, both A and P bound. This is permitted, and they can even be bound to different values. However, when the function terminates, the local binding of A simply disappears (because the symbol A is not among the paramters of the function). Only the value bound to P emerges, and is bound to A, which still appears unbound at that point. The P binding disappears also, and the net effect is that A is now bound. The "proxy" binding of A thorugh the parameter P "wins" the conflict with the direct binding.

 

9.2 Definition Syntax

Function definition syntax comes in two flavors: vertical and horizontal. Horizontal definitions actually come in two forms, the distinction between which is hardly noticeable, and the need for which is made clear below.

A function definition begins with a @(define ...) directive. For vertical functions, this is the only element in a line.

The define symbol must be followed by a symbol, which is the name of the function being defined. After the symbol, there is a parenthesized optional argument list. If there is no such list, or if the list is specified as () or the symbol nil then the function has no parameters. Examples of valid define syntax are:


  @(define foo)
  @(define bar ())
  @(define match (a b c))

If the define directive is followed by more material on the same line, then it defines a horizontal function:


  @(define match-x)x@(end)

If the define is the sole element in a line, then it is a vertical function, and the function definition continues below:


  @(define match-x)
  x
  @(end)

The difference between the two is that a horizontal function matches characters within a line, whereas a vertical function matches lines within a stream. The former match-x matches the character x, advancing to the next character position. The latter match-x matches a line consisting of the character x, advancing to the next line.

Material between @(define) and @(end) is the function body. The define directive may be followed directly by the @(end) directive, in which case the function has an empty body.

Functions may be nested within function bodies. Such local functions have dynamic scope. They are visible in the function body in which they are defined, and in any functions invoked from that body.

The body of a function is an anonymous block. (See BLOCKS above).

 

9.3 Two Forms of The Horizontal Function

If a horizontal function is defined as the only element of a line, it may not be followed by additional material. The following construct is erroneous:


  @(define horiz (x))@foo:@bar@(end)lalala

This kind of definition is actually considered to be in the vertical context, and like other directives that have special effects and that do not match anything, it does not consume a line of input. If the above syntax were allowed, it would mean that the line would not only define a function but also match lalala. This would, in turn, would mean that the @(define)...@(end) is actually in horizontal mode, and so it matches a span of zero characters within a line (which means that is would require a line of input to match: a surprising behavior for a non-matching directive!)

A horizontal function can be defined in an actual horizontal context. This occurs if its is in a line where it is preceded by other material. For instance:


  X@(define fun)...@(end)Y

This is a query line which must match the text XY. It also defines the function fun. The main use of this form is for nested horizontal functions:


  @(define fun)@(define local_fun)...@(end)@(end)

 

9.4 Vertical-Horizontal Overloading

A function of the same name may be defined as both vertical and horizontal. Both functions are available at the same time. Which one is used by a call is resolved by context. See the section Vertical Versus Horizontal Calls below.

 

9.5 Call Syntax

A function is invoked by compound directive whose first symbol is the name of that function. Additional elements in the directive are the arguments. Arguments may be symbols, or other objects like string and character literals, quasiliterals ore regular expressions.

Example:

code:
 @(define pair (a b))
 @a @b
 @(end)
 @(pair first second)
 @(pair "ice" cream)
data:
 one two
 ice milk
result:
 first="one"
 second="two"
 cream="milk"

The first call to the function takes the line "one two". The parameter a takes "one" and parameter b takes "two". These are rebound to the arguments first and second. The second call to the function binds the a parameter to the word "ice", and the b is unbound, because the corresponding argument cream is unbound. Thus inside the function, a is forced to match ice. Then a space is matched and b collects the text "milk". When the function returns, the unbound "cream" variable gets this value.

If a symbol occurs multiple times in the argument list, it constrains both parameters to bind to the same value. That is to say, all parameters which, in the body of the function, bind a value, and which are all derived from the same argument symbol must bind to the same value. This is settled when the function terminates, not while it is matching. Example:

code:
 @(define pair (a b))
 @a @b
 @(end)
 @(pair same same)
data:
 one two
result:
 [query fails]

Here the query fails because a and b are effectively proxies for the same unbound variable same and are bound to different values, creating a conflict which constitutes a match failure.

 

9.6 Vertical Versus Horizontal Calls

A function call which is the only element of the query line in which it occurs is ambiguous. It can go either to a vertical function or to the horizontal one. If both are defined, then it goes to the vertical one.

Example:

code:
 @(define which (x))@(bind x "horizontal")@(end)
 @(define which (x))
 @(bind x "vertical")
 @(end)
 @(which fun)
result:
 fun="vertical"

Not only does this call go to the vertical function, but it is in a vertical context.

If only a horizontal function is defined, then that is the one which is called, even if the call is the only element in the line. This takes place in a horizontal character-matching context, which requires a line of input which can be traversed:

Example:

code:
 @(define which (x))@(bind x "horizontal")@(end)
 @(which fun)
data:
 ABC
result:
 [query fails]

The query fails because since @(which fun) is in horizontal mode, it matches characters in a line. Since the function body consists only of @(bind ...) which doesn't match any characters, the function call requires an empty line to match. The line ABC is not empty, and so there is a matching failure. The following example corrects this:

Example:

code:
 @(define which (x))@(bind x "horizontal")@(end)
 @(which fun)
data:
 [empty line]
result:
 fun="horizontal"

A call made in a clearly horizontal context will prefer the horizontal function, and only fall back on the vertical one if the horizontal one doesn't exist. (In this fall-back case, the vertical function is called with empty data; it is useful for calling vertical functions which process arguments and produce values.)

In the next example, the call is followed by trailing material, placing it in a horizontal context. Leading material will do the same thing:

Example:

code:
 @(define which (x))@(bind x "horizontal")@(end)
 @(define which (x))
 @(bind x "vertical")
 @(end)
 @(which fun)B
data:
 B
result:
 fun="horizontal"

 

9.7 Local Variables

As described earlier, variables bound in a function body which are not parameters of the function are discarded when the function returns. However, that, by itself, doesn't make these variables local, because pattern functions have visibility to all variables in their calling environment. If a variable x exists already when a function is called, then an attempt to bind it inside a function may result in a failure. The local directive must be used in a pattern function to list which variables are local.

Example:


  @(define path (path))@\
    @(local x y)@\
    @(cases)@\
      (@(path x))@(path y)@(bind path `(@x)@y`)@\
    @(or)@\
      @{x /[.,;'!?][^ \t\f\v]/}@(path y)@(bind path `@x@y`)@\
    @(or)@\
      @{x /[^ .,;'!?()\t\f\v]/}@(path y)@(bind path `@x@y`)@\
    @(or)@\
      @(bind path "")@\
    @(end)@\
  @(end)

This is a horizontal function which matches a path, which lands into four recursive cases. A path can be parenthesized path followed by a path; it can be a certain character followed by a path, or it can be empty

This function ensures that the variables it uses internally, x and y, do not have anything to do with any inherited bindings for x and y.

Note that the function is recursive, which cannot work without x and y being local, even if no such bindings exist prior to the top-level invocation of the function. The invocation @(path x) causes x to be bound, which is visible inside the invocation @(path y), but that invocation needs to have its own binding of x for local use.

 

9.8 Nested Functions

Function definitions may appear in a function. Such definitions are visible in all functions which are invoked from the body (and not necessarily enclosed in the body). In other words, the scope is dynamic, not lexical. Inner definitions shadow outer definitions. This means that a caller can redirect the function calls that take place in a callee, by defining local functions which capture the references.

Example:

code:
 @(define which)
 @  (fun)
 @(end)
 @(define fun)
 @  (output)
 toplevel fun!
 @  (end)
 @(end)
 @(define callee)
 @  (define fun)
 @    (output)
 local fun!
 @    (end)
 @  (end)
 @  (which)
 @(end)
 @(callee)
 @(which)
output:
 local fun!
 toplevel fun!

Here, the function which is defined which calls fun. A toplevel definition of fun is introduced which outputs "toplevel fun!". The function callee provides its own local definition of fun which outputs "local fun!" before calling which. When callee is invoked, it calls which, whose @(fun) call is routed to callee's local definition. When which is called directly from the top level, its fun call goes to the toplevel definition.

 

10 MODULARIZATION

 

10.1 The load directive

The syntax of the load directive is:


  @(load
expr)

Where expr evaluates to a string giving the path of the file to load. Unless the path is absolute, it is interpreted relative to the directory of the source file from which the @(load) syntax was read. If there was no such source file (for instance, the script was read from standard input), then it is resolved relative to the current working directory.

If the file cannot be opened, then the .txr suffix is added and another attempt is made. Thus load expressions need not refer to the suffix. In the future, additional suffixes may be searched (compiled versions of a file).

Loading is performed at evaluation time; it is not a source file inclusion mechanism. A TXR script is read from beginning to end and parsed prior to being evaluated.

See also: the *self-path* variable in TXR Lisp.

 

11 OUTPUT

 

11.1 Introduction

A TXR query may perform custom output. Output is performed by output clauses, which may be embedded anywhere in the query, or placed at the end. Output occurs as a side effect of producing a part of a query which contains an @(output) directive, and is executed even if that part of the query ultimately fails to find a match. Thus output can be useful for debugging. An output clause specifies that its output goes to a file, pipe, or (by default) standard output. If any output clause is executed whose destination is standard output, TXR makes a note of this, and later, just prior to termination, suppresses the usual printing of the variable bindings or the word false.

 

11.2 The output directive

The syntax of the @(output) directive is:


  @(output [
destination ] { bool-keyword | keyword value }* )
  .
  . one or more output directives or lines
  .
  @(end)

The optional destination is a string which gives the path name of a file to open for output. If the name is - it instead denotes standard output, and if it begins with ! then the rest of the shell is treated as a shell command to which the output is piped.

The destination may be specified as a variable which holds text, as a string literal or as a quasiliteral

The keyword list consists of a mixture of boolean keywords which do not have an argument, or keywords with arguments.

The following boolean keywords are supported:

:nothrow
The output directive throws an exception if the output destination cannot be opened, unless the :nothrow keyword is present, in which case the situation is treated as a match failure.

Note that since command pipes are processes that report errors asynchronously, a failing command will not throw an immediate exception that can be suppressed with :nothrow. This is for synchronous errors, like trying to open a destination file, but not having permissions, etc.

:append
This keyword is meaningful for files, specifying append mode: the output is to be added to the end of the file rather than overwriting the file.

The following value keywords are supported by :append:

:filter
The argument can be a symbol, which specifies a filter to be applied to the variable substitutions occurring within the output clause. The argument can also be a list of filter symbols, which specifies that multiple filters are to be applied, in left to right order.

See the later sections Output Filtering below, and The Deffilter Directive.

:into
The argument of :into is a symbol which denotes a variable. The output will go into that variable. If the variable is unbound, it will be created. Otherwise, its contents are overwritten unless the :append keyword is used. If :append is used, then the new content will be appended to the previous content of the variable, after flattening the content to a list, as if by the flatten directive.

:named
The argument of :named is a symbol which denotes a variable. The file or pipe stream which is opened for the output is stored in this variable, and is not closed at the end of the output block. This allows a subsequent output block to continue output on the same stream, which is possible using the next two keywords, :continue or :finish. A new binding is established for the variable, even if it already has an existing binding.

:continue
A destination should not be specified if :continue is used. The argument of :continue is an expression, such as a variable name, that evaluates to a stream object. That stream object is used for the output block. At the end of the output block, the stream is flushed, but not closed. A usage example is given in the documentation for the Close Directive below.

:finish
A destination should not be specified if :finish is used. The argument of :finish is an expression, such as a variable name, that evaluates to a stream object. That stream object is used for the output block. At the end of the output block, the stream is closed. An example is given in the documentation for the Close Directive below.

 

11.3 Output Text

Text in an output clause is not matched against anything, but is output verbatim to the destination file, device or command pipe.

 

11.4 Output Variables

Variables occurring in an output clause do not match anything; instead their contents are output.

A variable being output can be any object. If it is of a type other than a list or string, it will be converted to a string as if by the tostring function in TXR Lisp.

A list is converted to a string in a special way: the elements are individually converted to a string and then they are catenated together. The default separator string is a single space: an alternate separation can be specified as an argument in the brace substitution syntax.

Lists may be output within @(repeat) or @(rep) clauses. Each nesting of these constructs removes one level of nesting from the list variables that it contains.

In an output clause, the @{name number} variable syntax generates fixed-width field, which contains the variable's text. The absolute value of the number specifies the field width. For instance -20 and 20 both specify a field width of twenty. If the text is longer than the field, then it overflows the field. If the text is shorter than the field, then it is left-adjusted within that field, if the width is specified as a positive number, and right-adjusted if the width is specified as negative.

An output variable may specify a filter which overrides any filter established for the output clause. The syntax for this is @{NAME :filter filterspec}. The filter specification syntax is the same as in the output clause. See Output Filtering below.

 

11.5 Output Variables: Indexing

Additional syntax is supported in output variables that does not appear in pattern matching variables.

A square bracket index notation may be used to extract elements or ranges from a variable, which works with strings, vectors and lists. Elements are indexed from zero. This notation is only available in brace-enclosed syntax, and looks like this:

@{name[expr]}
Extract the element at the position given by expr.

@{name[expr1..expr2]}
Extract a range of elements from the position given by expr1, up to one position less than the position given by expr2.

If the variable is a list, it is treated as a list substitution, exactly as if it were the value of an unsubscripted list variable. The elements of the list are converted to strings and catenated together wit ha separator string between them, the default one being a single space.

An alternate character may be given as a string argument in the brace notation.

Example:


  @(bind a ("a" "b" "c" "d"))
  @(output)
  @{a[1..3] "," 10}
  @(end)

The above produces the text "b,c" in a field 10 spaces wide. The [1..3] argument extracts a range of a; the "," argument specifies an alternate separator string, and 10 specifies the field width.

 

11.6 Output Substitutions

The brace syntax has another syntactic and semantic extension in output clauses. In place of the symbol, an expression may appear. The value of that expression is substituted.

Example:


 @(bind a "foo")
 @(output)
 @{`@a:` -10}

Here, the quasiliteral expression `@a:` is evaluated, producing the string "foo:". This string is printed right-adjusted in a 10 character field.

 

11.7 The repeat directive

The repeat directive generates repeated text from a "boilerplate", by taking successive elements from lists. The syntax of repeat is like this:


  @(repeat)
  .
  .
  main clause material, required
  .
  .
  special clauses, optional
  .
  .
  @(end)

repeat has four types of special clauses, any of which may be specified with empty contents, or omitted entirely. They are described below.

repeat takes arguments, also described below.

All of the material in the main clause and optional clauses is examined for the presence of variables. If none of the variables hold lists which contain at least one item, then no output is performed, (unless the repeat specifies an @(empty) clause, see below). Otherwise, among those variables which contain non-empty lists, repeat finds the length of the longest list. This length of this list determines the number of repetitions, R.

If the repeat contains only a main clause, then the lines of this clause is output R times. Over the first repetition, all of the variables which, outside of the repeat, contain lists are locally rebound to just their first item. Over the second repetition, all of the list variables are bound to their second item, and so forth. Any variables which hold shorter lists than the longest list eventually end up with empty values over some repetitions.

Example: if the list A holds "1", "2" and "3"; the list B holds "A", "B"; and the variable C holds "X", then


  @(repeat)
  >> @C
  >> @A @B
  @(end)

will produce three repetitions (since there are two lists, the longest of which has three items). The output is:


  >> X
  >> 1 A
  >> X
  >> 2 B
  >> X
  >> 3

The last line has a trailing space, since it is produced by "@A @B", where B has an empty value. Since C is not a list variable, it produces the same value in each repetition.

The special clauses are:

@(single)
If the repeat produces exactly one repetition, then the contents of this clause are processed for that one and only repetition, instead of the main clause or any other clause which would otherwise be processed.

@(first)
The body of this clause specifies an alternative body to be used for the first repetition, instead of the material from the main clause.

@(last)
The body of this clause is used instead of the main clause for the last repetition.

@(empty)
If the repeat produces no repetitions, then the body of this clause is output. If this clause is absent or empty, the repeat produces no output.

@(mod n m)
The forms n and m are expressions that evaluate to integers. The value of m should be nonzero. The clause denoted this way is active if the repetition modulo m is equal to n. The first repetition is numbered zero. For instance the clause headed by @(mod 0 2) will be used on repetitions 0, 2, 4, 6, ... and @(mod 1 2) will be used on repetitions 1, 3, 5, 7, ...

@(modlast n m)
The meaning of n and m is the same as in @(mod n m), but one more condition is imposed. This clause is used if the repetition modulo m is equal to n, and if it is the last repetition.

The precedence among the clauses which take an iteration is: single > first > mod > modlast > last > main. That is if two or more of these clauses can apply to a repetition, then the leftmost one in this precedence list applies. For instance, if there is just a single repetition, then any of these special clause types can apply to that repetition, since it is the only repetition, as well as the first and last one. In this situation, if there is a @(single) clause present, then the repetition is processed using that clause. Otherwise, if there is a @(first) clause present, that clause is used. Failing that, @(mod) is used if there is such a clause and its numeric conditions are satisfied. If there isn't, then @(modlast) clauses are considered, and if there are none, or none of them activate, then @(last) is considered. Finally if none of all these clauses are present or apply, then the repetition is processed using the main clause.

Repeat supports arguments.


  @(repeat [:counter
symbol] [:vars (symbol*)])

The :counter argument designates a symbol which will behave as an integer variable over the scope of the clauses inside the repeat. The variable provides access to the repetition count, starting at zero, incrementing with each repetition.

The :vars argument specifies a list of variables. The repeat directive will pick out from this list those variables which have bindings. It will assume that all these variables occur in the repeat block and are to be iterated. This syntax is needed for situations in which @(repeat) is not able to deduce the existence of a variable in the block. It does not dig very deeply to discover variables, and does not "see" variables that are referenced via embedded TXR Lisp expressions. For instance, the following produces no output:


  @(bind list ("a" "b" "c"))
  @(output)
  @(repeat)
  @(format nil "<~a>" list)
  @(end)
  @(end)

Although the list variable appears in the repeat block, it is embedded in a TXR Lisp construct. That construct will never be evaluated because no repetitions take place: the repeat construct doesn't find any variables and so doesn't iterate. The remedy is to provide a little help via the :vars parameter:


  @(bind list ("a" "b" "c"))
  @(output)
  @(repeat :vars (list))
  @(format nil "<~a>" list)
  @(end)
  @(end)

Now the repeat block iterates over list and the output is:


  <a>
  <b>
  <c>

 

11.8 Nested repeat directives

If a repeat clause encloses variables which hold multidimensional lists, those lists require additional nesting levels of repeat (or rep). It is an error to attempt to output a list variable which has not been decimated into primary elements via a repeat construct.

Suppose that a variable X is two-dimensional (contains a list of lists). X must be twice nested in a repeat. The outer repeat will traverse the lists contained in X. The inner repeat will traverse the elements of each of these lists.

A nested repeat may be embedded in any of the clauses of a repeat, not only the main clause.

 

11.9 The rep directive

The rep directive is similar to repeat. Whereas repeat is line oriented, rep generates material within a line. It has all the same clauses, but everything is specified within one line:


  @(rep)... main material ... .... special clauses ...@(end)

More than one @(rep) can occur within a line, mixed with other material. A @(rep) can be nested within a @(repeat) or within another @(rep).

Also, @(rep) accepts the same :counter and :vars arguments.

 

11.10 repeat and rep Examples

Example 1: show the list L in parentheses, with spaces between the elements, or the word EMPTY if the list is empty:


  @(output)
  @(rep)@L @(single)(@L)@(first)(@L @(last)@L)@(empty)EMPTY@(end)
  @(end)

Here, the @(empty) clause specifies EMPTY. So if there are no repetitions, the text EMPTY is produced. If there is a single item in the list L, then @(single)(@L) produces that item between parentheses. Otherwise if there are two or more items, the first item is produced with a leading parenthesis followed by a space by @(first)(@L and the last item is produced with a closing parenthesis: @(last)@L). All items in between are emitted with a trailing space by the main clause: @(rep)@L.

Example 2: show the list L like Example 1 above, but the empty list is ().


  @(output)
  (@(rep)@L @(last)@L@(end))
  @(end)

This is simpler. The parentheses are part of the text which surrounds the @(rep) construct, produced unconditionally. If the list L is empty, then @(rep) produces no output, resulting in (). If the list L has one or more items, then they are produced with spaces each one, except the last which has no space. If the list has exactly one item, then the @(last) applies to it instead of the main clause: it is produced with no trailing space.

 

11.11 The close directive

The syntax of the close directive is:


  @(close
expr)

Where expr evaluates to a stream. The close directive can be used to explicitly close streams created using @(output ... :named var) syntax, as an alternative to @(output :finish expr).

Examples:

Write two lines to "foo.txt" over two output blocks using a single stream:


  @(output "foo.txt" :named foo)
  Hello,
  @(end)
  @(output :continue foo)
  world!
  @(end)
  @(close foo)

The same as above, using :finish rather than :continue so that the stream is closed at the end of the second block:


  @(output "foo.txt" :named foo)
  Hello,
  @(end)
  @(output :finish foo)
  world!
  @(end)

 

11.12 Output Filtering

Often it is necessary to transform the output to preserve its meaning under the convention of a given data format. For instance, if a piece of text contains the characters < or >, then if that text is being substituted into HTML, these should be replaced by &lt; and &gt;. This is what filtering is for. Filtering is applied to the contents of output variables, not to any template text. TXR implements named filters. Built-in filters are named by keywords, given below. User-defined filters are possible, however. See notes on the deffilter directive below.

Instead of a filter name, the syntax (fun name) can be used. This denotes that the function called name is to be used as a filter. This is discussed in the next section Function Filters below.

Built-in filters named by keywords:

:to_html
Filter text to HTML, representing special characters using HTML ampersand sequences. For instance > is replaced by &gt;.

:from_html
Filter text with HTML codes into text in which the codes are replaced by the corresponding characters. For instance &gt; is replaced by >.

:upcase
Convert the 26 lower case letters of the English alphabet to upper case.

:downcase
Convert the 26 upper case letters of the English alphabet to lower case.

:frompercent
Decode percent-encoded text. Character triplets consisting of the % character followed by a pair of hexadecimal digits (case insensitive) are are converted to bytes having the value represented by the hexadecimal digits (most significant nybble first). Sequences of one or more such bytes are treated as UTF-8 data and decoded to characters.

:topercent
Convert to percent encoding according to RFC 3986. The text is first converted to UTF-8 bytes. The bytes are then converted back to text as follows. Bytes in the range 0 to 32, and 127 to 255 (note: including the ASCII DEL), bytes whose values correspond to ASCII characters which are listed by RFC 3986 as being in the "reserved set", and the byte value corresponding to the ASCII % character are encoded as a three-character sequence consisting of the % character followed by two hexadecimal digits derived from the byte value (most significant nybble first, upper case). All other bytes are converted directly to characters of the same value without any such encoding.

:fromurl
Decode from URL encoding, which is like percent encoding, except that if the unencoded + character occurs, it is decoded to a space character. Of course %20 still decodes to space, and %2B to the + character.

:tourl
Encode to URL encoding, which is like percent encoding except that a space maps to + rather than %20. The + character, being in the reserved set, encodes to %2B.

:tonumber
Converts strings to numbers. Strings that contain a period, e or E are converted to floating point as if by the Lisp function flo-str. Otherwise they are converted to integer as if using int-str with a radix of 10. Non-numeric junk results in the object nil.

:tointeger
Converts strings to integers as if using int-str with a radix of 10. Non-numeric junk results in the object nil.

:tofloat
Converts strings to floating-point values as if using the function flo-str. Non-numeric junk results in the object nil.

:hextoint
Converts strings to integers as if using int-str with a radix of 16. Non-numeric junk results in the object nil.

Examples:

To escape HTML characters in all variable substitutions occurring in an output clause, specify :filter :to_html in the directive:


  @(output :filter :to_html)
  ...
  @(end)

To filter an individual variable, add the syntax to the variable spec:


  @(output)
  @{x :filter :to_html}
  @(end)

Multiple filters can be applied at the same time. For instance:


  @(output)
  @{x :filter (:upcase :to_html)}
  @(end)

This will fold the contents of x to upper case, and then encode any special characters into HTML. Beware of combinations that do not make sense. For instance, suppose the original text is HTML, containing codes like &quot;. The compound filter (:upcase :from_html) will not work because &quot; will turn to &QUOT; which no longer be recognized by the :from_html filter, sonce the entity names in HTML codes are case-sensitive.

Capture some numeric variables and convert to numbers:


  @date @time @temperature @pressure
  @(filter :tofloat temperature pressure)
  @;; temperature and pressure can now be used in calculations

 

11.13 Function Filters

A function can be used as a filter. For this to be possible, the function must conform to certain rules:

1.
The function must take two special arguments, which may be followed by additional arguments.

2.
When the function is called, the first argument will be bound to a string, and the second argument will be unbound. The function must produce a value by binding it to the second argument. If the filter is to be used as the final filter in a chain, it must produce a string.

For instance, the following is a valid filter function:


  @(define foo_to_bar (in out))
  @  (next :string in)
  @  (cases)
  foo
  @    (bind out "bar")
  @  (or)
  @    (bind out in)
  @  (end)
  @(end)

This function binds the out parameter to "bar" if the in parameter is "foo", otherwise it binds the out parameter to a copy of the in parameter. This is a simple filter.

To use the filter, use the syntax (:fun foo_to_bar) in place of a filter name. For instance in the bind directive:


  @(bind "foo" "bar" :lfilt (:fun foo_to_bar))

The above should succeed since the left side is filtered from "foo" to "bar", so that there is a match.

Of course, function filters can be used in a chain:


  @(output :filter (:downcase (:fun foo_to_bar) :upcase))
  ...
  @(end)

Here is a split function which takes an extra argument which specifies the separator:


  @(define split (in out sep))
  @  (next :list in)
  @  (coll)@(maybe)@token@sep@(or)@token@(end)@(end)
  @  (bind out token)
  @(end)

Furthermore, note that it produces a list rather than a string. This function separates the argument in into tokens according to the separator text carried in the variable sep.

Here is another function, join, which catenates a list:


  @(define join (in out sep))
  @  (output :into out)
  @  (rep)@in@sep@(last)@in@(end)
  @  (end)
  @(end)

Now here is these two being used in a chain:


  @(bind text "how,are,you")
  @(output :filter (:fun split ",") (:fun join "-"))
  @text
  @(end)

Output:


  how-are-you

When the filter invokes a function, it generates the first two arguments internally to pass in the input value and capture the output. The remaining arguments from the (:fun ...) construct are also passed to the function. Thus the string objects "," and "-" are passed as the sep argument to split and join.

Note that split puts out a list, which join accepts. So the overall filter chain operates on a string: a string goes into split, and a string comes out of join.

 

11.14 The deffilter directive

The deffilter directive allows a query to define a custom filter, which can then be used in output clauses to transform substituted data.

This directive's syntax is illustrated in this example:

code:
 @(deffilter rot13
    ("a" "n")
    ("b" "o")
    ("c" "p")
    ("d" "q")
    ("e" "r")
    ("f" "s")
    ("g" "t")
    ("h" "u")
    ("i" "v")
    ("j" "w")
    ("k" "x")
    ("l" "y")
    ("m" "z")
    ("n" "a")
    ("o" "b")
    ("p" "c")
    ("q" "d")
    ("r" "e")
    ("s" "f")
    ("t" "g")
    ("u" "h")
    ("v" "i")
    ("w" "j")
    ("x" "k")
    ("y" "l")
    ("z" "m"))
 @(collect)
 @line
 @(end)
 @(output :filter rot13)
 @(repeat)
 @line
 @(end)
 @(end)
data:
 hey there!
output:
 url gurer!

The deffilter symbol must be followed by the name of the filter to be defined, followed by forms which evaluate to lists of strings. Each list must be at least two elements long and specifies one or more texts which are mapped to a replacement text. For instance, the following specifies a telephone keypad mapping from upper case letters to digits.


  @(deffilter alpha_to_phone ("E" "0")
                             ("J" "N" "Q" "1")
                             ("R" "W" "X" "2")
                             ("D" "S" "Y" "3")
                             ("F" "T" "4")
                             ("A" "M" "5")
                             ("C" "I" "V" "6")
                             ("B" "K" "U" "7")
                             ("L" "O" "P" "8")
                             ("G" "H" "Z" "9"))


  @(deffilter foo (`@a` `@b`) ("c" `->@d`))


  @(bind x ("from" "to"))
  @(bind y ("---" "+++"))
  @(deffilter sub x y)

The last deffilter above equivalent to @(deffilter sub (from to) (--- +++)).

Filtering works using a longest match algorithm. The input is scanned from left to right, and the longest piece of text is identified at every character position which matches a string on the left hand side, and that text is replaced with its associated replacement text. The scanning then continues at the first character after the matched text.

If none of the strings matches at a given character position, then that character is passed through the filter untranslated, and the scan continues at the next character in the input.

Filtering is not in-place but rather instantiates a new text, and so replacement text is not re-scanned for more replacements.

If a filter definition accidentally contains two or more repetitions of the same left hand string with different right hand translations, the later ones take precedence. No warning is issued.

 

11.15 The filter directive

The syntax of the filter directive is:


  @(filter FILTER { VAR }+ )

A filter is specified, followed by one or more variables whose values are filtered and stored back into each variable.

Example: convert a, b, and c to upper case and HTML encode:


  @(filter (:upcase :to_html) a b c)

 

12 EXCEPTIONS

 

12.1 Introduction

The exceptions mechanism in TXR is another disciplined form of non-local transfer, in addition to the blocks mechanism (see BLOCKS above). Like blocks, exceptions provide a construct which serves as the target for a dynamic exit. Both blocks and exceptions can be used to bail out of deep nesting when some condition occurs. However, exceptions provide more complexity. Exceptions are useful for error handling, and TXR in fact maps certain error situations to exception control transfers. However, exceptions are not inherently an error-handling mechanism; they are a structured dynamic control transfer mechanism, one of whose applications is error handling.

An exception control transfer (simply called an exception) is always identified by a symbol, which is its type. Types are organized in a subtype-supertype hierarchy. For instance, the file-error exception type is a subtype of the error type. This means that a file error is a kind of error. An exception handling block which catches exceptions of type error will catch exceptions of type file-error, but a block which catches file-error will not catch all exceptions of type error. A query-error is a kind of error, but not a kind of file-error. The symbol t is the supertype of every type: every exception type is considered to be a kind of t. (Mnemonic: t stands for type, as in any type).

Exceptions are handled using @(catch) clauses within a @(try) directive.

In addition to being useful for exception handling, the @(try) directive also provides unwind protection by means of a @(finally) clause, which specifies query material to be executed unconditionally when the try clause terminates, no matter how it terminates.

 

12.2 The try directive

The general syntax of the try directive is


  @(try)
  ... main clause, required ...
  ... optional catch clauses ...
  ... optional finally clause
  @(end)

A catch clause looks like:


  @(catch TYPE [ PARAMETERS ])
  .
  .
  .

and also this simple form:


  @(catch)
  .
  .
  .

which catches all exceptions, and is equivalent to @(catch t).

A finally clause looks like:


  @(finally)
  ...
  .
  .

The main clause may not be empty, but the catch and finally may be.

A try clause is surrounded by an implicit anonymous block (see BLOCKS section above). So for instance, the following is a no-op (an operation with no effect, other than successful execution):


  @(try)
  @(accept)
  @(end)

The @(accept) causes a successful termination of the implicit anonymous block. Execution resumes with query lines or directives which follow, if any.

try clauses and blocks interact. For instance, an accept from within a try clause invokes a finally.

code:
 @(block foo)
 @  (try)
 @    (accept foo)
 @  (finally)
 @     (output)
 bye!
 @     (end)
 @  (end)
output:
 bye!

How this works: the try block's main clause is @(accept foo). This causes the enclosing block named foo to terminate, as a successful match. Since the is nested within this block, it too must terminate in order for the block to terminate. But the try has a finally clause, which executes unconditionally, no matter how the try block terminates. The finally clause performs some output, which is seen.

 

12.3 The finally clause

A try directive can terminate in one of three ways. The main clause may match successfully, and possibly yield some new variable bindings. The main clause may fail to match. Or the main clause may be terminated by a non-local control transfer, like an exception being thrown or a block return (like the block foo example in the previous section).

No matter how the try clause terminates, the finally clause is processed.

The finally clause is itself a query which binds variables, which leads to questions: what happens to such variables? What if the finally block fails as a query? As well as: what if a finally clause itself initiates a control transfer? Answers follow.

Firstly, a finally clause will contribute variable bindings only if the main clause terminates normally (either as a successful or failed match). If the main clause of the try block successfully matches, then the finally block continues matching at the next position in the data, and contributes bindings. If the main clause fails, then the finally block tries to match at the same position where the main clause failed.

The overall try directive succeeds as a match if either the main clause or the finally clause succeed. If both fail, then the try directive is a failed match.

Example:

code:
 @(try)
 @a
 @(finally)
 @b
 @(end)
 @c
data:
 1
 2
 3
result:
 a="1"
 b="2"
 c="3"

In this example, the main clause of the try captures line "1" of the data as variable a, then the finally clause captures "2" as b, and then the query continues with the @c line after try block, so that c captures "3".

Example:

code:
 @(try)
 hello @a
 @(finally)
 @b
 @(end)
 @c
data:
 1
 2
result:
 b="1"
 c="2"

In this example, the main clause of the try fails to match, because the input is not prefixed with However, the finally clause matches, binding b to "1". This means that the try block is a successful match, and so processing continues with @c which captures "2".

When finally clauses are processed during a non-local return, they have no externally visible effect if they do not bind variables. However, their execution makes itself known if they perform side effects, such as output.

A finally clause guards only the main clause and the catch clauses. It does not guard itself. Once the finally clause is executing, the try block is no longer guarded. This means if a nonlocal transfer, such as a block accept or exception, is initiated within the finally clause, it will not re-execute the finally clause. The finally clause is simply abandoned.

The disestablishment of blocks and try clauses is properly interleaved with the execution of finally clauses. This means that all surrounding exit points are visible in a finally clause, even if the finally clause is being invoked as part of a transfer to a distant exit point. The finally clause can make a control transfer to an exit point which is more near than the original one, thereby "hijacking" the control transfer. Also, the anonymous block established by the try directive is visible in the finally clause.

Example:


  @(try)
  @  (try)
  @    (next "nonexistent-file")
  @  (finally)
  @    (accept)
  @  (end)
  @(catch file-error)
  @  (output)
  file error caught
  @  (end)
  @(end)

In this example, the @(next) directive throws an exception of type file-error, because the given file does not exist. The exit point for this exception is the @(catch file-error) clause in the outer-most try block. The inner block is not eligible because it contains no catch clauses at all. However, the inner try block has a finally clause, and so during the processing of this exception which is headed for @(catch file-error), the finally clause performs an anonymous accept. The exit point for that accept is the anonymous block surrounding the inner try. So the original transfer to the catch clause is thereby abandoned. The inner try terminates successfully due to the accept, and since it constitutes the main clause of the outer try, that also terminates successfully. The "file error caught" message is never printed.

 

12.4 catch clauses

catch clauses establish their associated try blocks as potential exit points for exception-induced control transfers (called "throws").

A catch clause specifies an optional list of symbols which represent the exception types which it catches. The catch clause will catch exceptions which are a subtype of any one of those exception types.

If a try block has more than one catch clause which can match a given exception, the first one will be invoked.

When a catch is invoked, it is of course understood that the main clause did not terminate normally, and so the main clause could not have produced any bindings.

catch clauses are processed prior to finally.

If a catch clause itself throws an exception, that exception cannot be caught by that same clause or its siblings in the same try block. The catch clauses of that block are no longer visible at that point. Nevertheless, the catch clauses are still protected by the finally block. If a catch clause throws, or otherwise terminates, the finally block is still processed.

If a finally block throws an exception, then it is simply aborted; the remaining directives in that block are not processed.

So the success or failure of the try block depends on the behavior of the catch clause or the clause, if there is one. If either of them succeed, then the try block is considered a successful match.

Example:

code:
 @(try)
 @  (next "nonexistent-file")
 @  x
 @  (catch file-error)
 @a
 @(finally)
 @b
 @(end)
 @c
data:
 1
 2
 3
result:
 a="1"
 b="2"
 c="3"

Here, the try block's main clause is terminated abruptly by a file-error exception from the @(next) directive. This is handled by the catch clause, which binds variable a to the input line "1". Then the finally clause executes, binding b to "2". The try block then terminates successfully, and so @c takes "3".

 

12.5 catch Clauses with Parameters

A catch clause may have parameters following the type name, like this:


  @(catch pair (a b))

To write a catch-all with parameters, explicitly write the master supertype t:


  @(catch t (arg ...))

Parameters are useful in conjunction with throw. The built-in error exceptions carry one argument, which is a string containing the error message. Using throw, arbitrary parameters can be passed from the throw site to the catch site.

 

12.6 The throw directive

The throw directive generates an exception. A type must be specified, followed by optional arguments. For example,


  @(throw pair "a" `@file.txt`)

throws an exception of type pair, with two arguments, being "a" and the expansion of the quasiliteral `@file.txt`.

The selection of the target catch is performed purely using the type name; the parameters are not involved in the selection.

Binding takes place between the arguments given in throw and the target catch.

If any catch parameter, for which a throw argument is given, is a bound variable, it has to be identical to the argument, otherwise the catch fails. (Control still passes to the catch, but the catch is a failed match).

code:
 @(bind a "apple")
 @(try)
 @(throw e "banana")
 @(catch e (a))
 @(end)
result:
 [query fails]

If any argument is an unbound variable, the corresponding parameter in the catch is left alone: if it is an unbound variable, it remains unbound, and if it is bound, it stays as is.

code:
 @(try)
 @(trow e "honda" unbound)
 @(catch e (car1 car2))
 @car1 @car2
 @(end)
data:
 honda toyota
result:
 car1="honda"
 car2="toyota"

If a catch has fewer parameters than there are throw arguments, the excess arguments are ignored:

code:
 @(try)
 @(throw e "banana" "apple" "pear")
 @(catch e (fruit))
 @(end)
result:
 fruit="banana"

If a catch has more parameters than there are throw arguments, the excess parameters are left alone. They may be bound or unbound variables.

code:
 @(try)
 @(trow e "honda")
 @(catch e (car1 car2))
 @car1 @car2
 @(end)
data:
 honda toyota
result:
 car1="honda"
 car2="toyota"

A throw argument passing a value to a catch parameter which is unbound causes that parameter to be bound to that value.

throw arguments are evaluated in the context of the throw, and the bindings which are available there. Consideration of what parameters are bound is done in the context of the catch.

code:
 @(bind c "c")
 @(try)
 @(forget c)
 @(bind (a c) ("a" "lc"))
 @(throw e a c)
 @(catch e (b a))
 @(end)
result:
 c="c"
 b="a"
 a="lc"

In the above example, c has a toplevel binding to the string "c", but then becomes unbound via forget within the try construct, and rebound to the value "lc". Since the try construct is terminated by a throw, these modifications of the binding environment are discarded. Hence, at the end of the query, variable c ends up bound to the original value "c". The throw still takes place within the scope of the bindings set up by the try clause, so the values of a and c that are thrown are "a" and "lc". However, at the catch site, variable a does not have a binding. At that point, the binding to "a" established in the try has disappeared already. Being unbound, the catch parameter a can take whatever value the corresponding throw argument provides, so it ends up with "lci".

 

12.7 The defex directive

The defex directive allows the query writer to invent custom exception types, which are arranged in a type hierarchy (meaning that some exception types are considered subtypes of other types).

Subtyping means that if an exception type B is a subtype of A, then every exception of type B is also considered to be of type A. So a catch for type A will also catch exceptions of type B. Every type is a supertype of itself: an A is a kind of A. This of course implies that every type is a subtype of itself also. Furthermore, every type is a subtype of the type t, which has no supertype other than itself. Type nil is is a subtype of every type, including itself. The subtyping relationship is transitive also. If A is a subtype of B, and B is a subtype of C, then A is a subtype of C.

defex may be invoked with no arguments, in which case it does nothing:


  @(defex)

It may be invoked with one argument, which must be a symbol. This introduces a new exception type. Strictly speaking, such an introduction is not necessary; any symbol may be used as an exception type without being introduced by @(defex):


  @(defex a)

Therefore, this also does nothing, other than document the intent to use a as an exception.

If two or more argument symbols are given, the symbols are all introduced as types, engaged in a subtype-supertype relationship from left to right. That is to say, the first (leftmost) symbol is a subtype of the next one, which is a subtype of the next one and so on. The last symbol, if it had not been already defined as a subtype of some type, becomes a direct subtype of the master supertype t. Example:


  @(defex d e)
  @(defex a b c d)

The first directive defines d as a subtype of e, and e as a subtype of t. The second defines a as a subtype of b, b as a subtype of c, and c as a subtype of d, which is already defined as a subtype of e. Thus a is now a subtype of e. The the above can be condensed to:


  @(defex a b c d e)

Example:

code:
 @(defex gorilla ape primate)
 @(defex monkey primate)
 @(defex human primate)
 @(collect)
 @(try)
 @(skip)
 @(cases)
 gorilla @name
 @(throw gorilla name)
 @(or)
 monkey @name
 @(throw monkey name)
 @(or)
 human @name
 @(throw human name)
 @(end)@#cases
 @(catch primate (name))
 @kind @name
 @(output)
 we have a primate @name of kind @kind
 @(end)@#output
 @(end)@#try
 @(end)@#collect
data:
 gorilla joe
 human bob
 monkey alice
output:
 we have a primate joe of kind gorilla
 we have a primate bob of kind human
 we have a primate alice of kind monkey

Exception types have a pervasive scope. Once a type relationship is introduced, it is visible everywhere. Moreover, the defex directive is destructive, meaning that the supertype of a type can be redefined. This is necessary so that something like the following works right:


  @(defex gorilla ape)
  @(defex ape primate)

These directives are evaluated in sequence. So after the first one, the ape type has the type t as its immediate supertype. But in the second directive, ape appears again, and is assigned the primate supertype, while retaining gorilla as a subtype. This situation could be diagnosed as an error, forcing the programmer to reorder the statements, but instead TXR obliges. However, there are limitations. It is an error to define a subtype-supertype relationship between two types if they are already connected by such a relationship, directly or transitively. So the following definitions are in error:


  @(defex a b)
  @(defex b c)
  @(defex a c)@# error: a is already a subtype of c, through b


  @(defex x y)
  @(defex y x)@# error: circularity; y is already a supertype of x.

 

12.8 The assert directive

The assert directive requires the remaining query or sub-query which follows it to match. If the remainder fails to match, the assert directive throws an exception. If the directive is simply


  @(assert)

Then it throws an assertion of type assert, which is a subtype of error. The assert directive also takes arguments similar to the throw directive. The following assert directive, if it triggers, will throw an exception of type foo, with arguments 1 and "2":


  @(assert foo 1 "2")

Example:


  @(collect)
  Important Header
  ----------------
  @(assert)
  Foo: @a, @b
  @(end)

Without the assertion in places, if the Foo: @a, @b part does not match, then the entire interior of the @(collect) clause fails, and the collect continues searching for another match.

With the assertion in place, if the text "Important Header" and its underline match, then the remainder of the collect body must match, otherwise an exception is thrown. Now the program will not silently skip over any Important Header sections due to a problem in its matching logic. This is particularly useful when the matching is varied with numerous cases, and they must all be handled.

There is a horizontal assert directive also. For instance:


  abc@(assert)d@x

asserts that if the prefix "abc" is matched, then it must be followed by a successful match for "d@x", or else an exception is thrown.

 

13 TXR LISP

The TXR language contains an embedded Lisp dialect called TXR Lisp.

This language is exposed in TXR in several ways.

Firstly, in any situation that calls for an expression, a Lisp expression can be used, if it is preceded by the @ character. The Lisp expression is evaluated and its value becomes the value of that expression. Thus, TXR directives are embedded in literal text using @, and Lisp expressions are embedded in directives using @ also.

Secondly, the @(do) directive can be used for evaluating one or more Lisp forms, such that their value is thrown away. This is useful for evaluating some Lisp code for the sake of its side effect, such as defining a variable, updating a hash table, et cetera.

Thirdly, the @(require) directive can be used to evaluate Lisp expressions as part of the matching logic of the TXR pattern language. The return value of the rightmost expression is examined. If it is nil, then the @(require) directive triggers a match failure. Otherwise, matching proceeds.

Lastly, TXR Lisp expressions can be evaluated via the command line, using the -e and -p options.

Examples:

Bind variable a to the integer 4:


  @(bind a @(+ 2 2))

Bind variable b to the standard input stream:


  @(bind a @*stdin*)

Define several Lisp functions inside @(do):


  @(do
    (defun add (x y) (+ x y))


    (defun occurs (item list)
      (cond ((null list) nil)
            ((atom list) (eql item list))
            (t (or (eq (first list) item)
                   (occurs item (rest list)))))))

Trigger a failure unless previously bound variable answer is greater than 42:


  @(require (> (str-int answer) 42)

 

13.1 Overview

TXR Lisp is a small and simple dialect, like Scheme, but much more similar to Common Lisp than Scheme. It has separate value and function binding namespaces, like Common Lisp (and thus is a Lisp-2 type dialect), and represents boolean true and false with the symbols t and nil (note the case sensitivity of identifiers denoting symbols!) Furthermore, the symbol nil is also the empty list, which terminates nonempty lists.

TXR Lisp has lexically scoped local variables and dynamic global variables, similarly to Common Lisp, including the convention that defvar marks symbols for dynamic binding in local scopes. Lexical closures are supported. Funtions are lexically scoped in TXR Lisp; they can be defined in pervasive global environment using defun or in local scopes using flet and labels.

 

13.2 Additional Syntax

Much of the TXR Lisp syntax has been introduced in the previous sections of the manual, since directive forms are based on it. There is some additional syntax that is useful in TXR Lisp programming.

 

13.3 Symbol Tokens

The symbol tokens in TXR Lisp, called a lident (Lisp identifier) has a similar syntax to the bident (braced identifier) in the TXR pattern language. It may consist of all the same characters, as well as the / (slash) character which may not be used in a bident. Thus a lident may consist of these characters, in addition to letters and numbers:


 ! $ % & * + - < = > ? \ _ ~ /

and of course, may not look like a number. A lone / is a symbol in TXR Lisp. The token /abc/ is also a symbol, and not a regular expression, like it is in the braced variable syntax. Within TXR Lisp, regular expressions are written with a leading #.

 

13.4 Consing Dot

Unlike other major Lisp dialects, TXR Lisp allows a consing dot with no forms preceding it. This construct simply denotes the form which follows the dot. That is to say, the parser implements the following transformation:


  (. expr) -> expr

This is convenient in writing function argument lists that only take variable arguments. Instead of the syntax:


  (defun fun args ...)

the following syntax can be used:


  (defun fun (. args) ...)

When a lambda form is printed, it is printed in the following style.


  (lambda nil ...) -> (lambda () ...)
  (lambda sym ...) -> (lambda (. sym) ...)
  (lambda (sym) ...) -> (lambda (sym) ...)

In no other circumstances is nil printed as (), or an atom sym as (. sym).

 

13.5 Quote and Quasiquote

'expr

The quote character in front of an expression is used for suppressing evaluation, which is useful for forms that evaluate to something other than themselves. For instance if '(+ 2 2) is evaluated, the value is the three-element list (+ 2 2), whereas if (+ 2 2) is evaluated, the value is 4. Similarly, the value of 'a is the symbol a itself, whereas the value of a is the contents of the variable a.

^qq-template

The caret in front of an expression is a quasiquote. A quasiquote is like a quote, but with the possibility of substitution of material.

Under a quasiquote, form is considered to be a quasiquote template. The template is considered to be a literal structure, except that it may contain the notations ,expr and ,*expr which denote non-constant parts.

A quasiquote gets translated into code which, when evaluated, constructs the structure implied by qq-template, taking into account the unquotes and splices.

A quasiquote also processes nested quasiquotes specially.

If qq-template does not contain any unquotes or splices (which match its level of nesting), or is simply an atom, then ^qq-template is equivalent to 'qq-template . in other words, it is like an ordinary quote. For instance ^(a b ^(c ,d)) is equivalent to '(a b ^(c ,d)). Although there is an unquote ,d it belongs to the inner quasiquote ^(c ,d), and the outer quasiquote does not have any unquotes of its own, making it equivalent to a quote.

Dialect note: in Common Lisp and Scheme, ^form is written `form, and quasiquotes are also informally known as backquotes. In TXR, the backquote character ` used for quasi string literals.

,expr

The comma character is used within a qq-template to denote an unquote. Whereas the quote suppresses evaluation, the comma introduces an exception: an element of a form which is evaluated. For example, list ^(a b c ,(+ 2 2) (+ 2 2)) is the list (a b c 4 (+ 2 2)). Everything in the quasiquote stands for itself, except for the ,(+ 2 2) which is evaluated.

Note: if a variable is called *x*, then the syntax ,*x* means ,* x*: splice the value of x*. In this situation, whitespace between the comma and the variable name should be used: , *x*.

,*expr

The comma-star operator is used within quasiquote list to denote a splicing unquote. The form which follows ,* must evaluate to a list. That list is spliced into the structure which the quasiquote denotes. For example: '(a b c ,*(list (+ 3 3) (+ 4 4) d)) evaluates to (a b c 6 8 d). The expression (list (+ 3 3) (+ 4 4)) is evaluated to produce the list (6 8), and this list is spliced into the quoted template.

Dialect note: in other Lisp dialects, the equivalent syntax is usually ,@ (comma at). The @ character already has an assigned meaning, so * is used.

 

13.6 Quasiquoting non-List Objects

Quasiquoting is supported over hash table and vector literals (see Vectors and Hashes below). A hash table or vector literal can be quoted, like any object, for instance:


  '#(1 2 3)

The #(1 2 3) literal is turned into a vector atom right in the TXR parser, and this atom is being quoted: this is (quote atom) syntactically, which evaluates to atom.

When a vector is quasi-quoted, this is a case of ^atom which evaluates to atom.

A vector can be quasiquoted, for example:


  ^#(1 2 3)

Of ourse, unquotes can occur within it.


  (let ((a 42))
    ^#(1 ,a 3)) ; value is #(1 42 3)

In this situation, the ^#(...) notation produces code which constructs a vector.

The vector in the following example is also a quasivector. It contains unquotes, and though the quasiquote is not directly applied to it, it is embedded in a quasiquote:


  (let ((a 42))
    ^(a b c #(d ,a))) ; value is (a b c #(d 42))

Hash table literals have two parts: the list of hash construction arguments and the key-value pairs. For instance:


   #H((:equal-based) (a 1) (b 2))

where (:equal-based) is the list of construction arguments and the pairs (a 1) and (b 2) are the key/value entries. Hash literals may be quasiquoted. In quasiquoting, the arguments and pairs are treated as separate syntax; it is not one big list. So the following is not a possible way to express the above hash:


  ;; not supported: splicing across the entire syntax
  (let ((hash-syntax '((:equal-based) (a 1) (b 2))))
    ^#H(,*hash-syntax))

This is correct:


  ;; fine: splicing hash arguments and contents separately
  (let ((hash-args '(:equal-based))
        (hash-contents '((a 1) (b 2))))
    ^#H(,hash-args ,*hash-contents))

 

13.7 Quasiquoting combined with Quasiliterals

When a quasiliteral is embedded in a quasiquote, it is possible to use splicing to insert material into the quasiliteral.

Example:


  (eval (let ((a 3)) ^`abc @,a @{,a} @{(list 1 2 ,a)}`))


  -> "abc 3 3 1 2 3"

 

13.8 Vectors

#(...)

A hash token followed by a list denotes a vector. For example #(1 2 a) is a three-element vector containing the numbers 1 and 2, and the symbol a.

 

13.9 Hashes

#H((hash-argument*) (key value)*)

The notation #H followed by a nested list syntax denotes a hash table literal. The first item in the syntax is a list of keywords. These are the same keywords as are used when calling the function hash to construct a hash table. Allowed keywords are: :equal-based, :weak-keys and :weak-values. An empty list can be specified as nil or (), which defaults to a hash table based on the eql function, with no weak semantics.

 

13.10 The .. notation

In TXR Lisp, there is a special "dotdot" notation consisting of a pair of dots. This can be written between successive atoms or compound expressions, and is a shorthand for cons.

That is to say, A .. B translates to (cons A B), and so for instance (a b .. (c d) e .. f . g) means (a (cons b (c d)) (cons e f) . g).

This is a syntactic sugar useful in certain situations in which a cons is used to represent a pair of numbers or other objects. For instance, if L is a list, then [L 1 .. 3] computes a sublist of L consisting of elements 1 through 2 (counting from zero).

Restrictions:

The notation must be enclosed in a list. For instance a..b is not, by itself, an expression, but (a..b) is. This is important if Lisp data is being parsed from a string or stream using the read function. If the data "a..b" is parsed, the symbol a will be extracted, leaving "..a", which, if parsed, produces a syntax error since it consists of a "dotdot" token followed by a symbol, which is not valid syntax.

The notation cannot occur in the dot position; that is, the syntax (a . b .. c) is invalid. The dotdot operator can only be used between the non-dot-position elements of a list.

 

13.11 The DWIM Brackets

TXR Lisp has a square bracket notation. The syntax [...] is a shorthand way of writing (dwim ...). The [] syntax is useful for situations where the expressive style of a Lisp-1 dialect is useful.

For instance if foo is a variable which holds a function object, then [foo 3] can be used to call it, instead of (call foo 3). If foo is a vector, then [foo 3] retrieves the fourth element, like (vecref foo 3). Indexing over lists, strings and hash tables is possible, and the notation is assignable.

Furthermore, any arguments enclosed in [] which are symbols are treated according to a modified namespace lookup rule.

More details are given in the documentation for the dwim operator.

 

13.12 Compound Forms

In TXR Lisp, there are two types of compound forms: the Lisp-2 style compound forms, denoted by ordinary lists that are expressed with parentheses. There are Lisp-1 style compound forms denoted by the DWIM Brackets, discussed in the previous section.

The first position of an ordinary Lisp-2 style compound form, is expected to have a function or operator name. Then arguments follow. There may also be an expression in the dotted position, if the form is a function call.

The following are Lisp functions and variables built-in to TXR.

If the form is a function call then the arguments are evaluated. If any of the arguments are symbols, they are treated according to Lisp-2 namespacing rules.

Additionally, if there is an expression in the dotted position, it is also evaluated. It should evaluate to a sequence: a list, vector or string. The elements of the sequence generate additional arguments for the function call. Note, however, that a compound form cannot be used in the dot position, for obvious reasons, namely that (a b c . (foo z)) does not mean that there is a compound form in the dot position, but denotes an alternate spelling for (a b c foo z), where foo behaves as a variable.

The DWIM brackets are similar, except that the first position is an arbitrary expression which is evaluated according to the same rules as the remaining positions. The first expression must evaluate to a function, or else to some other object for which the DWIM syntax is defined, such as a vector, string, list or hash. Operators are not supported. The dotted syntax for application of additional arguments from a list or vector is supported in the DWIM brackets just like in the parentheses.

Examples:


  ;; a contains 3
  ;; b contains 4
  ;; c contains #(5 6 7)
  ;; s contains "xyz"


  (foo a b . c) ;; calls (foo 3 4 5 6 7)
  (foo a)       ;; calls (foo 3)
  (foo . s)     ;; calls (foo #\x #\y #\z)


  [foo a b . c] ;; calls (foo 3 4 5 6 7)


  [c 1]         ;; indexes into vector #(5 6 7) to yield 6

Dialect Note:

In some other Lisp dialects, the improper list syntax is not supported; a function called apply (or similar) must be used for application even if the expression which gives the trailing arguments is a symbol. Moreover, applying sequences other than lists is not supported.

 

13.13 Regular Expressions

In TXR Lisp, the / character can occur in symbol names, and the / token is a symbol. Therefore the /regex/ syntax is not used for denoting regular expressions; rather, the #/regex/ syntax is used.

 

13.14 Generalization of List Accessors

In ancient Lisp in the 1960's, it was not possible to apply the operations car and cdr to the nil symbol (empty list), because it is not a cons cell. In the InterLisp dialect, this restriction was lifted: these operations were extended to accept nil (and return nil). The convention was adopted in other Lisp dialects such as MacLisp and eventually in Common Lisp. Thus there exists an object which is not a cons, yet which takes car and cdr.

In TXR Lisp, this relaxation is extended further. For the sake of convenience, the operations car and cdr, are made to work with strings and vectors:


  (cdr "") -> nil
  (car "") -> nil


  (car "abc") -> #\a
  (cdr "abc") -> "bc"


  (cdr #(1 2 3)) -> #(2 3)
  (car #(1 2 3)) -> 1

The ldiff function is also extended in a special way. When the right parameter is a string or vector, then it uses the equal equality test rather than eq for detecting the tail of the list.


  (ldiff "abcd" "cd") -> (#\a #\b)

The ldiff operation starts with "abcd" and repeatedly applies cdr to produce "bcd" and "cd", until the suffix is equal to the second argument: (equal "cd" "cd") yields true.

Operations based on car, cdr and ldiff, such as keep-if and remq extend to strings and vectors.

Most derived list processing operations such as remq or mapcar obey the following rule: the returned object follows the type of the leftmost input list object. For instance, if one or more sequences are processed by mapcar, and the leftmost one is a character string, the function is expected to return characters, which are converted to a character string. However, in the event that the objects produced cannot be assembled into that type of sequence, a list is returned instead.

For example [mapcar list "ab" "12"] returns ((#\a #\b) (#\1 #\2)), because a string cannot hold lists of characters. However [mappend list "ab" "12"] returns "a1b2".

The lazy versions of these functions such as mapcar* do not have this behavior; they produce lazy lists.

 

13.15 Callable Objects

In TXR Lisp, sequences (strings, vectors and lists) and hashes can be used as functions everywhere, not just with the DWIM brackets. Sequences work as one or two-argument functions. With a single argument, an element is selected by position and returned. With two arguments, a range is extracted and returned. Hashes also work as one or two argument functions, corresponding to the arguments of the gethash function.

Moreover, when a sequence is used as a function of one argument, and the argument is a cons cell rather than an integer, then the call becomes a two-argument call in which the car and cdr of the cell are passed as separate arguments. This allows for syntax like (call "abc" 0..1).

Example 1:


  (mapcar "abc" '(2 0 1)) -> (#\c #\a #\b)

Here, mapcar treats the string "abc" as a function of one argument (since there is one list argument). This function maps the indices 0, 1 and 2 to the corresponding characters of string "abc". Through this function, the list of integer indices (2 0 1) is taken to the list of characters (#\c #\a #\b).

Example 2:


  (call '(1 2 3 4) 1..3) -> (2 3)

Here, the shorthand 1 .. 3 denotes (cons 1 3). A cons cell as an argument to a sequence performs range extraction: taking a slice starting at index 1, up to and not including index 3, as if by the call (sub '(1 2 3 4) 1 3).

Example 3:


  (call '(1 2 3 4) '(0 2)) -> (1 2)

A list of indices applied to a sequence is equivalent to using the select function, as if (select '(1 2 3 4) '(0 2)) were called.

 

13.16 Special Variables

Similarly to Common Lisp, TXR Lisp is lexically scoped by default, but also has dynamically scoped (a.k.a "special") variables.

When a variable is defined with defvar, it is introduced as a global (global) binding, regardless of where in the scope the defvar form occurs.

Furthermore, at the time the defvar form is evaluated, the symbol which names the variable is tagged as special.

When a symbol is tagged as special, it behaves differently when it is used in a lexical binding construct like let, and all other such constructs such as function parameter lists. Such a binding is not the usual lexical binding, but a "rebinding" of the global variable. Over the dynamic scope of the form, the global variable takes on the value given to it by the rebinding. When the form terminates, the prior value of the variable is restored. (This is true no matter how the form terminates; even if by an exception.)

Because of this "pervasive special" behavior of a symbol that has been used as the name of a global variable, a good practice is to make global variables have visually distinct names via the "earmuffs" convention: beginning and ending the name with an asterisk.

Certain variables in TXR's library break this convention; however, they at least have distinct prefixes, examples being example s-ifmt, log-emerg and sig-hup.

Example:


  (defvar *x* 42)     ;; *x* has a value of 42


  (defun print-x ()
    (format t "~a\n" *x*))


  (let ((*x* "abc"))  ;; this overrides *x*
    (print-x))        ;; *x* is now "abc" and so that is printed


  (print-x)           ;; *x* is 42 again and so "42" is printed

 

14 TXR LISP OPERATOR AND FUNCTION LIBRARY

A compound expression with a symbol as its first element, if intended to be evaluated, denotes either an operator invocation or a function call. This depends on whether the symbol names an operator or a function.

When the form is an operator invocation, the interpretation of the meaning of that form is under the complete control of that operator.

If the compound form is a function call, the remaining forms, if any, denote argument expressions to the function. They are evaluated in left to right order to produce the argument values, which are passed to the function. An exception is thrown if there are not enough arguments, or too many. Programs can define named functions with the defun operator

Some operators are macros. There exist predefined macros in the library, and macro operators can also be user-defined using the macro-defining operator defmacro. Operators that are not macros are called special operators.

Macro operators work as functions which are given the source code of the form. They analyze the form, and translate it to another form which is substituted in their place. This happens during a code walking phase called the expansion phase, which is applied to Lisp code prior to evaluation. All macros are expanded in the expansion phase, resulting in code which contains only function calls and the executable forms of the operators.

(Special operators can also perform code transformations during the expansion phase, but that is not considered macroexpansion, but rather an adjustment of the representation of the operator into an required executable form.)

The following sections list all of the special operators, macros and functions in TXR Lisp.

In these sections Syntax is indicated using these conventions:

word
A symbol in fixed-width-italic font denotes some syntactic unit: it may be a symbol or compound form. The syntactic unit is explained in the Description section.

{syntax}* word*
This indicates a repetition of zero or more of the given syntax enclosed in the braces or syntactic unit.

{syntax}+ word+
This indicates a repetition of one or more of the given syntax enclosed in the braces or syntactic unit.

[syntax] [word]
Square brackets indicate optional syntax.

alternative1 | alternative2 | ... | alternativeN
Multiple syntactic variations allowed in one place are indicated as bar-separated items.

 

14.1 Control Flow and Sequencing

 

14.1.1 Operators progn and prog1

Syntax:


  (progn
form*)
  (prog1
form*)

Description:

The progn operator evaluates forms in order, and returns the value of the last form. The return value of the form (progn) is nil.

The prog1 operator evaluates forms in order, and returns the value of the first form. The return value of the form (prog1) is nil.

Various other operators such as let also arrange for the evaluation of a body of forms, the value of the last of which is returned. These operators are said to feature an implicit progn.

 

14.1.2 Operator cond

Syntax:


  (cond {(
test form*)}*)

Description:

The cond operator provides a multi-branching conditional evaluation of forms. Enclosed in the cond form are groups of forms expressed as lists. Each group must be a list of at least one form.

The forms are processed from left to right as follows: the first form, test, in each group is evaluated. If it evaluates true, then the remaining forms in that group, if any, are also evaluated. Processing then terminates and the result of the last form in the group is taken as the result of cond. If test is the only form in the group, then result of test is taken as the result of cond.

If the first form of a group yields nil, then processing continues with the next group, if any. If all form groups yield nil, then the cond form yields nil. This holds in the case that the syntax is empty: (cond) yields nil.

 

14.1.3 Macros caseq, caseql and casequal

Syntax:


  (caseq
test-form normal-clause* [else-clause])
  (caseql
test-form normal-clause* [else-clause])
  (caseqqual
test-form normal-clause* [else-clause])

Description:

These three macros arrange for the evaluation of of test-form, whose value is then compared against the key or keys in each normal-clause in turn. When the value matches a key, then the remaining forms of normal-clause are evaluated, and the value of the last form is returned; subsequent clauses are not evaluated. When the value doesn't match any of the keys of a normal-clause then the next normal-clause is tested. If all these clauses are exhausted, and there is no else-clause, then the value nil is returned. Otherwise, the forms in the else-clause are evaluated, and the value of the last one is returned.

The syntax of a normal-clause takes on these two forms:


  (
key form*)

where key may be an atom which denotes a single key, or else a list of keys. There is a restriction that the symbol t may not be used as key. The form (t) may be used as a key to match that symbol.

The syntax of an else-clause is:


  (t
form*)

which resembles a form that is often used as the final clause in the cond syntax.

The three forms of the case construct differ from what type of test they apply between the value of test-form and the keys. The caseq macro generates code which uses the eq function's equality. The caseql macro uses eql, and casequal uses equal.

Example


  (let ((command-symbol (casequal command-string
                          (("q" "quit") 'quit)
                          (("a" "add") 'add)
                          (("d" "del" "delete") 'delete)
                          (t 'unknown))))
    ...)

 

14.1.4 Macros when and unless

Syntax:


  (when
expression form*)
  (unless
expression form*)

Description:

The when macro operator evaluates expression. If expression yields true, and there are additional forms, then each form is evaluated. The value of the last form is becomes the result value of the when form. If there are no forms, then the result is nil.

The unless operator is similar to when, except that it reverses the logic of the test. The forms, if any, are evaluated if, and only if expression is false.

 

14.1.5 Macros while and until

Syntax:


  (while
expression form*)
  (until
expression form*)

Description:

The while macro operator provides a looping construct. It evaluates expression. If expression yields nil, then the evaluation of the while form terminates, producing the value nil. Otherwise, if there are additional forms, then each form is evaluated. Next, evaluation returns to expression, repeating all of the previous steps.

The until macro operator is similar to while, except that the until form terminates when expression evaluates true, rather than false.

These operators arrange for the evaluation of all their enclosed forms in an anonymous block. Any of the forms, or expression, may use the return operator to terminate the loop, and optionally to specify a result value for the form.

The only way these forms can yield a value other than nil is if the return operator is used to terminate the implicit anonymous block, and is given an argument, which becomes the result value.

 

14.1.6 Operator/function if

Syntax:


  (if
cond t-form [e-form])
  [if
cond then [else]]

Description:

There exist both an if operator and an if function. A list form with the symbol if in the fist position is interpreted as an invocation of the if operator. The function can be accessed using the DWIM bracket notation and in other ways.

The if operator provides a simple two-way-selective evaluation control. The cond form is evaluated. If it yields true then t-form is evaluated, and that form's return value becomes the return value of the if. If cond yields false, then e-form is evaluated and its return value is taken to be that of if. If e-form is omitted, then the behavior is as if e-form were specified as nil.

The if function provides no evaluation control. All of arguments are evaluated from left to right. If the cond argument is true, then it returns the then argument, otherwise it returns the value of the else argument if present, otherwise it returns nil.

 

14.1.7 Operator/function and

Syntax:


  (and
form*)
  [and
arg*]

Description:

There exist both an and operator and an and function. A list form with the symbol and in the fist position is interpreted as an invocation of the operator. The function can be accessed using the DWIM bracket notation and in other ways.

The and operator provides three functionalities in one. It computes the logical "and" function over several forms. It controls evaluation (a.k.a. "short-circuiting"). It also provides an idiom for the convenient substitution of a value in place of nil when some other values are all true.

The and operator evaluates as follows. First, a return value is established and initialized to the value t. The forms, if any, are evaluated from left to right. The return value is overwritten with the result of each form. Evaluation stops when all forms are exhausted, or when nil is stored in the return value. When evaluation stops, the operator yields the return value.

The and function provides no evaluation control; it receives all of its arguments fully evaluated. If it is given no arguments, it returns t. If it is given one or more arguments, and any of them are nil, it returns nil. Otherwise it returns the value of the last argument.

Examples:


  (and) -> t
  (and (> 10 5) (stringp "foo")) -> t
  (and 1 2 3) -> 3  ;; short-hand for (if (and 1 2) 3).

 

14.1.8 Operator/function or

Syntax:


  (or
form*)
  [or
arg*]

Description:

There exist both an or operator and an or function. A list form with the symbol or in the fist position is interpreted as an invocation of the operator. The function can be accessed using the DWIM bracket notation and in other ways.

The or operator provides three functionalities in one. It computes the logical "or" function over several forms. It controls evaluation (a.k.a. "short-circuiting"). The behavior of or also provides an idiom for the selection of the first non-nil value from a sequence of forms.

The or operator evaluates as follows. First, a return value is established and initialized to the value nil. The forms, if any, are evaluated from left to right. The return value is overwritten with the result of each form. Evaluation stops when all forms are exhausted, or when a true value is stored into the return value. When evaluation stops, the operator yields the return value.

The or function provides no evaluation control; it receives all of its arguments fully evaluated. If it is given no arguments, it returns nil. If all of its arguments are nil, it also returns nil. Otherwise, it returns the value of the first argument which isn't nil.

Examples:


  (or) -> nil
  (or 1 2) -> 1
  (or nil 2) -> 2
  (or (> 10 20) (stringp "foo")) -> t

 

14.1.9 Operator unwind-protect

Syntax:


  (unwind-protect
protected-form cleanup-form*)

Description:

The operator evaluates protected-form in such a way that no matter how the execution of protected-form terminates, the cleanup-forms will be executed.

The cleanup-forms, however, are not protected. If a cleanup-form terminates via some non-local jump, the subsequent cleanup-forms are not evaluated.

cleanup-forms themselves can "hijack" a non-local control transfer such as an exception. If a cleanup-form is evaluated during the processing of a dynamic control transfer such as an exception, and that cleanup-form initiates its own dynamic control transfer, the original control transfer is aborted and replaced with the new one.

Example:


  (block foo
    (unwind-protect
      (progn (return-from foo 42)
             (format t "not reached!\n"))
      (format t "cleanup!\n")))

In this example, the protected progn form terminates by returning from block foo. Therefore the form does not complete and so the output "not reached!" is not produced. However, the cleanup form executes, producing the output "cleanup!".

 

14.1.10 Operator block

Syntax:


  (block
name body-form*)

Description:

The block operator introduces a named block around the execution of some forms. The name argument must be a symbol. Since a block name is not a variable binding, keyword symbols are permitted, and so are the symbols t and nil. A block named by the symbol nil is slightly special: it is understood to be an anonymous block.

Blocks in TXR Lisp have dynamic scope. This means that the following situation is allowed:


  (defun func () (return-from foo 42))
  (block foo (func))

The function can return from the foo block even though the foo block does not lexically surround foo.

Thus blocks in TXR Lisp provide dynamic non-local returns, as well as returns out of lexical nesting.

Dialect Note:

In Common Lisp, blocks are lexical. A separate mechanism consisting of catch and throw operators performs non-local transfer based on symbols. The TXR Lisp example:


  (defun func () (return-from foo 42))
  (block foo (func))

is not allowed in Common Lisp, but can be transliterated to:


  (defun func () (throw 'foo 42))
  (catch 'foo (func))

Note that foo is quoted in CL. This underscores the dynamic nature of the construct. throw itself is a function and not an operator.

 

14.1.11 Operators return and return-from

Syntax:


  (return [
value])
  (return-from
name [value])

Description:

The return operator must be dynamically enclosed within an anonymous block (a block named by the symbol nil). It immediately terminates the evaluation of the innermost anonymous block which encloses it, causing it to return the specified value. If the value is omitted, the anonymous block returns nil.

The return-from operator must be dynamically enclosed within a named block whose name matches the name argument. It immediately terminates the evaluation of the innermost such block, causing it to return the specified value. If the value is omitted, that block returns nil.

Example:


    (block foo
      (let ((a "abc\n")
            (b "def\n"))
        (pprint a *stdout*)
        (return-from foo 42)
        (pprint b *stdout*)))

Here, the output produced is "abc". The value of b is not printed because. return-from terminates block foo, and so the second pprint form is not evaluated.

 

14.2 Evaluation

 

14.2.1 Operator dwim

Syntax:


  (dwim
argument*)
  [
argument*]

Description:

The dwim operator's name is an acronym: DWIM may be taken to mean "Do What I Mean", or alternatively, "Dispatch, in a Way that is Intelligent and Meaningful".

The notation [...] is a shorthand equivalent to (dwim ...) and is usually the preferred way for writing dwim expressions.

The dwim operator takes a variable number of arguments, which are all evaluated in the same way: the first argument is not evaluated differently from the remaining arguments.

This means that the first argument isn't a function name, but an ordinary expression which can simply compute a function object (or, more generally, a callable object).

Furthermore, for those arguments of dwim which are symbols (after all macro-expansion is performed on the arguments), the evaluation rules are altered. For the purposes of resolving symbols to values, the function and variable binding namespaces are considered to be merged into a single space, creating a situation that is very similar to a Lisp-1 style dialect.

This special Lisp-1 evaluation is not recursively applied. All arguments of dwim which, after macro expansion, are not symbols are evaluated using the normal Lisp-2 evaluation rules. Thus, the DWIM operator must be used in every expression where the Lisp-1 rules for reducing symbols to values are desired.

After macro expansion, the first argument of dwim may not be an operator such as let, or the name of a macro. Prior to macroexpansion, any argument of dwim may be a symbol macro.

If a symbol has bindings both in the variable and function namespace in scope, and is referenced by a dwim argument, this constitutes a conflict which is resolved according to two rules. When nested scopes are concerned, then an inner binding shadows an outer binding, regardless of their kind. An inner variable binding for a symbol shadows an outer or global function binding, and vice versa.

If a symbol is bound to both a function and variable in the global namespace, then the variable binding is favored.

Macros do not participate in the special scope conflation. This is apparent in two ways. Firstly, the space of symbol macros is not folded together with the space of regular macros. An argument of dwim that is a symbol might be symbol macro, variable or function, but it cannot be interpreted as the name of a regular macro. Secondly, function bindings cannot shadow symbol macros. If a function is defined in an inner scope relative to a symbol macro for the same symbol, that symbol macro still hides the function when it appears in a dwim form. Symbol macros are expanded without any consideration for the dwim operator, which is not treated specially by the expansion phase.

Thus, the following is invalid:

[let ((f (+ 2 2))) f]

the expression looks for a function or variable called let, which does not exist.

How many arguments are required by the dwim operator depends on the type of object to which the first argument expression evaluates. The possibilities are:

[function argument*]
Call the given function object to the given arguments.

[symbol argument*]
If the first expression evaluates to a symbol, that symbol is resolved in the function namespace, and then the resulting function, if found, is called with the given arguments.

[sequence index]
Retrieve an element from sequence, given by the integer index.

The following equivalences hold:


  [seq index] <--> (ref seq index)


  (set [seq index] new) <--> (refset seq index new)

This form accepts update operators like inc and flip.

[sequence from-index..to-below-index]
Retrieve the specified range of elements. The range of elements is specified in the car and cdr fields of a cons cell, for which the .. (dotdot) syntactic sugar is useful. See the section on Range Indexing below.

The following equivalences hold:


  [seq from..to] <--> (sub seq from to)


  (set [seq from..to] new) <--> (refset seq new from to)

This form does not accept update operators like inc and flip.

[sequence index-list]
Elements specified by index-list are extracted from sequence and returned as a sequence of the same kind as sequence. This form is equivalent to (select sequence where-index) except when the target of an assignment operation.

The following equivalences hold:


  [seq list] <--> (select seq list)


  (set [seq list] new) <--> (refset seq new list)

This form does not accept update operators like inc and flip.

Note that unlike the select function, this does not support [hash index-list] because since hash keys may be lists, that syntax is indistinguishable from a simple hash lookup where index-list is the key.

[hash key default-value]
Retrieve a value from the hash table corresponding to key, or default-value if there is no such entry.

The first argument may not be an operator such as let, only a function.

The places denoted by the dwim operator can be assigned. There are some restrictions. List, string and vector ranges can only be replaced using the set operator. The other operators like push do not apply. Characters in a string can only be assigned with set or incremented with inc and dec.

The source of a range assignment can be a string, vector or list, regardless of whether the target is a string, vector or list. If the target is a string, the replacement sequence must be a string, or a list or vector of characters.

Range Indexing

Vector and list range indexing is based from zero. The first element element zero. Furthermore, the value -1 refers to the last element of the vector or list, and -2 to the second last and so forth. So the range 1 .. -2 means "everything except for the first element and the last two".

The symbol t represents the position one past the end of the vector, string or list, so 0 .. t denotes the entire list or vector, and the range t .. t represents the empty range just beyond the last element. It is possible to assign to t .. t. For instance:


  (defvar list '(1 2 3))
  (set [list t .. t] '(4)) ;; list is now (1 2 3 4)

The value zero has a "floating" behavior when used as the end of a range. If the start of the range is a negative value, and the end of the range is zero, the zero is interpreted as being the position past the end of the sequence, rather than the first element. For instance the range -1..0 means the same thing as -1..t. Zero at the start of a range always means the first element, so that 0..-1 refers to all the elements except for the last one.

Notes:

The dwim operator allows for a Lisp-1 flavor of programming in TXR Lisp, which is principally a Lisp-2 dialect.

A Lisp-1 dialect is one in which an expression like (a b) treats both a and b as expressions subject to the same evaluation rules—at least, when a isn't an operator or an operator macro. This means that the symbols a and b are resolved to values in the same namespace. The form denotes a function call if the value of variable a is a function object. Thus in a Lisp-1, named functions do not exist as such: they are just variable bindings. In a Lisp-1 (car 1 2) means that there is a variable called car, which holds a function. In a Lisp-2 (car 1 2) means that there is a function called car, and so (car car car) is possible, because there can be also a variable called car which holds a cons cell object, rather than the car function.

The Lisp-1 approach is useful for functional programming, because it eliminates cluttering occurrences of the call and fun operators. For instance:


  ;; regular notation


  (call foo (fun second) '((1 a) (2 b)))


  ;; [] notation


  [foo second '((1 a) (2 b))]

Lisp-1 dialects can also provide useful extensions by giving a meaning to objects other than functions in the first position of a form, and the dwim/[...] syntax does exactly this.

However, Lisp-2 also has its advantages, whereas Lisp-1 also has disadvantages. Lisp-2 integrates with macros and operators more naturally than Lisp-1. Macros come in two flavors: symbol macros and operator macros. This distinction integrates better into a two namespace model, in which an operator macro is recognized in the first position of a form, and a symbol macro in other positions. Moreover, operators and operator macros are not functions; they cannot work by reducing a symbol to a macro function under ordinary evaluation rules. A Lisp-1 implementation must inspect the leftmost expression to determine whether or not it is an operator, which breaks the principle that all expressions are to be evaluated in the same way. Moreover, macros and operators are never indirected upon. If x is an operator or macro, it never makes sense for x to occur other than in the leftmost position of a form. In Lisp-1 dialects, the situation of using the name of an operator as a variable other than in the first position, as in (list let if) has no meaning, indicating that the single namespace model serves no purpose for macros and operators.

TXR Lisp banishes operators and macros (except symbol macros) from Lisp-1 evaluation, thereby achieving a more pure model of Lisp-1 evaluation than actual Lisp-1 languages, while retaining all the advantages of Lisp-2. In short, a "best of both worlds" situation is achieved in TXR Lisp.

 

14.2.2 Function identity

Syntax:


  (identity
value)

Description:

The identity function returns its argument.

Notes:

The identity function is useful as a functional argument, when a transformation function is required, but no transformation is actually desired.

 

14.2.3 Function eval

Syntax:


  (eval
form [env])

Description:

The eval function treats the form object as a Lisp expression, which is evaluated. The side effects implied by the form are performed, and the value which it produces is returned. The optional env object specifies an environment for resolving the function and variable references encountered in the expression. If this argument is omitted nil then evaluation takes place in the global environment.

See also: the make-env function.

 

14.2.4 Function make-env

Syntax:


  (make-env [
variable-bindings [function-bindings [next-env]]])

Description:

The make-env function creates an environment object suitable as the env parameter.

The variable-bindings and function-bindings parameters, if specified, should be association lists, mapping symbols to objects. The objects in function-bindings should be functions, or objects callable as functions.

The next-env argument, if specified, should be an environment.

Note: bindings can also be added to an environment using the env-vbind and env-fbind functions.

 

14.2.5 Functions env-vbind and env-fbind

Syntax:


  (env-vbind
env symbol value)
  (env-fbind
env symbol value)

Description:

These functions bind a symbol to a value in either the function or variable space of environment env.

Values established in the function space should be functions or objects that can be used as functions such as lists, strings, arrays or hashes.

If symbol already exists in the environment, in the given space, then its value is updated with value.

 

14.3 Mutation

 

14.3.1 Operators inc dec set push pop flip and del

Syntax:


  (inc
place [delta])
  (dec
place [delta])
  (set
place new-value)
  (push
item place)
  (pop
place)
  (flip
place)
  (del
place)

Description:

These destructive operators update the value of a place. A place is a storage location which is denoted by a form. Place forms are identical to value accessing forms. That is to say, any form recognized as a place by these operators can be evaluated by itself to retrieve the value of the storage location. However, the converse is false: not all forms which access storage location are recognized as places.

With are exceptions noted below, it is an error if a place does not exist. For instance, a variable being assigned must exist.

Literal objects which are directly specified in the source code are considered part of the program body. Modifying parts of these objects therefore gives rise to self-modifying code. The behavior of self-modifying code is not specified.

The inc and dec update the place by adding or subtracting, respectively, a displacement to or from that number. If the delta expression is specified, then it is evaluated and its value is used as the increment. Otherwise, a default increment of 1 is used. The prior value of the place and the delta must be suitable operands for the + and - functions. (inc x) is equivalent to (set x (+ 1 x)), except that expression x is evaluated only once to determine the storage location. The inc and dec operators return the new value that was stored.

The set operator overwrites the previous value of a place with a new value, and also returns that value.

The. push and pop operators operate on a place which holds a list. The push operator updates the list by replacing it with a new list which has a new item at the front, followed by the previous list. The item is returned. The pop operator performs the reverse operation: it removes the first item from the list and returns it. (push y x) is similar to


  (let ((temp y)) (set x (cons temp x)) temp)

except that x is evaluated only once to determine the storage place, and no such temporary variable is visible to the program. Similarly, (pop x) is much like


  (let ((temp (car x))) (set x (cdr x)) temp)

except that x is evaluated only once, and no such temporary variable is visible to the program.

The flip operator toggles a place between true and false. If the place contains a value other than nil, then its value is replaced with nil. If it contains nil, it is replaced with t.

The del operator does not modify the value of a place, but rather deletes the place itself. Index values and ranges of lists denoted using the dwim operator indexing notation can be subject to a deletion, as can hash table entries denoted using dwim or gethash. It is an error to try to delete other kinds of places such as simple variables. The del operator returns the value of the place that was deleted. Deleting from a sequence means removing the element or elements. Deleting a hash place means removing the corresponding entry from the hash table.

Currently, these forms are recognized as places:

symbol
A .meta symbol place denotes a variable. If the variable does not exist, it is an error.

(car cons)
(cdr cons)
These places denote the corresponding slots of a cons cell. The cons form must be an expression which evaluates to a cons.

(gethash hash key default-value)
The gethash place denotes a value stored in a hash table. The form hash must evaluate to a hash table. If the place does not exist in the hash table under the given key, then the destructive operation will create it. In that case, the default-value form is evaluated to determine the initial value of the place. Otherwise it is ignored.

(vecref vector index)
The vecref place denotes a vector element, allowing vector elements to be treated as assignment places.

(dwim obj ...)
[obj ...] ;; equivalent
The dwim/[] place denotes a vector element, list element, string, or hash table, depending on the type of obj.

 

14.4 Binding and Iteration

 

14.4.1 Operator defvar

Syntax:


  (defvar
sym value)

Description:

The defvar operator binds a variable in the global environment.

If the variable named sym already exists in the global environment, the form has no effect; the value form is not evaluated, and the value of the variable is unchanged.

If the variable does not exist, then it is introduced, with a value given by evaluating the value form. The value form is evaluated in the environment in which the defvar form occurs, not necessarily in the global environment.

The symbols t and nil may not be used as variables, and neither can be keyword symbols: symbols denoted by a leading colon.

In addition to creating a binding, the defvar operator also marks sym as the name of a special variable. This changes what it means to bind that symbol in a lexical binding construct such as the let operator, or a function parameter list. See the section "Special Variables" far above.

 

14.4.2 Operators let and let*

Syntax:


  (let ({
sym | (sym init-form)}*) body-form*)
  (let* ({
sym | (sym init-form)}*) body-form*)

Description:

The. let and let* operators introduce a new scope with variables and evaluate forms in that scope. The operator symbol, either let or let*, is followed by a list which can contain any mixture of variable name symbols, or (sym init-form) pairs. A symbol denotes the name of variable to be instantiated and initialized to the value nil. A symbol specified with an init-form denotes a variable which is initialized from the value of the init-form.

The symbols t and nil may not be used as variables, and neither can be keyword symbols: symbols denoted by a leading colon.

The difference between let and let* is that in let*, later init-forms have visibility over the variables established by earlier variables in the same let* construct. In plain let, the variables are not visible to any of the init-forms.

When the variables are established, then the body-forms are evaluated in order. The value of the last body-form becomes the return value of the let.

If there are no body-forms, then the return value nil is produced.

The list of variables may be empty.

Examples:


  (let ((a 1) (b 2)) (list a b)) -> (1 2)
  (let* ((a 1) (b (+ a 1))) (list a b (+ a b))) -> (1 2 3)
  (let ()) -> nil
  (let (:a nil)) -> error, :a and nil can't be used as variables

 

14.4.3 Operators for and for*

Syntax:


  ({
for | for } ({sym | (sym init-form)}*)
               
([test-form result-form*])
                (
inc-form*)
    
body-form*)

Description:

The for and for* operators combine variable binding with loop iteration. The first argument is a list of variables with optional initializers, exactly the same as in the let and let* operators. Furthermore, the difference between for and for* is like that between let and let* with regard to this list of variables.

The for and for* operators execute these steps:

1.
Establish bindings for the specified variables similarly to let and let*. The variable bindings are visible over the test-form, each result-form, each inc-form and each body-form.
2.
Establish an anonymous block over the remaining forms, allowing the return operator to be used to terminate the loop.
3.
Evaluate test-form. If test-form yields nil, then the loop terminates. Each result-form is evaluated, and the value of the last of these forms is is the result value of the loop. If there are no result-forms then the result value is nil. If the test-form is omitted, then the test is taken to be true, and the loop does not terminate.
4.
Otherwise, if test-form yields true, then each body-form is evaluated in turn. Then, each inc-form is evaluated in turn and processing resumes at step 2.

Furthermore, the for and for* operators establish an anonymous block, allowing the return operator to be used to terminate at any point.

 

14.4.4 Operators each, each*, collect-each, collect-each*, append-each and append-each*

Syntax:


  (each ({(
sym init-form)}*) body-form*)
  (each* ({(
sym init-form)}*) body-form*)
  (collect-each ({(
sym init-form)}*) body-form*)
  (collect-each* ({(
sym init-form)}*) body-form*)
  (append-each ({(
sym init-form)}*) body-form*)
  (append-each* ({(
sym init-form)}*) body-form*)

Description:

These operators establish a loop for iterating over the elements of one or more lists. Each init-form must evaluate to a list. The lists are then iterated in parallel over repeated evaluations of the body-forms, with each sym variable being assigned to successive elements of its list. The shortest list determines the number of iterations, so if any of the init-forms evaluate to an empty list, the body is not executed.

The body forms are enclosed in an anonymous block, allowing the return operator to terminate the loop prematurely and optionally specify the return value.

The collect-each and collect-each* variants are like each and each*, except that for each iteration, the resulting value of the body is collected into a list. When the iteration terminates, the return value of the collect-each or collect-each* operator is this collection.

The append-each and append-each* variants are like each and each*, except that for each iteration other than the last, the resulting value of the body must be a list. The last iteration may produce either an atom or a list. The objects produced by the iterations are combined together as if they were arguments to the append function, and the resulting value is the value of the append-each or append-each* operator.

The alternate forms denoted by the adorned symbols each*, collect-each* and append-each*, differ from each, collect-each and append-each* in the following way. The plain forms evaluate the init-forms in an environment in which none of the sym variables are yet visible. By contrast, the alternate forms evaluate each init-form in an environment in which bindings for the previous sym variables are visible. In this phase of evaluation, sym variables are list-valued: one by one they are each bound to the list object emanating from their corresponding init-form. Just before the first loop iteration, however, the sym variables are assigned the first item from each of their lists.

Example:


 ;; print numbers from 1 to 10 and whether they are even or odd
 (each* ((n (range 1 10)) ;; n list a list here!
         (even (collect-each ((n m)) (evenp m))))
   ;; n is an item here!
   (format t "~s is ~s\n" n (if even "even" "odd")))

Output:


 1 is odd
 2 is even
 3 is odd
 4 is even
 5 is odd
 6 is even
 7 is odd
 8 is even
 9 is odd
 10 is even

 

14.5 Function Objects and Named Functions

 

14.5.1 Operator defun

Syntax:


  (defun
name (param* [: opt-param*] [. rest-param])
    
body-form)

Description:

The defun operator introduces a new function in the global function namespace. The function is similar to a lambda, and has the same parameter syntax and semantics as the lambda operator.

Unlike in lambda, the body-forms of a defun are surrounded by a block. The name of this block is the same as the name of the function, making it possible to terminate the function and return a value using (return-from name value). For more information, see the definition of the block operator.

A function may call itself by name, allowing for recursion.

 

14.5.2 Operator lambda

Syntax:


  (lambda (
param* [: opt-param*] [. rest-param])
    
body-form)
  (lambda
rest-param
    
body-form)

Description:

The lambda operator produces a value which is a function. Like in most other Lisps, functions are objects in TXR Lisp. They can be passed to functions as arguments, returned from functions, aggregated into lists, stored in variables, et cetera.

The first argument of lambda is the list of parameters for the function. It may be empty, and it may also be an improper list (dot notation) where the terminating atom is a symbol other than nil. It can also be a single symbol.

The second and subsequent arguments are the forms making up the function body. The body may be empty.

When a function is called, the parameters are instantiated as variables that are visible to the body forms. The variables are initialized from the values of the argument expressions appearing in the function call.

The dotted notation can be used to write a function that accepts a variable number of arguments. There are two ways write a function that accepts only a variable argument list and no required arguments:


  (lambda (.
rest-param) ...)
  (lambda
rest-param ...)

(These notations are syntactically equivalent because the list notation (. X) actually denotes the object X which isn't wrapped in any list).

The keyword symbol : (colon) can appear in the parameter list. This is the symbol in the keyword package whose name is the empty string. This symbol is treated specially: it serves as a separator between required parameters and optional parameters. Furthermore, the : symbol has a role to play in function calls: it can be specified as an argument value to an optional parameter by which the caller indicates that the optional argument is not being specified. It will be processed exactly that way.

An optional parameter can also be written in the form (name expr [sym]). In this situation, if the call does not specify a value for the parameter (or specifies a value as the keyword : (colon)) then the parameter takes on the value of the expression expr. If sym is specified, then sym will be introduced as an additional binding with a boolean value which indicates whether or not the optional parameter had been specified by the caller.

The initializer expressions are evaluated an environment in which all of the previous parameters are visible, in addition to the surrounding environment of the lambda. For instance:


  (let ((default 0))
    (lambda (str : (end (length str)) (counter default))
      (list str end counter)))

In this lambda, the initializing expression for the optional parameter end is (length str), and the str variable it refers to is the previous argument. The initializer for the optional variable counter is the expression default, and it refers to the binding established by the surrounding let. This reference is captured as part of the lambda's lexical closure.

Examples:

Counting function:
This function, which takes no arguments, captures the variable counter. Whenever this object is called, it increments counter by 1 and returns the incremented value.


  (let ((counter 0))
    (lambda () (inc counter)))

Function that takes two or more arguments:
The third and subsequent arguments are aggregated into a list passed as the single parameter z:


  (lambda (x y . z) (list 'my-arguments-are x y z))

Variadic function:


  (lambda args (list 'my-list-of-arguments args))

Optional arguments:


  [(lambda (x : y) (list x y)) 1] -> (1 nil)
  [(lambda (x : y) (list x y)) 1 2] -> (1 2)

 

14.5.3 Function call

Syntax:


  (call
function argument*)

Description:

The call function invokes function, passing it the given arguments, if any.

Examples:

Apply arguments 1 2 to a lambda which adds them to produce 3:


  (call (lambda (a b) (+ a b)) 1 2)

Useless use of call on a named function; equivalent to (list 1 2):


  (call (fun list) 1 2)

 

14.5.4 Operator fun

Syntax:


  (fun
function-name)

Description:

The fun operator retrieves the function object corresponding to a named function in the current lexical environment.

The function-name is a symbol denoting a named function: a built in function, or one defined by defun.

Note: the fun operator does not see macro bindings. It is possible to retrieve a global macro expander using symbol-function.

Dialect Note:

A lambda expression is not a function name in TXR Lisp. The syntax (fun (lambda ...)) is invalid.

 

14.5.5 Functions symbol-function and symbol-value

Syntax:


  (symbol-function
symbol)
  (symbol-value
symbol)

Description:

The symbol-function function retrieves the value of the global function binding of the given symbol if it has one: that is, the function object bound to the symbol. If symbol has no global function binding, then the value of the global macro binding is returned. If that doesn't exist, then the value of a global special operator binding is returned, and if that doesn't exist, then nil is returned.

The symbol-value function retrieves the value of a global variable, if it exists, otherwise nil.

Note: a function binding is a function, but a macro or special operator binding binding isn't. The value of a macro binding is a list of the following form:


  (#<environment object>
macro-parameter-list body-form*)

The value of a special operator binding is a "C pointer" object, whose printed representation looks like:


  #<cptr: 808be4f>

These details may change in future version of TXR.

Dialect note:

Forms which call symbol-function or symbol-value are currently not assignable places. Only the defun operator defines functions.

 

14.5.6 Functions boundp and fboundp

Syntax:


  (boundp
symbol)
  (fboundp
symbol)

Description:

boundp returns t if the symbol has a variable binding in the global environment, otherwise nil.

fboundp returns t if the symbol has a function or macro binding in the global environment, or if it is an operator, otherwise nil.

 

14.5.7 Function func-get-form

Syntax:


  (func-get-form
func)

Description:

The func-get-form function retrieves a source code form of func, which must be an interpreted function. The source code form has the syntax (name arglist body-form*) .

 

14.5.8 Function func-get-env

Syntax:


  (func-get-env
func)

Description:

The func-get-env function retrieves the environment object associated with function func. The environment object holds the captured bindings of a lexical closure.

 

14.5.9 Function functionp

Syntax:


  (functionp
obj)

Description:

The functionp function returns t if obj is a function, otherwise it returns nil.

 

14.5.10 Function interp-fun-p

Syntax:


  (interp-fun-p
obj)

Description:

The interp-fun-p function returns t if obj is an interpreted function, otherwise it returns nil.

 

14.5.11 Macros flet and labels

Syntax:


  (flet ({(
name param-list function-body-form*)}*)
    
body-form*)


  (labels ({(
name param-list function-body-form*)}*)
    
body-form*)

Description:

The flet and labels macros bind local, named functions in the lexical scope. The difference between flet and labels is that a function defined by labels can see itself, and therefore recurse directly by name. Moreover, if multiple functions are defined by the same labels construct, they all see each other. By contrast, a flet-defined function does not have itself in scope and cannot recurse. Multiple functions in the same flet do not have each other's names in their scopes.

More formally, the function-body-form-s and param-list of the functions defined by labels are in a scope in which all of the function names being defined by that same labels construct are visible.

Under both labels and flet, the local functions that are defined are lexically visible to the main xbody-form.

Note that labels and flet are properly scoped with regard to macros. During macro expansion, they shadow, macros defined by macrolet and defmacro.

See also: the macrolet operator.

Examples:


  ;; Wastefully slow algorithm for determining even-ness.
  ;; Note:
  ;; - mutual recursion between labels-defined functions
  ;; - inner is-even bound by labels shadows the outer
  ;;   one bound by defun so the (is-even n) call goes
  ;;   to the local function.


  (defun is-even (n)
   (labels ((is-even (n)
              (if (zerop n) t (is-odd (- n 1))))
            (is-odd (n)
              (if (zerop n) nil (is-even (- n 1)))))
     (is-even n)))

 

14.6 Object Type And Equivalence

 

14.6.1 Function typeof

Syntax:


  (typeof
value)

Description:

The typeof function returns a symbol representing the type of value.

The core types are identified by the following symbols:

cons
Cons cell.

str
String.

lit
Literal string embedded in the TXR executable image.

chr
Character.

fixnum
Fixnum integer: an integer that fits into the value word, not having to be heap allocated.

sym
Symbol.

pkg
Symbol package.

fun
Function.

vec
Vector.

lcons
Lazy cons.

lstr
Lazy string.

env
Function/variable binding environment.

bignum
A bignum integer: arbitrary precision integer that is heap-allocated.

There are additional kinds of objects, such as streams.

 

14.6.2 Functions null, not, and false

Syntax:


  (null
value)
  (not
value)
  (false
value)

Description:

The null, not and false functions are synonyms. They tests whether value is the object nil. They return t if this is the case, nil otherwise.

Examples:


  (null '()) -> t
  (null nil) -> t
  (null ()) -> t
  (false t) -> nil


  (if (null x) (format t "x is nil!"))


  (let ((list '(b c d)))
    (if (not (memq 'a list))
      (format t "list ~s does not contain the symbol a\n")))

 

14.6.3 Function true

Syntax:


  (true
value)

Description:

The true function is the complement of the null, not and false functions.

It return t if the value is any object other than nil. If value is nil, it returns nil.

Note: programs should avoid explicitly testing values with true. For instance (if x ...) should be favored over (if (true x) ...).

Example:


   ;; Compute indices where the list '(1 nil 2 nil 3)
   ;; has true values:
   [where '(1 nil 2 nil 3) true] -> (1 3)

 

14.6.4 Functions eq, eql and equal

Syntax:


  (eq
left-obj right-obj)
  (eql
left-obj right-obj)
  (equal
left-obj right-obj)

Description:

The principal equality test functions eq, eql and equal test whether two objects are equivalent, using different criteria. They return t if the objects are equivalent, and nil otherwise.

The eq function uses the strictest equivalence test, called implementation equality. The eq function returns t if, and only if, left-obj and right-obj are actually the same object. The eq test is is implemented by comparing the raw bit pattern of the value, whether or not it is an immediate value or a pointer to a heaped object. Two character values are eq if they are the same character, and two fixnum integers are eq if they have the same value. All other object representations are actually pointers, and are eq if, and only, if they point to the same object in memory. So, for instance, two bignum integers might not be eq even if they have the same numeric value, two lists might not be eq even if all their corresponding elements are eq and two strings might not be eq even if they hold identical text.

The. eql function is slightly less strict than eq. The difference between eql and eq is that if left-obj and right-obj are numbers which are of the same kind and have the same numeric value, eql returns t, even if they are different objects. Note that an integers and a floating-point number are not eql even if one has a value which converts to the other: thus, (eql 0.0 0) yields nil; the comparison operation which finds these numbers equal is the (= 0.0 0). For all other object types, eql behaves like eq.

The equal function is less strict still than eql. In general, it recurses into some kinds of aggregate objects to perform a structural equivalence check.

Firstly, if left-obj and right-obj are eql then they are also equal, though of course the converse isn't necessarily the case.

If two objects are both cons cells, then they are equal if their car fields are equal and their cdr fields are equal.

If two objects are vectors, they are equal if they have the same length, and their corresponding elements are equal.

If two objects are strings, they are equal if they are textually identical.

If two objects are functions, they are equal if they have equal environments, and if they have the same code. Two compiled functions are considered to have the same code if and only if they are pointers to the same function. Two interpreted functions are considered to have the same code if their list structure is equal.

Two hashes are equal if they use the same equality (both are :equal-based, or both are the default :eql-based), if their associated user data elements are equal (see the function get-hash-userdata), if their sets of keys are identical, and if the data items associated with corresponding keys from each respective hash are equal objects.

For some aggregate objects, there is no special semantics. Two arguments which are symbols, packages, or streams are equal if and only if they are the same object.

Certain object types have a custom equal function.

 

14.7 Basic List Library

 

14.7.1 Function cons

Syntax:


  (cons
car-value cdr-value)

Description:

The cons function allocates, initializes and returns a single cons cell. A cons cell has two fields called car and cdr, which are accessed by functions of the same name, or by the functions first and rest, which are synonyms for these.

Lists are made up of conses. A (proper) list is either the symbol nil denoting an empty list, or a cons cell which holds the first item of the list in its car, and the list of the remaining items in cdr. The expression (cons 1 nil) allocates and returns a single cons cell which denotes the one-element list (1). The cdr is nil, so there are no additional items.

A cons cell whose cdr is an atom other than nil is printed with the dotted pair notation. For example the cell produced by (cons 1 2) is denoted (1 . 2). The notation (1 . nil) is perfectly valid as input, but the cell which it denotes will print back as (1). The notations are equivalent.

The dotted pair notation can be used regardless of what type of object is the cons cell's cdr. so that for instance (a . (b c)) denotes the cons cell whose car is the symbol a a and whose cdr is the list (b c). This is eactly the same thing as (a b c). In other words (a b ... l m . (n o ... w . (x y z))) is exactly the same as (a b ... l m n o ... w x yz).

Every list, and more generally cons cell tree structure, can be written in a "fully dotted" notation, such that there are as many dots as there are cells. For instance the cons structure of the nested list (1 (2) (3 4 (5))) can be made more explicit using (1 . ((2 . nil) . ((3 . (4 . ((5 . nil) . nil))) . nil)))). The structure contains eight conses, and so there are eight dots in the fully dotted notation.

The number of conses in a linear list like (1 2 3) is simply the number of items, so that list in particular is made of three conses. Additional nestings require additional conses, so for instance (1 2 (3)) requires four conses. A visual way to count the conses from the printed representation is to count the atoms, then add the count of open parentheses, and finally subtract one.

A list terminated by an atom other than nil is called an improper list, and the dot notation is extended to cover improper lists. For instance (1 2 . 3) is an improper list of two elements, terminated by 3, and can be constructed using (cons 1 (cons 2 3)). The fully dotted notation for this list is (1 . (2 . 3)).

 

14.7.2 Function atom

Syntax:


  (atom
value)

Description:

The atom function tests whether value is an atom. It returns t if this is the case, nil otherwise. All values which are not cons cells are atoms.

(atom x) is equivalent to (not (consp x)).

Examples:


  (atom 3) -> t
  (atom (cons 1 2)) -> nil
  (atom "abc") -> t
  (atom '(3)) -> nil

 

14.7.3 Function consp

Syntax:


  (consp
value)

Description:

The consp function tests whether value is a cons. It returns t if this is the case, nil otherwise.

(consp x) is equivalent to (not (atom x)).

Non-empty lists test positive under consp because a list is represented as a reference to the first cons in a chain of one or more conses.

Note that a lazy cons is a cons and satisfies the consp test. See the function make-lazy-cons.

Examples:


  (consp 3) -> nil
  (consp (cons 1 2)) -> t
  (consp "abc") -> nil
  (consp '(3)) -> t

 

14.7.4 Functions car and first

Syntax:


  (car
object)
  (first
object)

Description:

The functions car and first are synonyms.

If obejct is a cons cell, these functions retrieve the car field of that cons cell. (car (cons 1 2)) yields 1.

For programming convenience, object may be of several other kinds in addition to conses.

(car nil) is allowed, and returns nil.

object may also be a vector or a string. If it is an empty vector or string, then nil is returned. Otherwise the first character of the string or first element of the vector is returned.

 

14.7.5 Functions cdr and rest

Syntax:


  (cdr
object)
  (rest
object)

Description:

The functions cdr and rest are synonyms.

If obejct is a cons cell, these functions retrieve the cdr field of that cons cell. (cdr (cons 1 2)) yields 2.

For programming convenience, object may be of several other kinds in addition to conses.

(cdr nil) is allowed, and returns nil.

object may also be a vector or a string. If it is a non-empty string or vector containing at least two items, then the remaining part of the object is returned, with the first element removed. For example (cdr abc) yields bc. If object is is a one-element vector or string, or an empty vector or string, then nil is returned. Thus (cdr a) and (cdr ) both result in nil.

Example:

Walk every element of the list (1 2 3) using a for loop:


    (for ((i '(1 2 3))) (i) ((set i (cdr i)))
      (print (car i) *stdout*)
      (print #\newline *stdout*))

The variable i marches over the cons cells which make up the "backbone" of the list. The elements are retrieved using the car function. Advancing to the next cell is achieved using (cdr i). If i is the last cell in a (proper) list, (cdr i) yields nil and so i becomes nil, the loop guard expression i fails and the loop terminates.

 

14.7.6 Functions rplaca and rplacd

Syntax:


  (rplaca
cons new-car-value)
  (rplacd
cons new-cdr-value)

Description:

The rplaca and rplacd functions assign new values into the car and cdr fields of the cell cons. Note that (rplaca x y) is the same as the more generic (set (car x) y), and likewise (rplacd x y) can be written as (set (cdr x) y).

It is an error if cons is not a cons or lazy cons. In particular, whereas (car nil) is correct, (rplaca nil ...) is erroneous.

The rplaca and rplacd functions return cons.

Note: TXR versions 89 and earlier, these functions returned the new value. The behavior was undocumented.

 

14.7.7 Functions second, third, fourth, fifth and sixth

Syntax:


  (first
list)
  (second
list)
  (third
list)
  (fourth
list)
  (fifth
list)
  (sixth
list)

Description:

These functions access the elements of a proper list by position.

If the list is shorter than implied, these functions return nil.

Examples:


  (third '(1 2)) -> nil
  (second '(1 2)) -> 2
  (third '(1 2 . 3)) -> **error**

 

14.7.8 Functions append, nconc and append*

Syntax:


  (append [
list* last-arg])
  (nconc [
list* last-arg])
  (append* [
list* last-arg])

Description:

The append function creates a new list which is a catenation of the list arguments. All arguments are optional; (append) produces the empty list.

If a single argument is specified, then append simply returns the value of that argument. It may be any kind of object.

If N arguments are specified, where N > 1, then the first N-1 arguments must be proper lists. Copies of these lists are catenated together. The last argument N, shown in the above syntax as last-arg, may be any kind of object. It is installed into the cdr field of the last cons cell of the resulting list. Thus, if argument N is also a list, it is catenated onto the resulting list, but without being copied. Argument N may be an atom other than nil; in that case append produces an improper list.

The nconc function works like append, but avoids consing. It destructively manipulates (that is to say, mutates) incoming lists to catenate them, and so must be used with care.

The append* function works like append, but returns a lazy list which produces the catenation of the lists on demand. If some of the arguments are themselves lazy lists which are infinite, then append* can return immediately, whereas append will get caught in an infinite loop trying to produce a catenation and eventually exhaust available memory. (However, the last argument to append may be an infinite lazy list, because append does not traverse the last argument.)

Examples:


  ;; An atom is returned.
  (append 3) -> 3


  ;; A list is also just returned: no copying takes place.
  ;; The eq function can verify that the same object emerges
  ;; from append that went in.
  (let ((list '(1 2 3)))
    (eq (append list) list)) -> t


  (append '(1 2 3) '(4 5 6) 7) -> '(1 2 3 4 5 6 . 7))


  ;; the (4 5 6) tail of the resulting list is the original
  ;; (4 5 6) object, shared with that list.


  (append '(1 2 3) '(4 5 6)) -> '(1 2 3 4 5 6)


  (append nil) -> nil


  ;; (1 2 3) is copied: it is not the last argument
  (append '(1 2 3) nil) -> (1 2 3)


  ;; empty lists disappear
  (append nil '(1 2 3) nil '(4 5 6)) -> (1 2 3 4 5 6)
  (append nil nil nil) -> nil


  ;; atoms and improper lists other than in the last position
  ;; are erroneous
  (append '(a . b) 3 '(1 2 3)) -> **error**

 

14.7.9 Function list

Syntax:


  (list
value*)

Description:

The list function creates a new list, whose elements are the argument values.

Examples:


  (list) -> nil
  (list 1) -> (1)
  (list 'a 'b) -> (a b)

 

14.7.10 Function list*

Syntax:


  (
list* value*)

Description:

The list* function is a generalization of cons. If called with exactly two arguments, it behaves exactly like cons: (list* x y) is identical to (cons x y). If three or more arguments are specified, the leading arguments specify additional atoms to be consed to the front of the list. So for instance (list* 1 2 3) is the same as (cons 1 (cons 2 3)) and produces the improper list (1 2 . 3). Generalizing in the other direction, list* can be called with just one argument, in which case it returns that argument, and can also be called with no arguments in which case it returns nil.

Examples:


  (list*) -> nil
  (list* 1) -> 1
  (list* 'a 'b) -> (a . b)
  (list* 'a 'b 'c) -> (a b . c)

Dialect Note:

Note that unlike in some other Lisp dialects, the effect of (list* 1 2 x) can also be obtained using (list 1 2 . x). However, (list* 1 2 (func 3)) cannot be rewritten as (list 1 2 . (func 3)) because the latter is equivalent to (list 1 2 func 3).

 

14.7.11 Function sub-list

Syntax:


  (sub-list
list [from [to]])

Description:

This function is like the sub function, except that it operates strictly on lists.

For a description of the arguments and semantics, refer to the sub function.

 

14.7.12 Function replace-list

Syntax:


  (replace-list
list item-sequence [from [to]])

Description:

The replace-list function is like the replace function, except that the first argument must be a list.

For a description of the arguments and semantics, refer to the replace function.

 

14.7.13 Functions listp and proper-listp

Syntax:


  (listp
value)
  (proper-listp
value)

Description:

The listp and proper-listp functions test, respectively, whether value is a list, or a proper list, and return t or nil accordingly.

The listp test is weaker, and executes without having to traverse the object. (listp x) is equivalent to (or (null x) (consp x)). The empty list nil is a list, and a cons cell is a list.

The proper-listp function returns t only for proper lists. A proper list is either nil, or a cons whose cdr is a proper list. proper-listp traverses the list, and its execution will not terminate if the list is circular.

 

14.7.14 Function length-list

Syntax:


  (length-list
list)

Description:

The length-list function returns the length of list, which may be a proper or improper list. The length of a list is the number of conses in that list.

 

14.7.15 Function copy-list

Syntax:


  (copy-list
list)

Description:

The copy-list function which returns a list similar to list, but with a newly allocated cons cell structure.

If list is an atom, it is simply returned.

Otherwise, list is a cons cell, and copy-list returns the same object as the expression (cons (car list) (copy-list (cdr list))).

Note that the object (car list) is not deeply copied, but only propagated by reference into the new list. copy-list produces a new list structure out of the same items that are in list.

Dialect Note:

Common Lisp does not allow the argument to be an atom, except for the empty list nil.

 

14.7.16 Function copy-cons

Syntax:


  (copy-cons
cons)

Description:

This function creates a fresh cons cell, whose car and cdr fields are copied from cons.

 

14.7.17 Functions reverse and nreverse

Syntax:


  (reverse
list)
  (nreverse
list)

Description:

Description:

The functions reverse and nreverse produce an object which contains the same items as proper list list, but in reverse order. If list is nil, then both functions return nil.

The reverse function is non-destructive: it creates a new list.

The nreverse function creates the structure of the reversed list out of the cons cells of the input list, thereby destructively altering it (if it contains more than one element). How nreverse uses the material from the original list is unspecified. It may rearrange the cons cells into a reverse order, or it may keep the structure intact, but transfer the car values among cons cells into reverse order. Other approaches are possible.

 

14.7.18 Function ldiff

Syntax:


  (ldiff
list sublist)

Description:

The values list and sublist are proper lists.

The ldiff function determines whether sublist is a structural suffix of list (meaning that it actually is a suffix, and is not merely equal to one).

This is true if list and sublist are the same object, or else, recursively, if sublist is a suffix of (cdr list).

The object nil is the sublist of every list, including itself.

The ldiff function returns a new list consisting of the elements of the prefix of list which come before the sublist suffix. The elements are in the same order as in list. If sublist is not a suffix of list, then a copy of list is returned.

These functions also work more generally on sequences. The list and sublist arguments may be strings or vectors. In this case, the suffixing matching behavior is relaxed to one of structural equivalence. See the relevant examples below.

Examples:


  ;;; unspecified: the compiler could make
  ;;; '(2 3) a suffix of '(1 2 3),
  ;;; or they could be separate objects.
  (ldiff '(1 2 3) '(2 3)) -> either (1) or (1 2 3)


  ;; b is the (1 2) suffix of a, so the ldiff is (1)
  (let ((a '(1 2 3)) (b (cdr a)))
    (ldiff a b))
  -> (1)


  ;; string and vector behavior
  (ldiff "abc" "bc") -> "a"


  (ldiff "abc" nil) -> "abc"


  (ldiff #(1 2 3) #(3)) -> #(1 2)


  ;; mixtures do not have above behavior
  (ldiff #(1 2 3) '(3)) -> #(1 2 3)


  (ldiff '(1 2 3) #(3)) -> #(1 2 3)


  (ldiff "abc" #(#\b #\c)) -> "abc"

 

14.7.19 Function last

Syntax:


  (last
list)

Description:

If list is a nonempty proper or improper list, the last function returns the last cons cell in the list: that cons cell whose cdr field is a terminating atom.

If list is nil, then nil is returned.

 

14.7.20 Functions flatten and flatten*

Syntax:


  (flatten
list)
  (flatten*
list)

Description:

The flatten function produces a list whose elements are all of the non-nil atoms contained in the structure of list. The flatten* function works like flatten except that flatten creates and returns a complete flattened list, whereas flatten* produces a lazy list which is instantiated on demand. This is particularly useful when the input structure is itself lazy.

Examples:


  (flatten '(1 2 () (3 4))) -> (1 2 3 4)


  ;; equivalent to previous, since
  ;; nil is the same thing as ()
  (flatten '(1 2 nil (3 4))) -> (1 2 3 4)


  (flatten nil) -> nil


  (flatten '(((()) ()))) -> nil

 

14.7.21 Functions memq, memql and memqual

Syntax:


  (memq
object list)
  (memql
object list)
  (memqual
object list)

Description:

The memq, memql and memqual functions search list for a member which is, respectively, eq, eql or to object. (See the eq, eql and equal functions above.)

If no such element found, nil is returned.

Otherwise, that suffix of list is returned whose first element is the matching object.

 

14.7.22 Functions member and member-if

Syntax:


  (member
key sequence [testfun [keyfun]])
  (member-if
predfun sequence [keyfun])

Description:

The member and member-if functions search through sequence for an item which matches a key, or satisfies a predicate function, respectively.

The keyfun argument specifies a function which is applied to the elements of the sequence to produce the comparison key. If this argument is omitted, then the untransformed elements of the sequence themselves are examined.

The member function's testfun argument specifies the test function which is used to compare the comparison keys taken from the sequence to the search key. If this argument is omitted, then the equal function is used. If member does not find a matching element, it returns nil. Otherwise it returns the suffix of sequence which begins with the matching element.

The member-if function's predfun argument specifies a predicate function which is applied to the successive comparison keys pulled from the sequence by applying the key function to successive elements. If no match is found, then nil is returned, otherwise what is returned is the suffix of sequence which begins with the matching element.

 

14.7.23 Functions remq, remql and remqual

Syntax:


  (remq
object list)
  (remql
object list)
  (remqual
object list)

Description:

The remq, remql and remqual functions produce a new list based on list, removing the items which are eq, eql or equal to object.

The input list is unmodified, but the returned list may share substructure with it. If no items are removed, it is possible that the return value is list itself.

 

14.7.24 Functions remq, remql* and remqual*

Syntax:


  (remq*
object list)
  (remql*
object list)
  (remqual*
object list)

Description:

The remq*, remql* and remqual* functions are lazy versions of remq, remql and remqual. Rather than computing the entire new list prior to returning, these functions return a lazy list.

Caution: these functions can still get into infinite looping behavior. For instance, in (remql* 0 (repeat '(0))), remql will keep consuming the 0 values coming out of the infinite list, looking for the first item that does not have to be deleted, in order to instantiate the first lazy value.

Examples:


  ;; Return a list of all the natural numbers, excluding 13,
  ;; then take the first 100 of these.
  ;; If remql is used, it will loop until memory is exhausted,
  ;; because (range 1) is an infinite list.


  [(remql* 13 (range 1)) 0..100]

 

14.7.25 Functions countqual, countql and countq

Syntax:


  (countq
object list)
  (countql
object list)
  (countqual
object list)

Description:

The countq, countql and countqual functions count the number of objects in list which are eq, eql or equal to object, and return the count.

 

14.8 Applicative List Processing

 

14.8.1 Functions remove-if, keep-if, remove-if* and keep-if*

Syntax:


  (remove-if
predicate-function list [key-function])
  (keep-if
predicate-function list [key-function])
  (remove-if*
predicate-function list [key-function])
  (keep-if*
predicate-function list [key-function])

Description:

The remove-if function produces a list whose contents are those of list but with those elements removed which satisfy predicate-function. Those elements which are not removed appear in the same order. The result list may share substructure with the input list, and may even be the same list object if no items are removed.

The optional key-function specifies how each element from the list is transformed to an argument to predicate-function. If this argument is omitted then the predicate function is applied to the elements directly, a behavior which is identical to key-function being (fun identity).

The keep-if function is exactly like remove-if, except the sense of the predicate is inverted. The function keep-if retains those items which remove-if will delete, and removes those that remove-if will preserve.

The remove-if* and keep-if* functions are like remove-if and keep-if, but produce lazy lists.

Examples:


  ;; remove any element numerically equal to 3.
  (remove-if (op = 3) '(1 2 3 4 3.0 5)) -> (1 2 4 5)


  ;; remove those pairs whose first element begins with "abc"
  [remove-if (op equal [@1 0..3] "abc")
             '(("abcd" 4) ("defg" 5))
             car]
  -> (("defg" 5))


  ;; equivalent, without test function
  (remove-if (op equal [(car @1) 0..3] "abc")
             '(("abcd" 4) ("defg" 5)))
  -> (("defg" 5))

 

14.8.2 Function count-if

Syntax:


  (count-if
predicate-function list [key-function])

Description:

The count-if function counts the number of elements of list which satisfy predicate-function and returns the count.

The optional key-function specifies how each element from the list is transformed to an argument to predicate-function. If this argument is omitted then the predicate function is applied to the elements directly, a behavior which is identical to key-function being (fun identity).

 

14.8.3 Functions posqual, posql and posq

Syntax:


  (posq
object list)
  (posql
object list)
  (posqual
object list)

Description:

The posq, posql and posqual functions return the zero-based position of the first item in list which is, respectively, eq, eql or equal to object.

 

14.8.4 Functions posqual, posql and posq

Syntax:


  (posq
object list)
  (posql
object list)
  (posqual
object list)

Description:

The posq, posql and posqual functions return the zero-based position of the first item in list which is, respectively, eq, eql or equal to object.

 

14.8.5 Functions pos and pos-if

Syntax:


  (pos
key list [testfun [keyfun]])
  (pos-if
predfun list [keyfun])

Description:

The pos and pos-if functions search through list for an item which matches key, or satisfies predicate function predfun, respectively. They return the zero-based position of the matching item.

The keyfun argument specifies a function which is applied to the elements of list to produce the comparison key. If this argument is omitted, then the untransformed elements of list are examined.

The pos function's testfun argument specifies the test function which is used to compare the comparison keys from list to key. If this argument is omitted, then the equal function is used. The position of the first element list whose comparison key (as retrieved by keyfun) matches the search (under testfun) is returned. If no such element is found, nil is returned.

The pos-if function's predfun argument specifies a predicate function which is applied to the successive comparison keys taken from list by applying keyfun to successive elements. The position of the first element for which predfun yields true is returned. If no such element is found, nil is returned.

 

14.8.6 Functions pos-max and pos-min

Syntax:


  (pos-max > sequence [
testfun [keyfun]])
  (pos-min > sequence [
testfun [keyfun]])

Description:

The pos-min and pos-max functions implement exactly the same algorithm; they differ only in their defaulting behavior with regard to the testfun argument. If testfun is not given, then the pos-max function defaults testfun to the > ("greater than") function, whereas pos-min defaults it to the < ("less than") function.

If sequence is empty, both functions return nil.

Without a testfun argument, the pos-max function finds the zero-based position index of the numerically maximum value occurring in sequence, whereas pos-min without a testfun argument finds the index of the minimum value.

If a testfun argument is given, the two functions are equivalent. The testfun function must be callable with two arguments. If testfun behaves like a greater-than comparison, then pos-max and pos-min return the index of the maximum element. If testfun behaves like a less-than comparison, then the functions return the index of the minimum element.

The keyfun argument defaults to the identity function. Each element from sequence is passed through this one-argument function, and the resulting value is used in its place.

 

14.8.7 Function where

Syntax:


  (where
object function)

Description:

If object is a sequence, the where function returns a list of the numeric indices of those of its elements which satisfy function. The numeric indices appear in increasing order.

If object is a hash, the where function returns an unordered list of keys which have values which satisfy function.

function must be a function that can be called with one argument. For each element of object, function is called with that element as an argument. If a non-nil value is returned, then the zero-based index of that element is added to a list. Finally, the list is returned.

 

14.8.8 Function select

Syntax:


  (select
object index-list)

Description:

The select function returns an object, of the same kind as object, which consists of those elements of object which are identified by the indices in index-list.

If object is a sequence, then index-list consists of numeric indices. The select function stops processing object upon encountering an index inside index-list which is out of range. (Rationale: without this strict behavior, select would not be able to terminate if index-list is infinite.)

If object is a list, then index-list must contain monotonically increasing numeric values, even if no value is out of range, since the select function makes a single pass through the list based on the assumption that indices are ordered. (Rationale: optimization.)

If object is a hash, then index-list is a list of keys. A new hash is returned which contains those elements of object whose keys appear in index-list. All of index-list is processed, even if it contains keys which are not in object.

 

14.8.9 Funtion partition

Syntax:


  (partition
sequence {index-list |index |function})

Description:

If sequence is empty, then partition returns an empty list, and the second argument is ignored; if it is function, it is not called.

Otherwise, partition returns a lazy list of partitions of the sequence

sequence. Partitions are consecutive, non-empty sub-strings of sequence, of the same kind as sequence.

If the second argument is of the form index-list, it is a sequence of strictly increasing, positive integers. First, any leading zeros in this sequence are dropped. The partition function then divides sequence according to the indices in index list. The first partition begins with the first element of sequence. The second partition begins at the first position in index-list, and so on. Indices beyond the length of the sequence are ignored.

If index-list is empty then a one-element list containing the entire sequence is returned.

If the second argument is a function, then this function is applied to sequence, and the return value of this call is then used in place of the second argument, which must be an index or index-list.

If the second argument is an atom other than a function, it is assumed to be an integer index, and is turned into an index-list of one element.

Examples:


  (partition '(1 2 3) 1) -> ((1) (2 3))


  ;; split the string where there is a "b"
  (partition "abcbcbd" (op where @1 (op eql #\b))) -> ("a" "bc"
                                                       "bc" "bd")

 

14.8.10 Function partition*

Syntax:


  (partition*
sequence {index-list |index |function})

Description:

If sequence is empty, then partition* returns an empty list, and the second argument is ignored; if it is function, it is not called.

If the second argument is of the form index-list, which is a sequence of strictly increasing non-negative integers, then partition* produces lazy list of pieces taken from sequence. The pieces are formed by deleting from sequence the elements at the positions given in index-list. The pieces are the non-empty sub-strings between the deleted elements.

If index-list is empty then a one-element list containing the entire

sequence is returned.

If the second argument is a function, then this function is applied to sequence, and the return value of this call is then used in place of the second argument, which must be an index or index-list.

If the second argument is an atom other than a function, it is assumed to be an integer index, and is turned into an index-list of one element.

Examples:


  (partition* '(1 2 3 4 5) '(0 2 4)) -> ((1) (3) (5))


  (partition* "abcd" '(0 3)) -> "bc"


  (partition* "abcd" '(0 1 2 3)) -> nil

 

14.8.11 Function tree-find

Syntax:


  (tree-find
obj tree test-function)

Description:

The tree-find function searches tree for an occurrence of obj. Tree can be any atom, or a cons. If tree it is a cons, it is understood to be a proper list whose elements are also trees.

The equivalence test is performed by test-function which must take two arguments, and has conventions similar to eq, eql or equal.

tree-find works as follows. If tree is equivalent to obj under test-function, then t is returned to announce a successful finding. If this test fails, and tree is an atom, nil is returned immediately to indicate that the find failed. Otherwise, tree is taken to be a proper list, and tree-find is recursively applied to each element of the list in turn, using the same obj and test-function arguments, stopping at the first element which returns a non-nil value.

 

14.8.12 Functions find and find-if

Syntax:


  (find
key sequence [testfun [keyfun]])
  (find-if
predfun sequence [keyfun])

Description:

The find and find-if functions search through a sequence for an item which matches a key, or satisfies a predicate function, respectively.

The keyfun argument specifies a function which is applied to the elements of sequence to produce the comparison key. If this argument is omitted, then the untransformed elements of the sequence are searched.

The find function's testfun argument specifies the test function which is used to compare the comparison keys from sequence to the search key. If this argument is omitted, then the equal function is used. The first element from the list whose comparison key (as retrieved by keyfun) matches the search (under testfun) is returned. If no such element is found, nil is returned.

The find-if function's predfun argument specifies a predicate function which is applied to the successive comparison keys pulled from the list by applying keyfun to successive elements. The first element for which predfun yields true is returned. If no such element is found, nil is returned.

 

14.8.13 Functions find-max and find-min

Syntax:


  (find-max
sequence [testfun [keyfun]])
  (find-min
sequence [testfun [keyfun]])

Description:

The find-min and find-max function implement exactly the same algorithm; they differ only in their defaulting behavior with regard to the testfun argument. If testfun is not given, then the find-max function defaults it to the > function, whereas find-min defaults it to the < function.

Without a testfun argument, the find-max function finds the numerically maximum value occurring in sequence, whereas pos-min without a testfun argument finds the minimum value.

If a testfun argument is given, the two functions are equivalent. The testfun function must be callable with two arguments. If testfun behaves like a greater-than comparison, then find-max and find-min both return the maximum element. If testfun behaves like a less-than comparison, then the functions return the minimum element.

The keyfun argument defaults to the identity function. Each element from sequence is passed through this one-argument function, and the resulting value is used in its place for the purposes of the comparison. However, the original element is returned.

 

14.8.14 Function set-diff

Syntax:


  (set-diff
seq1 seq2 [testfun [keyfun]])

Description:

The set-diff function treats the sequences seq1 and seq2 as if they were sets and computes the set difference: a sequence which contains those elements in seq1 which do not occur in seq2.

set-diff returns a sequence of the same kind as seq1.

Element equivalence is determined by a combination of testfun and keyfun. Elements are compared pairwise, and each element of a pair is passed through keyfun function to produce a comparison value. The comparison values are compared using testfun. If keyfun is omitted, then the untransformed elements themselves are compared, and if testfun is omitted, then the equal function is used.

If seq1 contains duplicate elements which do not occur in seq2 (and thus are preserved in the set difference) then these duplicates appear in the resulting sequence. Furthermore, the order of the items from seq1 is preserved.

 

14.8.15 Functions mapcar, mappend mapcar* and mappend*

Syntax:


  (mapcar
function sequence*)
  (mappend
function sequence*)
  (mapcar*
function sequence*)
  (mappend*
function sequence*)

Description:

When given only one argument, the mapcar function returns nil. function is never called.

When given two arguments, the mapcar function applies function to each elements of sequence and returns a sequence of the resulting values in the same order as the original values. The returned sequence is the same kind as sequence, if possible. If the accumulated values cannot be elements of that type of sequence, then a list is returned.

When additional sequences are given as arguments, this filtering behavior is generalized in the following way: mapcar traverses the sequences in parallel, taking a value from each sequence as an argument to the function. If there are two lists, function is called with two arguments and so forth. The traversal is limited by the length of the shortest sequence. The return values of the function are collected into a new sequence which is returned. The returned sequence is of the same kind as the leftmost input sequence, unless the accumulated values cannot be elements of that type of sequence, in which case a list is returned.

The mappend function works like mapcar, with the following difference. Rather than accumulating the values returned by the function into a sequence, mappend expects the items returned by the function to be sequences which are catenated with append, and the resulting sequence is returned. The returned sequence is of the same kind as the leftmost input sequence, unless the values cannot be elements of that type of sequence, in which case a list is returned.

The mapcar* and mappend* functions work like mapcar and mappend, respectively. However, they return lazy lists rather than generating the entire output list prior to returning.

Caveats:

Like mappend, mappend* must "consume" empty lists. For instance, if the function being mapped puts out a sequence of nils, then the result must be the empty list nil, because (append nil nil nil nil ...) is nil.

But suppose that mappend* is used on inputs which are infinite lazy lists, such that the function returns nil values indefinitely. For instance:


  ;; Danger: infinite loop!!!
  (mappend* (fun identity) (repeat '(nil))) 

The mappend* function is caught in a loop trying to consume and squash an infinite stream of nils, and so doesn't return.

Examples:


  ;; multiply every element by two
  (mapcar (lambda (item) (* 2 item)) '(1 2 3)) -> (4 6 8)


  ;; "zipper" two lists together
  (mapcar (lambda (le ri) (list le ri)) '(1 2 3) '(a b c)) '((1 a) (2 b) (3 c)))


  ;; like append, mappend allows a lone atom or a trailing atom:
  (mappend (fun identity) 3) -> (3)
  (mappend (fun identity) '((1) 2)) -> (1 . 2)


  ;; take just the even numbers
  (mappend (lambda (item) (if (evenp x) (list x))) '(1 2 3 4 5))
  -> (2 4)

 

14.8.16 Function mapdo

Syntax:


  (mapdo
function sequence*)

Description:

The mapdo function is similar to mapcar, but always returns nil. It is useful when function performs some kind of side effect, hence the "do" in the name, which is a mnemonic for the execution of imperative actions.

When only the function argument is given, function is never called, and nil is returned.

If a single sequence argument is given, then mapdo iterates over sequence, invoking function on each element.

If two or more sequence arguments are given, then mapdo iterates over the sequences in parallel, extracting parallel tuples of items. These tuples are passed as arguments to function, which must accept as many arguments as there are sequences.

 

14.8.17 Functions transpose and zip

Syntax:


  (transpose
sequence)
  (zip
sequence*)

Description:

The transpose function performs a transposition on sequence. This means that the elements of sequence must be sequences. These sequences are understood to be columns; transpose exchanges rows and columns, returning a sequence of the rows which make up the columns. The returned sequence is of the same kind as sequence, and the rows are also the same kind of sequence as the first column of the original sequence. The number of rows returned is limited by the shortest column among the sequences.

All of the input sequences (the elements of sequence) must have elements which are compatible with the first sequence. This means that if the first element of sequence is a string, then the remaining sequences must be strings, or else sequences of characters, or of strings.

The zip function takes variable arguments, and is equivalent to calling transpose on a list of the arguments. The following equivalences hold:

Syntax:


   (zip . x) <--> (transpose x)


   [apply zip x] <--> (transpose x)

Examples:


  ;; transpose list of lists
  (transpose '((a b c) (c d e))) ->  ((a c) (b d) (c e))


  ;; transpose vector of strings:
  ;; - string columns become string rows
  ;; - vector input becomes vector output
  (transpose #("abc" "def" "ghij")) -> #("adg" "beh" "cfi")


  ;; error: transpose wants to make a list of strings
  ;; but 1 is not a character
  (transpose #("abc" "def" '(1 2 3))) ;; error!


  ;; String elements are catenated:
  (transpose #("abc" "def" ("UV" "XY" "WZ"))) -> #("adUV" "beXY" "cfWZ")


  (zip '(a b c) '(c d e)) ->  ((a c) (b d) (c e))

 

14.8.18 Function interpose

Syntax:


  (interpose
sep sequence)

Description:

The interpose function returns a sequence of the same type as sequence, in which the elements from sequence appear with the sep value inserted between them.

If sequence is an empty sequence or a sequence of length 1, then a sequence identical to sequence is returned. It may be a copy of sequence or it may be sequence itself.

If sequence is a character string, then the value sep must be a character.

It is permissible for sequence, or for a suffix of sequence to be a lazy list, in which case interpose returns a lazy list, or a list with a lazy suffix.

Examples:


  (interpose #\- "xyz") -> "x-y-z"
  (interpose t nil) -> nil
  (interpose t #()) -> #()
  (interpose #\a "") -> ""
  (interpose t (range 0 0)) -> (0)
  (interpose t (range 0 1)) -> (0 t 1)
  (interpose t (range 0 2)) -> (0 t 1 t 2)

 

14.8.19 Functions conses and conses*

Syntax:


  (conses
list)
  (conses*
list)

Description:

These functions return a list whose elements are the conses which make up list. The conses* function does this in a lazy way, avoiding the computation of the entire list: it returns a lazy list of the conses of list. The conses function computes the entire list before returning.

The input list may be proper or improper.

The first cons of list is that list itself. The second cons is the rest of the list, or (cdr list). The third cons is (cdr (cdr list)) and so on.

Example:


  (conses '(1 2 3)) -> ((1 2 3) (2 3) (3))

Dialect Note:

These functions are useful for simulating the maplist function found in other dialects like Common Lisp.

TXR Lisp's (conses x) can be expressed in Common Lisp as (maplist #'identity x).

Conversely, the Common Lisp operation (maplist function list) can be computed in TXR Lisp as (mapcar function (conses list)).

More generally, the Common Lisp operation


  (maplist function list0 list1 ... listn)

can be expressed as:


  (mapcar function (conses list0)
                   (conses list1) ... (conses listn))

 

14.8.20 Functions apply and iapply

Syntax:


  (apply
function [arg* trailing-args])
  (iapply
function [arg* trailing-args])

Description:

The apply function invokes function, optionally passing to it an argument list. The return value of the apply call is that of function.

If no arguments are present after function, then function is invoked without arguments.

If one argument is present after function, then it is interpreted as trailing-args. If this is a sequence (a list, vector or string), then the elements of the sequence are passed as individual arguments to function. If trailing-args is not a sequence, then function is invoked with an improper argument list, terminated by the trailing-args atom.

If two or more arguments are present after function, then the last of these arguments is interpreted as trailing-args. The previous arguments represent leading arguments which are applied to function, prior to the arguments taken from trailing-args.

The iapply function ("improper apply") is similar to apply, except with regard to the treatment of trailing-args. Firstly, under iapply, if trailing-args is an atom other than nil (possibly a sequence, such as a vector or string), then it is treated as an ordinary argument: function is invoked with a proper argument list, whose last element is trailing-args. Secondly, if trailing-args is a list, but an improper list, then the terminating atom of trailing-args becomes an ordinary argument. Thus, in all possible cases, iapply treats an extra non-nil atom as an argument, and never calls function with an improper argument list.

Examples:


  ;; '(1 2 3) becomes arguments to list, thus (list 1 2 3).
  (apply (fun list) '(1 2 3)) -> (1 2 3)


  ;; this effectively invokes (list 1 2 3 4)
  (apply (fun list) 1 2 '(3 4)) -> (1 2 3)


  ;; this effectively invokes (list 1 2 . 3)
  (apply (fun list) 1 2 3)) -> (1 2 . 3)


  ;; "abc" is separated into characters which become arguments of list
  (apply (fun list) "abc") -> (#\a #\b #\c)

Dialect Note:

Note that some uses of this function that are necessary in other Lisp dialects are not necessary in TXR Lisp. The reason is that in TXR Lisp, improper list syntax is accepted as a compound form, and performs application:


  (foo a b . x)

Here, the variables a and b supply the first two arguments for foo. In the dotted position, x must evaluate to a list or vector. The list or vector's elements are pulled out and treated as additional arguments for foo. Of course, this syntax can only be used if x is a symbolic form or an atom. It cannot be a compound form, because (foo a b . (x)) and (foo a b x) are equivalent structures.

 

14.8.21 Functions reduce-left and reduce-right

Syntax:


  (reduce-left
binary-function list
               [
init-value [key-function]])


  (reduce-right
binary-function list
                [
init-value [key-function]])

Description:

The reduce-left and reduce-right functions reduce lists of operands specified by list and init-value to a single value by the repeated application of binary-function.

An effective list of operands is formed by combining list and init-value. If key-function is specified, then the items of list are mapped to a new values through key-function. If init-value is supplied, then in the case of reduce-left, the effective list of operands is formed by prepending init-value to list. In the case of reduce-right, the effective operand list is produced by appending init-value to list.

The production of the effective list can be expressed like this, though this is not to be understood as the actual implementation:


  (append (if init-value-present (list init-value))
          [mapcar (or key-function identity) list]))))

In the reduce-right case, the arguments to append are reversed.

If the effective list of operands is empty, then binary-function is called with no arguments at all, and its value is returned. This is the only case in which binary-function is called with no arguments; in all remaining cases, it is called with two arguments.

If the effective list contains one item, then that item is returned.

Otherwise, the effective list contains two or more items, and is decimated as follows.

Note that an init-value specified as nil is not the same as a missing init-value; this means that the initial value is the object nil. Omitting init-value is the same as specifying a value of : (the colon symbol). It is possible to specify key-function while omitting an init-value argument. This is achieved by explicitly specifying : as the init-value argument.

Under reduce-left, the leftmost pair of operands is removed from the list and passed as arguments to binary-function, in the same order that they appear in the list, and the resulting value initializes an accumulator. Then, for each remaining item in the list, binary-function is invoked on two arguments: the current accumulator value, and the next element from the list. After each call, the accumulator is updated with the return value of binary-function. The final value of the accumulator is returned.

Under reduce-right, the list is processed right to left. The rightmost pair of elements in the effective list is removed, and passed as arguments to binary-function, in the same order that they appear in the list. The resulting value initializes an accumulator. Then, for each remaining item in the list, binary-function is invoked on two arguments: the next element from the list, in right to left order, and the current accumulator value. After each call, the accumulator is updated with the return value of binary-function. The final value of the accumulator is returned.

Examples:


  ;;; effective list is (1) so 1 is returned
  (reduce-left (fun +) () 1 nil)  ->  1


  ;;; computes (- (- (- 0 1) 2) 3)
  (reduce-left (fun -) '(1 2 3) 0 nil) -> -6


  ;;; computes (- 1 (- 2 (- 3 0)))
  (reduce-right (fun -) '(1 2 3) 0 nil) -> 2


  ;;; computes (* 1 2 3)
  (reduce-left (fun *) '((1) (2) (3)) nil (fun first)) -> 6


  ;;; computes 1 because the effective list is empty
  ;;; and so * is called with no arguments, which yields 1.
  (reduce-left (fun *) nil)

 

14.8.22 Function some, all and none

Syntax:


  (some
sequence [predicate-fun [key-fun]])
  (all
sequence [predicate-fun [key-fun]])
  (none
sequence [predicate-fun [key-fun]])

Description:

The some, all and none functions apply a predicate test function predicate-fun over a list of elements. If the argumenti key-fun is specified, then elements of sequence are passed into key-fun, and predicate-fun is applied to the resulting values. If key-fun is omitted, the behavior is as if key-fun is the identity function. If predicate-fun is omitted, the behavior is as if predicate-fun is the identity function.

These functions have short-circuiting semantics and return conventions similar to the and and or operators.

The some function applies predicate-fun to successive values produced by retrieving elements of list and processing them through key-fun. If the list is empty, it returns nil. Otherwise it returns the first non-nil return value returned by a call to predicate-fun and stops evaluating more elements. If predicate-fun returns nil for all elements, it returns nil.

The all function applies predicate-fun to successive values produced by retrieving elements of list and processing them through key-fun. If the list is empty, it returns t. Otherwise, if predicate-fun yields nil for any value, the function immediately returns without invoking predicate-fun on any more elements. If all the elements are processed, then the all function returns the value which predicate-fun yielded for the last element.

The none function applies predicate-fun to successive values produced by retrieving elements of list and processing them through key-fun. If the list is empty, it returns t. Otherwise, if predicate-fun yields non-nil for any value, the none function immediately returns nil. If predicate-fun yields nil for all values, the none function returns t.

Examples:


  ;; some of the integers are odd
  [some '(2 4 6 9) oddp] -> t


  ;; none of the integers are even
  [none '(1 3 4 7) evenp] -> t

 

14.8.23 Function multi

Syntax:


  (multi
function list*)

Description:

The multi function distributes an arbitrary list processing function multi over multiple lists given by the list arguments.

The list arguments are first transposed into a single list of tuples. Each successive element of this transposed list consists of a tuple of the successive items from the lists. The length of the transposed list is that of the shortest list argument.

The transposed list is then passed to function as an argument.

The functio is expected to produce a list of tuples, which are transposed again to produce a list of lists which is then returned.

Conceptually, the input lists are columns and function is invoked on a list of the rows formed from these columns. The output of function is a transformed list of rows which is reconstituted into a list of columns.

Example:


  ;; Take three lists in parallel, and remove from all of them
  ;; them the element at all positions where the third list
  ;; has an element of 20.


  (multi (op remove-if (op eql 20) @1 third)
         '(1 2 3)
         '(a b c)
         '(10 20 30))


  -> ((1 3) (a c) (10 30))


  ;; The (2 b 20) "row" is gone from the three "columns".


  ;; Note that the (op remove if (op eql 20) @1 third)
  ;; expression can be simplified using the ap operator:
  ;;
  ;; (op remove-if (ap eql @3 20))

 

14.9 Association Lists

Association lists are ordinary lists formed according to a special convention. Firstly, any empty list is a valid association list. A non-empty association list contains only cons cells as the key elements. These cons cells are understood to represent key/value associations, hence the name "association list".

 

14.9.1 Function assoc

Syntax:


  (assoc
key alist)

Description:

The assoc function searches an association list alist for a cons cell whose car field is equivalent to key (with equality determined by the equal function). The first such cons is returned. If no such cons is found, nil is returned.

 

14.9.2 Function assql

Syntax:


  (assql
key alist)

Description:

The assql function is just like assoc, except that the equality test is determined using the eql function rather than equal.

 

14.9.3 Function acons

Syntax:


  (acons
car cdr alist)

Description:

The acons function constructs a new alist by consing a new cons to the front of alist. The following equivalence holds:


  (acons car cdr alist) <--> (cons (cons car cdr) alist)

 

14.9.4 Function acons-new

Syntax:


  (acons-new
car cdr alist)

Description:

The acons-new function searches alist, as if using the assoc function, for an existing cell which matches the key provided by the car argument. If such a cell exists, then its cdr field is overwritten with the cdr argument, and then the alist is returned. If no such cell exists, then a new list is returned by adding a new cell to the input list consisting of the car and cdr values, as if by the acons function.

 

14.9.5 Function aconsql-new

Syntax:


  (aconsql-new
car cdr alist)

Description:

This function is like acons-new, except that the eql function is used for equality testing. Thus, the list is searched for an existing cell as if using the assql function rather than assoc.

 

14.9.6 Function alist-remove

Syntax:


  (alist-remove
alist keys)

Description:

The alist-remove function takes association list alist and produces a duplicate from which cells matching the specified keys have been removed. The keys argument is a list of the keys not to appear in the output list.

 

14.9.7 Function alist-nremove

Syntax:


  (alist-nremove
alist keys)

Description:

The alist-nremove function is like alist-remove, but potentially destructive. The input list alist may be destroyed and its structural material re-used to form the output list. The application should not retain references to the input list.

 

14.9.8 Function copy-alist

Syntax:


  (copy-alist
alist)

Description:

The copy-alist function duplicates alist. Unlike copy-list, which only duplicates list structure, copy-alist also duplicates each cons cell of the input alist. That is to say, each element of the output list is produced as if by the copy-cons function applied to the corresponding element of the input list.

 

14.10 Property Lists

 

14.10.1 Function prop

Syntax:


  (prop
plist key)

Description:

A property list a flat list of even length consisting of interleaved pairs of property names (usually symbols) and their values (arbitrary objects). An example property list is (:a 1 :b "two") which contains two properties, :a having value 1, and :b having value "two".

The prop function searches property list plist for key key. If the key is found, then the value next to it is returned. Otherwise nil is returned.

It is ambiguous whether nil is returned due to the property not being found, or due to the property being present with a nil value.

 

14.11 List Sorting

 

14.11.1 Function merge

Syntax:


  (merge
list1 list2 lessfun [keyfun])

Description:

The merge function merges two sorted lists list1 and list2 into a single sorted list. The semantics and defaulting behavior of the lessfun and keyfun arguments are the same as those of the sort function. The input lists are assumed to be sorted according to these functions.

This function is destructive. The application should not retain references to the input lists, since the output list is formed out of the structure of the input lists.

 

14.11.2 Function multi-sort

Syntax:


  (multi-sort
columns less-funcs [key-funcs])

Description:

The multi-sort function regards a list of lists to be the columns of a database. The corresponding elements from each list constitute a record. These records are to be sorted, producing a new list of lists.

The columns argument supplies the list of lists which comprise the columns of the database. The lists should ideally be of the same length. If the lists are of different lengths, then the shortest list is taken to be the length of the database. Excess elements in the longer lists are ignored, and do not appear in the sorted output.

The less-funcs argument supplies a list of comparison functions which are applied to the columns. Successive functions correspond to successive columns. If less-funcs is an empty list, then the sorted database will emerge in the original order. If less-funcs contains exactly one function, then the rows of the database is sorted according to the first column. The remaining columns simply follow their row. If less-funcs contains more than one function, then additional columns are taken into consideration if the items in the previous columns compare equal. For instance if two elements from column one compare equal, then the corresponding second column elements are compared using the second column comparison function.

The optional key-funcs argument supplies transformation functions through which column entries are converted to comparison keys, similarly to the single key function used in the sort function and others. If there are more key functions than less functions, the excess key functions are ignored.

 

14.12 Lazy Lists and Lazy Evaluation

 

14.12.1 Function make-lazy-cons

Syntax:


  (make-lazy-cons
function)

Description:

The function make-lazy-cons makes a special kind of cons cell called a lazy cons, or lcons. Lazy conses are useful for implementing lazy lists.

Lazy lists are lists which are not allocated all at once. Rather, their elements materialize when they are accessed, like magic stepping stones appearing under one's feet out of thin air.

A lazy cons has car and cdr fields like a regular cons, and those fields are initialized to nil when the lazy cons is created. A lazy cons also has an update function, the one which is provided as the function argument to make-lazy-cons.

When either the car and cdr fields of a cons are accessed for the first time, the function is automatically invoked first. That function has the opportunity to initialize the car and cdr fields. Once the function is called, it is removed from the lazy cons: the lazy cons no longer has an update function.

To continue a lazy list, the function can make another call to make-lazy-cons and install the resulting cons as the cdr of the lazy cons.

Example:


  ;;; lazy list of integers between min and max
  (defun integer-range (min max)
    (let ((counter min))
      ;; min is greater than max; just return empty list,
      ;; otherwise return a lazy list
      (if (> min max)
        nil
        (make-lazy-cons
          (lambda (lcons)
            ;; install next number into car
            (rplaca lcons counter)
            ;; now deal wit cdr field
            (cond
              ;; max reached, terminate list with nil!
              ((eql counter max)
               (rplacd lcons nil))
              ;; max not reached: increment counter
              ;; and extend with another lazy cons
              (t
                (inc counter)
                (rplacd lcons (make-lazy-cons
                                (lcons-fun lcons))))))))))

 

14.12.2 Function lconsp

Syntax:


  (lconsp
value)

Description:

The lconsp function returns t if value is a lazy cons cell. Otherwise it returns nil, even if value is an ordinary cons cell.

 

14.12.3 Function lcons-fun

Syntax:


  (lcons-fun
lazy-cons)

Description:

The lcons-fun function retrieves the update function of a lazy cons. Once a lazy cons has been accessed, it no longer has an update function and lcons-fun returns nil. While the update function of a lazy cons is executing, it is still accessible. This allows the update function to retrieve a reference to itself and propagate itself into another lazy cons (as in the example under make-lazy-cons).

 

14.12.4 Functions lazy-stream-cons and get-lines

Syntax:


  (lazy-stream-cons
stream)
  (get-lines
stream)

Description:

The lazy-stream-cons and get-lines functions are synonyms. Thus, the following description of lazy-stream-cons also applies to get-lines.

The lazy-stream-cons returns a lazy cons which generates a lazy list based on reading lines of text from input stream stream, which form the elements of the list. The get-line function is called on demand to add elements to the list.

The lazy-stream-cons function itself makes the first call to get-line on the stream. If this returns nil, then the stream is closed and nil is returned. Otherwise, a lazy cons is returned whose update function will install that line into the car field of the lazy cons, and continue the lazy list by making another call to lazy-stream-cons, installing the result into the cdr field.

lazy-stream-cons inspects the real-time property of a stream as if by the real-time-stream-p function. This determines which of two styles of lazy list are returned. For an ordinary (non-real-time) stream, the lazy list treats the end-of-file condition accurately: an empty file turns into the empty list nil, a one line file into a one-element list which contains that line and so on. This accuracy requires one line of lookahead which is not acceptable in real-time streams, and so a different type of lazy list is used, which generates an extra nil item after the last line. Under this type of lazy list, an empty input stream translates to the list (nil); a one-line stream translates to (linenil) and so forth.

 

14.12.5 Macro delay

Syntax:


  (delay
expression)

Description:

The delay operator arranges for the delayed (or "lazy") evaluation of expression. This means that the expression is not evaluated immediately. Rather, the delay expression produces a promise object.

The promise object can later be passed to the force function (described later in this document). The force function will trigger the evaluation of the expression and retrieve the value.

The expression is evaluated in the original scope, no matter where the force takes place.

The expression is evaluated at most once, by the first call to force. Additional calls to force only retrieve a cached value.

Example:


  @(do
    ;; list is popped only once: the value is computed
    ;; just once when force is called on a given promise
    ;; for the first time.


    (defun get-it (promise)
      (format t "*list* is ~s\n" *list*)
      (format t "item is ~s\n" (force promise))
      (format t "item is ~s\n" (force promise))
      (format t "*list* is ~s\n" *list*))


    (defvar *list* '(1 2 3))


    (get-it (delay (pop *list*))))


  Output:


  *list* is (1 2 3)
  item is 1
  item is 1
  *list* is (2 3)

 

14.12.6 Function force

Syntax:


  (force
promise)

Description:

The force function accepts a promise object produced by the delay macro. The first time force is invoked, the expression which was wrapped inside promise by the delay macro is evaluated (in its original lexical environment, regardless of where in the program the force call takes place). The value of expression is cached inside promise and returned, becoming the return value of the force function call. If the force function is invoked additional times on the same promise, the cached value is retrieved.

 

14.13 Lazy Sequences, Ranges, Permutations and Combinations

 

14.13.1 Functions generate and giterate

Syntax:


  (generate
while-fun gen-fun)
  (giterate
while-fun gen-fun [value])

Description:

The generate function produces a lazy list which dynamically produces items according to the following logic.

The arguments to generate are functions which do not take any arguments. The return value of generate is a lazy list.

When the lazy list is accessed, for instance with the functions car and cdr, it produces items on demand. Prior to producing each item, while-fun is called. If it returns a true boolean value (any value other than nil), then the gen-fun function is called, and its return value is incorporated as the next item of the lazy list. But if while-fun yields nil, then the lazy list immediately terminates.

Prior to returning the lazy list, generate invokes the while-fun one time. If while-fun yields nil, then generate returns the empty list nil instead of a lazy list. Otherwise, it instantiates a lazy list, and invokes the gen-func to populate it with the first item.

The giterate function is similar to generate, except that while-fun and gen-fun are functions of one argument rather than functions of no arguments. The optional value argument defaults to nil and is threaded through the function calls. That is to say, the lazy list returned is (value [gen-fun value] [gen-fun [gen-fun << value ]] ...).

The lazy list terminates when a value fails to satsify while-fun. That is to say, prior to generating each value, the lazy list tests the value using while-fun. If that function returns nil, then the item is not added, and the sequence terminates.

Note: giterate could be written in terms of generate like this:


  (defun giterate (w g v)
     (generate (lambda () [w v])
               (lambda () (prog1 v (set v [g v])))))

Example:


  (giterate (op > 5) (op + 1) 0) -> (0 1 2 3 4)

 

14.13.2 Function repeat

Syntax:


  (repeat
list [count])

Description:

If list is empty, then repeat returns an empty list.

If count is omitted, the repeat function produces an infinite lazy list formed by catenating together copies of list.

If count is specified and is zero or negative, then an empty list is returned.

Otherwise a list is returned consisting of count repetitions of list catenated together.

 

14.13.3 Macros gen and gun

Syntax:


  (gen
while-expression produce-item-expression)
  (gun
produce-item-expression)

Description:

The gen macro operator produces a lazy list, in a manner similar to the generate function. Whereas the generate function takes functional arguments, the gen operator takes two expressions, which is often more convenient.

The return value of gen is a lazy list. When the lazy list is accessed, for instance with the functions car and cdr, it produces items on demand. Prior to producing each item, the while-expression is evaluated, in its original lexical scope. If the expression yields a non-nil value, then produce-item-expression is evaluated, and its return value is incorporated as the next item of the lazy list. If the expression yields nil, then the lazy list immediately terminates.

The gen operator itself immediately evaluates while-expression before producing the lazy list. If the expression yields nil, then the operator returns the empty list nil. Otherwise, it instantiates the lazy list and invokes the produce-item-expression to force the first item.

The gun macro similarly creates a lazy list according to the following rules. Each successive item of the lazy list is obtained as a result of evaluating produce-item-expression. However, when produce-item-expression yields nil, then the list terminates (without adding that nil as an item).

Note 1: the form gun can be implemented as a macro-expanding to an instance of the gen operator, like this:


  (defmacro gun (expr)
    (let ((var (gensym)))
      ^(let (,var)
         (gen (set ,var ,expr)
              ,var))))

This exploits the fact that the set operator returns the value that is assigned, so the set expression is tested as a condition by gen, while having the side effect of storing the next item temporarily in a hidden variable.

In turn, gen can be implemented as a macro expanding to some lambda functions which are passed to the generate function:


  (defmacro gen (while-expr produce-expr)
    ^(generate (lambda () ,while-expr) (lambda () ,produce-expr)))

Note 2: gen can be considered as an acronym for Generate, testing Expression before Next item, whereas gun stands for Generate Until Null.

Example:


  @(do
    ;; Make a lazy list of integers up to 1000
    ;; access and print the first three.
    (let* ((counter 0)
           (list (gen (< counter 1000) (inc counter))))
      (format t "~s ~s ~s\n" (pop list) (pop list) (pop list))))


  Output:
  1 2 3

 

14.13.4 Functions range and range*

Syntax:


  (range [[[
from] to] step])
  (range* [[[
from] to] step])

Description:

The range and range* functions generate a lazy sequence of integers, with a fixed step between successive values.

The difference between range and range* is that range* excludes the endpoint. For instance (range 0 3) generates the list (0 1 2 3), whereas (range* 0 3) generates (0 1 2).

All arguments are optional. If the step argument is omitted, then it defaults to 1: each value in the sequence is greater than the previous one by 1. Positive or negative step sizes are allowed. There is no check for a step size of zero, or for a step direction which cannot meet the endpoint.

The to argument specifies the endpoint value, which, if it occurs in the sequence, is excluded from it by the range* function, but included by the range function. If to is missing, or specified as nil, then there is no endpoint, and the sequence which is generated is infinite, regardless of step.

If from is omitted, then the sequence begins at zero, otherwise from must be an integer which specifies the initial value.

The sequence stops if it reaches the endpoint value (which is included in the case of range, and excluded in the case of range*). However, a sequence with a stepsize greater than 1 or less than -1 might step over the endpoint value, and therefore never attain it. In this situation, the sequence also stops, and the excess value which surpasses the endpoint is excluded from the sequence.

 

14.13.5 Function perm

Syntax:


  (perm
seq [len])

Description:

The rperm function returns a lazy list which consists of all length len permutations of formed by items taken from seq. The permutations do not use any element of seq more than once.

Argument len, if present, must be a positive integer, and seq must be a sequence.

If len is not present, then its value defaults to the length of seq: the list of the full permutations of the entire sequence is returned.

The permutations in the returned list are sequences of the same kind as seq.

If len is zero, then a list containing one permutation is returned, and that permutations is of zero length.

If len exceeds the length of seq, then an empty list is returned, since it is impossible to make a single non-repeating permutation that requires more items than are available.

The permutations are lexicographically ordered.

 

14.13.6 Function rperm

Syntax:


  (rperm
seq len)

Description:

The rperm function returns a lazy list which consists of all the repeating permutations of length len formed by items taken from seq. "Repeating" means that the items from seq can appear more than once in the permutations.

The permutations which are returned are sequences of the same kind as seq.

Argument len must be a nonnegative integer, and seq must be a sequence.

If len is zero, then a single permutation is returned, of zero length. This is true regardless of whether seq is itself empty.

If seq is empty and len is greater than zero, then no permutations are returned, since permutations of a positive length require items, and the sequence has no items. Thus there exist no such permutations.

The first permutation consists of le repetitions of the first element of seq. The next repetition, if there is one, differs from the first repetition in that its last element is the second element of seq. That is to say, the permutations are lexicographically ordered.

Examples:


  (rperm "01" 4) -> ("000" "001" "010" "011"
                     "100" "101" "110" "111")


  (rperm #(1) 3) -> (#(1 1 1))


  (rperm '(0 1 2) 2) -> ((0 0) (0 1) (0 2) (1 0)
                         (1 1) (1 2) (2 0) (2 1) (2 2))

 

14.13.7 Function comb

Syntax:


  (comb
seq len)

Description:

The comb function returns a lazy list which consists of all length len non-repeating combinations formed by taking items taken from seq. "Non-repeating combinations" means that the combinations do not use any element of seq more than once. If seq contains no duplicates, then the combinations contain no duplicates.

Argument len must be a nonnegative integer, and seq must be a sequence or a hash table.

The combinations in the returned list are objects of the same kind as seq.

If len is zero, then a list containing one combination is returned, and that permutations is of zero length.

If len exceeds the number of elements in seq, then an empty list is returned, since it is impossible to make a single non-repeating combination that requires more items than are available.

If seq is a sequence, the returned combinations are lexicographically ordered. This requirement is not applicable when seq is a hash table.

 

14.13.8 Function rcomb

Syntax:


  (rcomb
seq len)

Description:

The comb function returns a lazy list which consists of all length len repeating combinations formed by taking items taken from seq. "Repeating combinations" means that the combinations can use an element of seq more than once.

Argument len must be a nonnegative integer, and seq must be a sequence.

The combinations in the returned list are sequences of the same kind as seq.

If len is zero, then a list containing one combination is returned, and that permutations is of zero length. This is true even if seq is empty.

If seq is empty, and len is nonzero, then an empty list is returned.

The combinations are lexicographically ordered.

 

14.14 Characters and Strings

 

14.14.1 Function mkstring

Syntax:


  (mkstring
length char)

Description:

The mkstring function constructs a string object of a length specified by the length parameter. Every position in the string is initialized with char, which must be a character value.

 

14.14.2 Function copy-str

Syntax:


  (copy-str
string)

Description:

The copy-str function constructs a new string whose contents are identical to string.

 

14.14.3 Function upcase-str

Syntax:


  (upcase-str
string)

Description:

The upcase-str function produces a copy of string such that all lower-case characters of the English alphabet are mapped to their upper case counterparts.

 

14.14.4 Function downcase-str

Syntax:


  (downcase-str
string)

Description:

The downcase-str function produces a copy of string such that all upper case characters of the English alphabet are mapped to their lower case counterparts.

 

14.14.5 Function string-extend

Syntax:


  (string-extend
string tail)

Description:

The string-extend function destructively increases the length of string, which must be an ordinary dynamic string. It is an error to invoke this function on a literal string or a lazy string.

The tail argument can be a character, string or integer. If it is a string or character, it specifies material which is to be added to the end of the string: either a single character or a sequence of characters. If it is an integer, it specifies the number of characters to be added to the string.

If tail is an integer, the newly added characters have indeterminate contents. The string appears to be the original one because of an internal terminating null character remains in place, but the characters beyond the terminating zero are indeterminate.

 

14.14.6 Function stringp

Syntax:


  (stringp
obj)

Description:

The stringp function returns t if obj is one of the several kinds of strings. Otherwise it returns nil.

 

14.14.7 Function length-str

Syntax:


  (length-str
string)

Description:

The length-str function returns the length string in characters. The argument must be a string.

 

14.14.8 Function search-str

Syntax:


  (search-str
haystack needle [start [from-end]])

Description:

The search-str function finds an occurrence of the string needle inside the haystack string and returns its position. If no such occurrence exists, it returns nil.

If a start argument is not specified, it defaults to zero. If it is a non-negative integer, it specifies the starting character position for the search. Negative values of start indicate positions from the end of the string, such that -1 is the last character of the string.

If the from-end argument is specified and is not nil, it means that the search is conducted right-to-left. If multiple matches are possible, it will find the rightmost one rather than the leftmost one.

 

14.14.9 Function search-str-tree

Syntax:


  (search-str-tree
haystack tree [start [from-end]])

Description:

The search-str-tree function is similar to search-str, except that instead of searching haystack for the occurrence of a single needle string, it searches for the occurrence of numerous strings at the same time. These search strings are specified, via the tree argument, as an arbitrarily structured tree whose leaves are strings.

The function finds the earliest possible match, in the given search direction, from among all of the needle strings.

If tree is a single string, the semantics is equivalent to search-str.

 

14.14.10 Function match-str

Syntax:


  (match-str
bigstring littlestring [start])

Description:

Without the start argument, the match-str function determines whether littlestring is a prefix of bigstring, returning a t or nil indication.

If the start argument is specified, and is a non-negative integer, then the function tests whether littlestring matches a prefix of that portion of bigstring which starts at the given position.

If the start argument is a negative integer, then match-str determines whether littlestring is a suffix of bigstring, ending on that position of bigstring, where -1 denotes the last character of bigstring, -2 the second last one and so on.

If start is -1, then this corresponds to testing whether littlestring is a suffix of bigstring.

 

14.14.11 Function match-str-tree

Syntax:


  (match-str-tree
bigstring tree [start])

Description:

The match-str-tree function is a generalization of match-str which matches multiple test strings against bigstring at the same time. The value reported is the longest match from among any of the strings.

The strings are specified as an arbitrarily shaped tree structure which has strings at the leaves.

If tree is a single string atom, then the function behaves exactly like match-str.

 

14.14.12 Function sub-str

Syntax:


  (sub-str
string [from [to]])

Description:

The sub-str function is like the more generic function sub, except that it operates only on strings. For a description of the arguments and semantics, refer to the sub function.

 

14.14.13 Function replace-str

Syntax:


  (replace-str
string item-sequence [from [to]])

Description:

The replace-str function is like the replace function, except that the first argument must be a string.

For a description of the arguments and semantics, refer to the replace function.

 

14.14.14 Function cat-str

Syntax:


  (cat-str
string-list [sep-string])

Description:

The cat-str function catenates a list of strings given by string-list into a single string. The optional sep-string argument specifies a separator string which is interposed between the catenated strings.

 

14.14.15 Function split-str

Syntax:


  (split-str
string sep)

Description:

The split-str function breaks the string into pieces, returning a list thereof. The sep argument must be either a string or a regular expression. It specifies the separator character sequence within string.

All non-overlapping matches for sep within string are identified in left to right order, and are removed from string. The string is broken into pieces according to the gaps left behind by the removed separators, and a list of the remaining pieces is returned.

If a match for sep is not found in the string at all, then the string is not split at all: a list of one element is returned containing the original string.

If sep matches the entire string, then a list of two empty strings is returned, except in the case that the original string is empty, in which case a list of one element is returned, containing the empty string.

Whenever two adjacent matches for sep occur, they are considered separate cuts with an empty piece between them.

This operation is nondestructive: string is not modified in any way.

 

14.14.16 Function split-str-set

Syntax:


  (split-str-set
string set)

Description:

The split-str-set function breaks the string into pieces, returning a list thereof. The sep argument must be a string. It specifies a set of characters. All occurrences of any of these characters within string are identified, and are removed from string. The string is broken into pieces according to the gaps left behind by the removed separators.

Adjacent occurrences of characters from set within string are considered to be separate gaps which come between empty strings.

This operation is nondestructive: string is not modified in any way.

 

14.14.17 Functions tok-str and tok-where

Syntax:


  (tok-str
string regex [keep-between])
  (tok-where
string regex)

Description:

The tok-str function searches string for tokens, which are defined as substrings of which match the regular expression regex in the longest possible way, and do not overlap. These tokens are extracted from the string and returned as a list.

Whenever regex matches an empty string, then an empty token is returned, and the search for another token within string resumes after advancing by one character position. So for instance, (tok-str abc #/a?/) returns the ("a" "" "" ""). After the token "a" is extracted from a non-empty match for the regex, the regex is considered to match three more times: before the "b", between "b" and "c", and after the "c".

If the keep-betwee argument is specified, and is not nil, then the behavior of tok-str changes in the following way. The pieces of string which are skipped by the search for tokens are included in the output. If no token is found in string, then a list of one element is returned, containing string. Generally, if N tokens are found, then the returned list consists of 2N + 1 elements. The first element of the list is the (possibly empty) substring which had to be skipped to find the first token. Then the token follows. The next element is the next skipped substring and so on. The last element is the substring of string between the last token and the end.

The tok-where function works similarly to tok-str, but instead of returning the extracted tokens themselves, it returns a list of the character position ranges within string where matches for regex occur. The ranges are pairs of numbers, represented as cons cells, where the first number of the pair gives the starting character position, and the second number is one position past the end of the match. If a match is empty, then the two numbers are equal.

The tok-where function does not support the keep-between parameter.

 

14.14.18 Function list-str

Syntax:


  (list-str
string)

Description:

The list-str function converts a string into a list of characters.

 

14.14.19 Function trim-str

Syntax:


  (trim-str
string)

Description:

The trim-str function produces a copy of string from which leading and trailing whitespace is removed. Whitespace consists of spaces, tabs, carriage returns, linefeeds, vertical tabs and form feeds.

 

14.14.20 Function chrp

Syntax:


  (chrp
obj)

Description:

Returns t if obj is a character, otherwise nil.

 

14.14.21 Function chr-isalnum

Syntax:


  (chr-isalnum
char)

Description:

Returns t if char is an alpha-numeric character, otherwise nil. Alpha-numeric means one of the upper or lower case letters of the English alphabet found in ASCII, or an ASCII digit. This function is not affected by locale.

 

14.14.22 Function chr-isalpha

Syntax:


  (chr-isalpha
char)

Description:

Returns t if char is an alphabetic character, otherwise nil. Alphabetic means one of the upper or lower case letters of the English alphabet found in ASCII. This function is not affected by locale.

 

14.14.23 Function chr-isascii

Syntax:


  (chr-isalpha
char)

Description:

This function returns t if the code of character char is in the range 0 to 127 inclusive. For characters outside of this range, it returns nil.

 

14.14.24 Function chr-iscntrl

Syntax:


  (chr-iscntrl
char)

Description:

This function returns t if the character char is a character whose code ranges from 0 to 31, or is 127. In other words, any non-printable ASCII character. For other characters, it returns nil.

 

14.14.25 Function chr-isdigit

Syntax:


  (chr-isdigit
char)

Description:

This function returns t if the character char is is an ASCII digit. Otherwise, it returns nil.

 

14.14.26 Function chr-isgraph

Syntax:


  (chr-isgraph
char)

Description:

This function returns t if char is a non-space printable ASCII character. It returns nil if it is a space or control character.

It also returns nil for non-ASCII characters: Unicode characters with a code above 127.

 

14.14.27 Function chr-islower

Syntax:


  (chr-islower
char)

Description:

This function returns t if char is an ASCII lower case letter. Otherwise it returns nil.

 

14.14.28 Function chr-isprint

Syntax:


  (chr-isprint
char)

Description:

This function returns t if char is an ASCII character which is not a control character. It also returns nil for all non-ASCII characters: Unicode characters with a code above 127.

 

14.14.29 Function chr-ispunct

Syntax:


  (chr-ispunct
char)

Description:

This function returns t if char is an ASCII character which is not a control character. It also returns nil for all non-ASCII characters: Unicode characters with a code above 127.

 

14.14.30 Function chr-isspace

Syntax:


  (chr-isspace
char)

Description:

This function returns t if char is an ASCII whitespace character: any of the characters in the set #\space, #\tab, #\linefeed, #\newline, #\return, #\vtab and #\page. For all other characters, it returns nil.

 

14.14.31 Function chr-isupper

Syntax:


  (chr-isupper
char )

Description:

This function returns t if char is an ASCII upper case letter. Otherwise it returns nil.

 

14.14.32 Function chr-isxdigit

Syntax:


  (chr-isxdigit
char)

Description:

This function returns t if char is a hexadecimal digit. One of the ASCII letters A through F, or their lower-case equivalents, or an ASCII digit 0 through 9.

 

14.14.33 Function chr-toupper

Syntax:


  (chr-toupper
char)

Description:

If character char is a lower case ASCII letter character, this function returns the upper case equivalent character. If it is some other character, then it just returns char.

 

14.14.34 Function chr-tolower

Syntax:


  (chr-tolower
char)

Description:

If character char is an upper case ASCII letter character, this function returns the lower case equivalent character. If it is some other character, then it just returns char.

 

14.14.35 Functions num-chr and chr-num

Syntax:


  (num-chr
char)
  (chr-num
num)

Description:

The argument char must be a character. The num-chr function returns that character's Unicode code point value as an integer.

The argument num must be a fixnum integer in the range 0 to #\x10FFFF. The argument is taken to be a Unicode code point value and the corresponding character object is returned.

 

14.14.36 Function chr-str

Syntax:


  (chr-str
str idx)

Description:

The chr-str function performs random access on string str to retrieve the character whose position is given by integer idx, which must be within range of the string.

The index value 0 corresponds to the first (leftmost) character of the string and so non-negative values up to one less than the length are possible.

Negative index values are also allowed, such that -1 corresponds to the last (rightmost) character of the string, and so negative values down to the additive inverse of the string length are possible.

An empty string cannot be indexed. A string of length one supports index 0 and index -1. A string of length two is indexed left to right by the values 0 and 1, and from right to left by -1 and -2.

Notes:

Direct use of chr-str is equivalent to the DWIM bracket notation except that str must be a string. The following relation holds:


  (chr-str s i) --> [s i]

since [s i] <--> (ref s i), this also holds:


  (chr-str s i) --> (ref s i)

 

14.14.37 Function chr-str-set

Syntax:


  (chr-str-set
str idx char)

Description:

The chr-str function performs random access on string str to overwrite the character whose position is given by integer idx, which must be within range of the string. The character at idx is overwritten with character char.

The idx argument works exactly as in chr-str.

The str argument must be a modifiable string.

Notes:

Direct use of chr-str is equivalent to the DWIM bracket notation except that str must be a string. The following relation holds:


  (chr-str-set s i c) --> (set [s i] c)

since (set [s i] c) <--> (refset s i c), this also holds:


  (chr-str s i) --> (refset s i c)

 

14.14.38 Function span-str

Syntax:


  (span-str
str set)

Description:

The span-str function determines the longest prefix of string str which consists only of the characters in string set, in any combination.

 

14.14.39 Function compl-span-str

Syntax:


  (compl-span-str
str set)

Description:

The compl-span-str function determines the longest prefix of string str which consists only of the characters which do not appear in set, in any combination.

 

14.14.40 Function break-str

Syntax:


  (break-str
str set)

Description:

The break-str function returns an integer which represents the position of the first character in string str which appears in string set.

If there is no such character, then nil is returned.

 

14.15 Lazy Strings

Lazy strings are objects that were developed for the TXR pattern matching language, and are exposed via TXR Lisp. Lazy strings behave much like strings, and can be substituted for strings. However, unlike regular strings, which exist in their entirety, first to last character, from the moment they are created, lazy strings do not exist all at once, but are created on demand. If character at index N of a lazy string is accessed, then characters 0 through N of that string are forced into existence. However, characters at indices beyond N need not necessarily exist.

A lazy string dynamically grows by acquiring new text from a list of strings which is attached to that lazy string object. When the lazy string is accessed beyond the end of its hitherto materialized prefix, it takes enough strings from the list in order to materialize the index. If the list doesn't have enough material, then the access fails, just like an access beyond the end of a regular string. A lazy string always takes whole strings from the attached list.

Lazy string growth is achieved via the lazy-str-force-upto function which forces a string to exist up to a given character position. This function is used internally to handle various situations.

The lazy-str-force function forces the entire string to materialize. If the string is connected to an infinite lazy list, this will exhaust all memory.

Lazy strings are specially recognized in many of the regular string functions, which do the right thing with lazy strings. For instance when sub-str is invoked on a lazy string, a special version of the sub-str logic is used which handles various lazy string cases, and can potentially return another lazy string. Taking a sub-str of a lazy string from a given character position to the end does not force the entire lazy string to exist, and in fact the operation will work on a lazy string that is infinite.

Furthermore, special lazy string functions are provided which allow programs to be written carefully to take better advantage of lazy strings. What carefully means is code that avoids unnecessarily forcing the lazy string. For instance, in many situations it is necessary to obtain the length of a string, only to test it for equality or inequality with some number. But it is not necessary to compute the length of a string in order to know that it is greater than some value.

 

14.15.1 Function lazy-str

Syntax:


  (lazy-str
string-list [terminator [limit-count]])

Description:

The lazy-str function constructs a lazy string which draws material from string-list which is a list of strings.

If the optional terminator argument is given, then it specifies a string which is appended to every string from string-list, before that string is incorporated into the lazy string. If terminator is not given, then it defaults to the string "\n", and so the strings from string-list are effectively treated as lines which get terminated by newlines as they accumulate into the growing prefix of the lazy string. To avoid the use of a terminator string, a null string terminator argument must be explicitly passed. In that case, the lazy string grows simply by catenating elements from string-list.

If the limit-count argument is specified, it must be a positive integer. It expresses a maximum limit on how many elements will be consumed from string-list in order to feed the lazy string. Once that many elements are drawn, the string ends, even if the list has not been exhausted.

 

14.15.2 Function lazy-stringp

Syntax:


  (lazy-stringp
obj)

Description:

The lazy-stringp function returns t if obj is a lazy string. Otherwise it returns nil.

 

14.15.3 Function lazy-str-force-upto

Syntax:


  (lazy-str-force-upto
lazy-str index)

Description:

The lazy-str-force-upto function tries to instantiate the lazy string such that the position given by index materializes. The index is a character position, exactly as used in the chr-str function.

Some positions beyond index may also materialize, as a side effect.

If the string is already materialized through to at least index, or if it is possible to materialize the string that far, then the value t is returned to indicate success.

If there is insufficient material to force the lazy string through to the index position, then nil is returned.

It is an error if the lazy-str argument isn't a lazy string.

 

14.15.4 Function lazy-str-force

Syntax:


  (lazy-str-force
lazy-str)

Description:

The lazy-str argument must be a lazy string. The lazy string is forced to fully materialize.

The return value is an ordinary, non-lazy string equivalent to the fully materialized lazy string.

 

14.15.5 Function lazy-str-get-trailing-list

Syntax:


  (lazy-str-get-trailing-list
string index)

Description:

The lazy-str-get-trailing-list function is a sort of inverse operation to the lazy string from its associated list.

First, the string is forced up through the position index.

Next, the materialized part of the string starting at position index, through to the end, is split into pieces on occurrences of the terminator character, which had been given as the terminator argument in the lazy-str constructor, and defaults to the newline character.

Finally, a list is returned consisting of the pieces produced by the split, to which is appended the remaining list of the string which has not yet been forced to materialize.

If index is a position which cannot be forced, then the lazy string's remaining list is returned, with single null string prepended to it.

 

14.15.6 Functions length-str->, length-str->=, length-str-< and length-str-<=

Syntax:


  (length-str->
string len)
  (length-str->=
string len)
  (length-str-<
string len)
  (length-str-<=
string len)

Description:

These functions compare the lengths of two strings. The following equivalences hold, as far as the resulting value is concerned:


  (length-str-> s l) <--> (> (length-str s) l)
  (length-str->= s l) <--> (>= (length-str s) l)
  (length-str-< s l) <--> (< (length-str s) l)
  (length-str-<= s l) <--> (<= (length-str s) l)

The difference between the functions and the equivalent forms is that if the string is lazy, the length-str function will fully force it in order to calculate and return its length.

These functions only force a string up to position len, so they are not only more efficient, but on infinitely long lazy strings they are usable.

length-str cannot compute the length of a lazy string with an unbounded length; it will exhaust all memory trying to force the string.

These functions can be used to test such as string whether it is longer or shorter than a given length, without forcing the string beyond that length.

 

14.15.7 Function cmp-str

Syntax:


  (cmp-str
left-string right-string)

Description:

The cmp-str function returns a negative integer if left-string is lexicographically prior to right-string, and a positive integer if the reverse situation is the case. Otherwise the strings are equal and zero is returned.

If either or both of the strings are lazy, then they are only forced to the minimum extent necessary for the function to reach a conclusion and return the appropriate value, since there is no need to look beyond the first character position in which they differ.

The lexicographic ordering is naive, based on the character code point values in Unicode taken as integers, without regard for locale-specific collation orders.

 

14.15.8 Functions str=, str<, str>, str>= and str<=

Syntax:


  (str=
left-string right-string)
  (str<
left-string right-string)
  (str>
left-string right-string)
  (str<=
left-string right-string)
  (str>=
left-string right-string)

Description:

These functions compare left-string and right-string lexicographically, as if by the cmp-str function.

The str= function returns t if the two strings are exactly the same, character for character, otherwise it returns nil.

The str< function returns t if left-string is lexicographically before right-string, otherwise nil.

The str> function returns t if left-string is lexicographically after right-string, otherwise nil.

The str< function returns t if left-string is lexicographically before right-string, or if they are exactly the same, otherwise nil.

The str< function returns t if left-string is lexicographically after right-string, or if they are exactly the same, otherwise nil.

 

14.15.9 Function string-lt

Syntax:


  (string-lt
left-str right-str)

Description:

The string-lt is a deprecated alias for str<.

 

14.16 Vectors

 

14.16.1 Function vector

Syntax:


  (vector
length [initval])

Description:

The vector function creates and returns a vector object of the specified length. The elements of the vector are initialized to initval, or to nil if initval is omitted.

 

14.16.2 Function vec

Syntax:


  (vec
arg*)

Description:

The vec function creates a vector out of its arguments.

 

14.16.3 Function vectorp

Syntax:


  (vectorp
obj)

Description:

The vectorp function returns t if obj is a vector, otherwise it returns nil.

 

14.16.4 Function vec-set-length

Syntax:


  (vec-set-length
vec len)

Description:

The vec-set-length modifies the length of vec, making it longer or shorter. If the vector is made longer, then the newly added elements are initialized to nil. The len argument must be nonnegative.

The return value is vec.

 

14.16.5 Function vecref

Syntax:


  (vecref
vec idx)

Description:

The vecref function performs indexing into a vector. It retrieves an element of vec at position idx, counted from zero. The idx value must range from 0 to one less than the length of the vector. The specified element is returned.

 

14.16.6 Function vec-push

Syntax:


  (vec-push
vec elem)

Description:

The vec-push function extends the length of a vector vec by one element, and sets the new element to the value elem.

The previous length of the vector (which is also the position of elem) is returned.

This function performs similarly to the generic function ref, except that the first argument must be a vector.

 

14.16.7 Function length-vec

Syntax:


  (length-vec
vec)

Description:

The length-vec function returns the length of vector vec. It performs similarly to the generic length function, except that the argument must be a vector.

 

14.16.8 Function size-vec

Syntax:


  (size-vec
vec)

Description:

The size-vec function returns the number of elements for which storage is reserved in the vector vec.

Notes:

The length of the vector can be extended up to this size without any memory allocation operations having to be performed.

 

14.16.9 Function vector-list

Syntax:


  (vector-list
list)

Description:

This function returns a vector which contains all of the same elements and in the same order as list list.

 

14.16.10 Function list-vector

Syntax:


  (list-vector
vec)

Description:

The list-vector function returns a list of the elements of vector vec.

 

14.16.11 Function copy-vec

Syntax:


  (copy-vec
vec)

Description:

The copy-vec function returns a new vector object of the same length as vec and containing the same elements in the same order.

 

14.16.12 Function sub-vec

Syntax:


  (sub-vec
vec [from [to]])

Description:

The sub-vec function is like the more generic function sub, except that it operates only on vectors.

For a description of the arguments and semantics, refer to the sub function.

 

14.16.13 Function replace-vec

Syntax:


  (replace-vec
vec item-sequence [from [to]])

Description:

The replace-vec is like the replace function, except that the first argument must be a vector.

For a description of the arguments and semantics, refer to the replace function.

 

14.16.14 Function cat-vec

Syntax:


  (cat-vec
vec-list)

Description:

The vec-list argument is a list of vectors. The cat-vec function produces a catenation of the vectors listed in vec-list. It returns a single large vector formed by catenating those vectors together in order.

 

14.17 Sequence Manipulation

 

14.17.1 Function seqp

Syntax:


  (seqp
object)

Description:

The function seqp returns t if object is a sequence, otherwise nil.

A sequence is defined as a list, vector or string. The object nil denotes the empty list and so is a sequence.

 

14.17.2 Function length

Syntax:


  (length
sequence)

Description:

The length function returns the number of items in sequence, and returns it. sequence may be a hash, in which case (hash-count sequence) is returned.

 

14.17.3 Function empty

Syntax:


  (empty
sequence)

Description:

Returns t if (length sequence) is zero, otherwise nil.

 

14.17.4 Function copy

Syntax:


  (copy
sequence)

Description:

The copy function duplicates a sequence. If sequence is nil, it returns nil. If sequence is a list, it returns (copy-list sequence). If sequence is a string, it returns (copy-str sequence). If sequence is a vector, it returns (copy-vec sequence). If sequence is a hash, it returns (copy-hash sequence).

Except in the case when sequence is nil, copy returns a value that is distinct from (not eq to) sequence. This is different from the behavior of [sequence 0..t] or (sub sequence 0 t) which recognize that they need not make a copy of sequence, and just return it.

Note however, that the elements of the returned sequence may be eq to elements of the original sequence. In other words, copy is a deeper copy than just duplicating the sequence value itself, but it is not a deep copy.

 

14.17.5 Function sub

Syntax:


  (sub
sequence [from [to]])

Description:

The sub function extracts a slice from input sequence sequence. The slice is a sequence of the same type as sequence.

If the from argument is omitted, it defaults to 0. If the to parameter is omitted, it defaults to t. Thus (sub a) means (sub a 0 t).

The following equivalence holds between the sub function and the DWIM-bracket syntax:


  ;; from is not a list
  (sub seq from to) <--> [seq from..to]

The description of the dwim operator—in particular, the section on Range Indexing—explains the semantics of the range specification.

If the sequence is a list, the output sequence may share substructure with the input sequence.

 

14.17.6 Function replace

Syntax:


  (replace
sequence replacement-sequence [from [to]])
  (replace
sequence replacement-sequence index-list)

Description:

The replace function has two invocation styles, distinguished by the type of the third argument. If the third argument is a list, then it is deemed to be the index-list parameter of the second form. Otherwise, if the third argument is missing, or is not a list, then it is deemed to be the from argument of the first form.

The first form of the replace function replaces a contiguous subsequence of the sequence with replacement-sequence. The replaced subsequence may be empty, in which case an insertion is performed. If replacement-sequence is empty (for example, the empty list nil), then a deletion is performed.

If the from and to arguments are omitted, their values default to 0 and t respectively.

The following equivalence holds between assignment to a place denoted by DWIM bracket syntax and first form of the replace function:


  (set seq (replace seq new fr to)) <--> (set [seq fr..to] new)

The description of the dwim operator—in particular, the section on Range Indexing—explains the semantics of the range specification.

This operation is destructive: it may work "in place" by modifying the original sequence. The caller should retain the return value and stop relying on the original input sequence.

The second form of the replace function replaces a subsequence of elements from sequence given by index-list, with their counterparts from replacement-sequence. This form of the replace function does not insert or delete; it simply overwrites elements. If replacement-sequence and idex-list are of different lengths, then the shorter of the two determines the maximum number of elements which are overwritten. Furthermore, similar restrictions apply on index-list as under the select function. Namely, the replacement stops when an index value in index-list is encountered which is out of range for sequence. furthermore, if sequence is a list, then index-list must be monotonically increasing.

The following equivalence holds between assignment to a place denoted by DWIM bracket syntax and second form of the replace function:


  (set seq (replace seq new ix-list)) <--> (set [seq ix-list] new)

 

14.17.7 Function search

Syntax:


  (search
haystack needle [testfun [keyfun])

Description:

The search function determines whether the sequence needle occurs as substring within haystack, under the given comparison function testfun and key function keyfun. If this is the case, then the zero-based position of the leftmost occurrence of key within haystack is returned. Otherwise nil is returned to indicate that key does not occur within haystack. If key is empty, then zero is always returned.

The arguments haystack and needle are sequences: lists, vectors or strings, in any combination.

If needle is not empty, then occurs at some position N within haystack if the first element of needle matches the element at position N of haystack, the second element of needle matches the element at position N+1 of haystack and so forth, for all elements of needle. A match between elements is determined by passing each element through keyfun, and then comparing the resulting values using testfun.

If testfun is supplied, it must be a function which can be called with two arguments. If it is not supplied, it defaults to eql.

If keyfun is supplied, it must be a function which can be called with one argument. If it is not supplied, it defaults to identity.

Examples:


  ;; fails because 3.0 doesn't match 3
  ;; under the default eql function
  [search #(1.0 3.0 4.0 7.0) '(3 4)] -> nil


  ;; occurrence found at position 1:
  ;; (3.0 4.0) matches (3 4) under =
  [search #(1.0 3.0 4.0 7.0) '(3 4) =] -> 1


  ;; "even odd odd odd even" pattern
  ;; matches at position 2
  [search #(1 1 2 3 5 7 8) '(2 1 1 1 2) : evenp] -> 2


  ;; Case insensitive string search
  [search "abcd" "CD" : chr-toupper] -> 2


  ;; Case insensitive string search
  ;; using vector of characters as key
  [search "abcd" #(#\C #\D) : chr-toupper] -> 2

 

14.17.8 Functions ref and refset

Syntax:


  (ref
seq index)
  (refset
seq index new-value)

Description:

The ref and refset functions perform array-like indexing into sequences. The ref function retrieves an element of seq, whereas refset overwrites an element of seq with a new value.

The index argument is based from zero, and negative values are permitted, with a special meaning as described in the Range Indexing section under the description of the dwim operator.

The refset function returns the new value.

The following equivalences hold between ref and refset, and the DWIM bracket syntax:


  (ref seq idx) <--> [seq idx]


  (refset seq idx new) <--> (set [seq idx] new)

The difference is that ref and refset are first class functions which can be used in functional programming as higher order functions, whereas the bracket notation is syntactic sugar, and set is an operator, not a function. Therefore the brackets cannot replace all uses of ref and refset.

 

14.17.9 Function update

Syntax:


  (update
sequence-or-hash function)

Description:

The update function replaces each elements in a sequence, or each value in a hash table, with the value of function applied to that element or value.

The sequence or hash table is returned.

 

14.17.10 Function sort

Syntax:


  (sort
sequence lessfun [keyfun])

Description:

The sort function destructively sorts sequence, producing a sequence which is sorted according to the lessfun and keyfun arguments.

The keyfun argument specifies a function which is applied to elements of the sequence to obtain the key values which are then compared using the lessfun. If keyfun is omitted, the identity function is used by default: the sequence elements themselves are their own sort keys.

The lessfun argument specifies the comparison function which determines the sorting order. It must be a binary function which can be invoked on pairs of keys as produced by the key function. It must return a non-nil value if the left argument is considered to be lesser than the right argument. For instance, if the numeric function < is used on numeric keys, it produces an ascending sorted order. If the function > is used, then a descending sort is produced.

The sort function is stable for sequences which are lists. This means that the original order of items which are considered identical is preserved. For strings and vectors, sort is not stable.

 

14.17.11 Function uniq

Syntax:


  (uniq
sequence)

Description:

The uniq function returns a sequence of the same kind as sequence, but with duplicates removed. Elements of sequence are considered equal under the equal function. The first occurrence of each element is retained, and the subsequent duplicates of that element, of any, are suppressed, such that the order of the elements is otherwise preserved.

 

14.17.12 Function tuples

Syntax:


  (tuples
length sequence [fill-value])

Description:

The tuples function produces a lazy list which represents a reorganization of the elements of sequence into tuples of length, where length must be a positive integer.

The length of the sequence might not be evenly divisible by the tuple length. In this case, if a fill-value argument is specified, then the last tuple is padded with enough repetitions of fill-value to make it have length elements. If fill-value is not specified, then the last tuple is left shorter than length.

The output of the function is a list, but the tuples themselves are sequences of the same kind as sequence. If sequence is any kind of list, they are lists, and not lazy lists.

Examples:


  (tuples 3 #(1 2 3 4 5 6 7 8) 0) -> (#(1 2 3) #(4 5 6) #(7 8 0))
  (tuples 3 "abc") -> ("abc")
  (tuples 3 "abcd") -> ("abc" "d")
  (tuples 3 "abcd" # -> ("abc" "dzz")
  (tuples 3 (list 1 2) # -> ((1 2 #)

 

14.17.13 Function partition-by

Syntax:


  (partition-by
function sequence)

Description:

If sequence is empty, then partition-by returns an empty list, and function is never called.

Otherwise, partition-by returns a lazy list of partitions of the sequence sequence. Partitions are consecutive, non-empty sub-strings of sequence, of the same kind as sequence.

The partitioning begins with the first element of sequence being placed into a partition.

The subsequent partitioning is done according to function, which is applied to each element of sequence. Whenever, for the next element, the function returns the same value as it returned for the previous element, the element is placed into the same partition. Otherwise, the next element is placed into, and begins, a new partition.

The return values of the calls to function are compared using the equal function.

Examples:


  [partition-by identity '(1 2 3 3 4 4 4 5)] -> ((1) (2) (3 3)
                                                 (4 4 4) (5))


  (partition-by (op = 3) #(1 2 3 4 5 6 7)) -> (#(1 2) #(3)
                                               #(4 5 6 7))

 

14.17.14 Function make-like

Syntax:


  (make-like
list ref-sequence)

Description:

The list argument must be a list. If ref-sequence is a sequence type, then list is converted to the same type of sequence and returned. Otherwise the original list is returned.

Note: the make-like function is a helper which supports the development of unoptimized versions of a generic function that accepts any type of sequence as input, and produces a sequence of the same type as output. The implementation of such a function can internally accumulate a list, and then convert the resulting list to the same type as an input value by using make-like.

 

14.17.15 Function nullify

Syntax:


  (nullify
sequence)

Description:

The nullify function returns nil if sequence is an empty sequence. Otherwise it returns sequence itself.

Note: the nullify function is a helper to support unoptimized generic programming over sequences. Thanks to the generic behavior of cdr, any sequence can be traversed using cdr functions, checking for the nil value as a terminator. This, however, breaks for empty sequences which are not lists, because they are not equal to nil: to car and cdr they look like a one-element sequence containing nil. The nullify function reduces all empty sequences to nil, thereby correcting the behavior of code which traverses sequences using cdr, and tests for termination with nil.

 

14.18 Math Library

 

14.18.1 Arithmetic functions + and -

Syntax:


  (+
number*)
  (-
number number*)
  (*
number*)

Description:

The +, - and * functions perform addition, subtraction and multiplication, respectively. Additionally, the - function performs additive inverse.

The + function requires zero or more arguments. When called with no arguments, it produces 0 (the identity element for addition), otherwise it produces the sum over all of the arguments.

Similarly, the * function requires zero or more arguments. When called with no arguments, it produces 1 (the identity element for multiplication). Otherwise it produces the product of all the arguments.

The semantics of - changes from subtraction to additive inverse when there is only one argument. The argument is treated as a subtrahend, against an implicit minuend of zero. When there are two or more argument, the first one is the minuend, and the remaining are subtrahends.

When there are three or more operands, these operations are performed as if by binary operations, in a left-associative way. That is to say, (+ a b c) means (+ (+ a b) c). The sum of a and b is computed first, and then this is added to c. Similarly (- a b c) means (- (- a b) c). First, b is subtracted from a, and then c is subtracted from that result.

The arithmetic inverse is performed as if it were subtraction from integer 0. That is, (- x) means the same thing as (- 0 x).

The operands of +, - and * can be characters, integers (fixnum and bignum), and floats, in nearly any combination.

If two operands have different types, then one of them is converted to the type of the one with the higher rank, according to this ranking: character < integer < float. For instance if one operand is integer, and the other float, the integer is converted to a float.

Restrictions:

Characters are not considered numbers, and participate in these operations in limited ways. Subtraction can be used to computed the displacement between the Unicode values of characters, and an integer displacement can be added to a character, or subtracted from a character. For instance (- #\9 #\0) is 9. The Unicode value of a character C can be found using (- C #\x0): the displacement from the NUL character.

The rules can be stated as a set of restrictions:

1
Two characters may not be added together.
2
A character may not be subtracted from an integer (which also rules out the possibility of computing the additive inverse of a character).
3
A character operand may not be opposite to a floating point operand in any operation.
4
A character may not be an operand of multiplication.

 

14.18.2 Functions /, trunc and mod

Syntax:


  (/ [
dividend] divisor)
  (trunc
dividend divisor)
  (mod
dividend divisor)

Description:

The arguments to these functions are numbers. Characters are not permitted.

The / function performs floating-point division. Each operands is first converted to floating-point type, if necessary. If dividend is omitted, then it is taken to be 1.0 and the function calculates the reciprocal.

The trunc function performs a division of dividend by divisor whose result is truncated to integer toward zero. If both operands are integers, then an integer division is performed and the result is an integer. If either operand is a floating point value, a floating point division occurs, and the result is truncated toward zero to a floating-point integral value.

The mod function performs a modulus operation. Firstly, the absolute value of divisor is taken to be a modulus. Then a residue of dividend with respect to modulus is calculated. The residue's sign follows that of the sign of divisor. That is, it is the smallest magnitude (closest to zero) residue of dividend with respect to the absolute value of divisor, having the same sign as divisor. If the operands are integer, the result is an integer. If either operand is of type float, then the result is a float. The modulus operation is then generalized into the floating point domain. For instance the expression (mod 0.75 0.5) yields a residue of 0.25 because 0.5 "goes into" 0.75 only once, with a "remainder" of 0.25.

 

14.18.3 Function gcd

Syntax:

Description:

The gcd function computes the greatest common divisor: the largest positive integer which divides both arguments.

Operands left and right must be integers, or else an exception is thrown.

The value of (gcd 0 x) is 0 for all x, including 0.

The value of (gcd x 123) is (abs x) for all x.

Negative operands are permitted; this operation effectively ignores sign, so that the value of (gcd x y) is the same as (gcd (abs x) (abs y)) for all x and y.

 

14.18.4 Function abs

Syntax:


  (abs
number)

Description:

The abs function computes the absolute value of number. If number is positive, it is returned. If number is negative, its additive inverse is returned: a positive number of the same type with exactly the same magnitude.

 

14.18.5 Functions floor and ceil

Syntax:


  (floor
number)
  (ceil
number)

Description:

The floor function returns the highest integer which does not exceed the value of number. The ceiling function returns the lowest integer which does not exceed the value of number.

If number an integer, it is simply returned.

If the argument is a float, then the value returned is a float. For instance (floor 1.1) returns 1.0 rather than 1.

 

14.18.6 Functions sin, cos, tan, asin, acos, atan and atan2

Syntax:


  (sin
radians)
  (cos
radians)
  (tan
radians)
  (atan
slope)
  (atan2
y x)
  (asin
num)
  (acos
num)

Description:

These trigonometric functions convert their argument to floating point and return a float result. The sin, cos and tan functions compute the sine and cosine and tangent of the radians argument which represents an angle expressed in radians. The atan, acos and asin are their respective inverse functions. The num argument to asin and acos must be in the range -1.0 to 1.0. The atan2 function converts the rectilinear coordinates x and y to an angle in polar coordinates in the range [0, 2π).

 

14.18.7 Functions exp, log, log10 and log2

Syntax:


  (exp
arg)
  (log
arg)
  (log10
arg)
  (log2
arg)

Description:

The exp function calculates the value of the transcendental number e raised to the exponent arg.

The log function calculates the base e logarithm of arg, which must be a positive value.

The log10 function calculates the base 10 logarithm of arg, which must be a positive value.

The log2 function calculates the base 2 logarithm of arg, which must be a positive value.

 

14.18.8 Functions expt, sqrt and isqrt

Syntax:


  (expt
base exponent*)
  (sqrt
arg)
  (isqrt
arg)

Description:

The expt function raises base to zero or more exponents given by the exponent arguments. (expt x) is equivalent to (expt x 1), and yields x for all x. For three or more arguments, the operation is right-associative. That is to say, (expt x y z) is equivalent to (expt x (expt y z)), similarly to the way nested exponents work in standard algebraic notation.

Exponentiation is done pairwise using a binary operation. If both operands to this binary operation are integers, then the result is an integer. If either operand is a float, then the other operand is converted to a float, and a floating point exponentiation is performed. Exponentiation that would produce a complex number is not supported.

The sqrt function produces a floating-point square root of arg, which is converted from integer to floating-point if necessary. Negative operands are not supported.

The isqrt function computes the integer square root of arg, which must be an integer. The integer square root is a value which is the greatest integer that is no greater than the real square root of arg. The input value must be an integer.

 

14.18.9 Function exptmod

Syntax:


  (exptmod
base exponent modulus)

Description:

The exptmod function performs modular exponentiation and accepts only integer arguments. Furthermore, exponent must be a non-negative and modulus must be positive.

The return value is base raised to exponent, and reduced to the least positive residue modulo modulus.

 

14.18.10 Function cum-norm-dist

Syntax:


  (cum-norm-dist
argument)

Description:

The cum-norm-dist function calculates an approximation to the cumulative normal distribution function: the integral, of the normal distribution function, from negative infinity to the argument.

 

14.18.11 Functions n-choose-k and n-perm-k

Syntax:


  (n-choose-k
n k)
  (n-perm-k
n k)

Description:

The n-choose-k function computes the binomial coefficient nCk which expresses the number of combinations of k items that can be chosen from a set of n, where combinations are subsets.

The n-perm-k function computes nPk: the number of permutations of size k that can be drawn from a set of n, where permutations are sequences, whose order is significant.

The calculations only make sense when n and k are nonnegative integers, and k does not exceed n. The behavior is not specified if these conditions are not met.

 

14.18.12 Functions fixnump, bignump, integerp, floatp and numberp

Syntax:


  (fixnump
object)
  (bignump
object)
  (integerp
object)
  (floatp
object)
  (numberp
object)

Description:

These functions test the type of object, returning t if it is an object of the implied type, nil otherwise. The fixnump, bignump and floatp functions return t if the object is of the basic type fixnum, bignum or float. The function integerp returns true of object is either a fixnum or a bignum. The function numberp returns t if object is either a fixnum, bignum or float.

 

14.18.13 Function zerop

Syntax:


  (zerop
number)

Description:

The zerop function tests number for equivalence to zero. The argument must be a number. It returns t for the integer value 0 and for the floating-point value 0.0. For other numbers, it returns nil.

 

14.18.14 Functions evenp and oddp

Syntax:


  (evenp
integer)
  (oddp
integer)

Description:

The evenp and oddp functions require integer arguments. evenp returns t if integer is even (divisible by two), otherwise it returns nil. oddp returns t if integer is not divisible by two (odd), otherwise it returns nil.

 

14.18.15 Functions >, <, >=, <= and =

Syntax:


  (>
number number*)
  (>
number number*)
  (>=
number number*)
  (<=
number number*)
  (=
number number*)

Description:

These relational functions compare characters and numbers for numeric equality or inequality. The arguments must be one or more numbers or characters.

If just one argument is given, then these functions all return t.

If two arguments are given then, they are compared as follows. First, if the numbers do not have the same type, then the one which has the lower ranking type is converted to the type of the other, according to this ranking: character < integer < float. For instance if a character and integer are compared, the character is converted to integer. Then a straightforward numeric comparison is applied.

Three or more arguments may be given, in which case the comparison proceeds pairwise from left to right. For instance in (< a b c), the comparison (< a b) is performed in isolation. If the comparison is false, then nil is returned, otherwise the comparison (< b c) is performed in isolation, and if that is false, nil is returned, otherwise t is returned. Note that it is possible for b to undergo two different conversions. For instance in the (< float character integer) comparison, character will first convert to a floating-point representation of its Unicode value so that it can be compared to float, and if that comparison succeeds, then in the second comparison, character will be converted to integer so that it can be compared to integer.

 

14.18.16 Function /=

Syntax:


  (/=
number*)

Description:

The arguments to /= may be numbers or characters. The /= function returns t if no two of its arguments are numerically equal. That is to say, if there exist some a and b which are distinct arguments such that (= a b) is true, then the function returns nil. Otherwise it returns t.

 

14.18.17 Functions max and min

Syntax:


  (max
first-arg arg*)
  (min
first-arg args*)

Description:

The max and min functions determine and return the highest or lowest value from among their arguments.

The arguments must be numbers or characters.

If only first-arg is given, that value is returned.

If two or more arguments are given, then (max a b) is equivalent to (if (>= a b) a b), and (min a b) is equivalent to (if (<= a b) a b). If the operands do not have the same type, then one of them is converted to the type of the other; however, the original unconverted values are returned. For instance (max 4 3.0) yields the integer 4, not 4.0.

If three or more arguments are given, max and min reduce the arguments in a left-associative manner. Thus (max a b c) means (max (max a b) c).

 

14.18.18 Functions int-str, flo-str and num-str

Syntax:


  (int-str
string [radix])
  (flo-str
string)
  (num-str
string)

Description:

These functions extract numeric values from character string string. Leading whitespace in string, if any, is skipped. If no digits can be successfully extracted, then nil is returned. Trailing material which does not contribute to the number is ignored.

The int-str function converts a string of digits in the specified radix to an integer value. If the radix isn't specified, it defaults to 10. For radices above 10, letters of the alphabet are used for digits: A represent a digit whose value is 10, B represents 11 and so forth until Z. For values of radix above 36, the returned value is unspecified. Upper and lower case letters are recognized. Any character which is not a digit of the specified radix is regarded as the start of trailing junk at which the extraction of the digits stops.

The flo-str function converts a floating-point decimal notation to a nearby floating point value. The material which contributes to the value is the longest match for optional leading space, followed by a mantissa which consists of an optional sign followed by a mixture of at least one digit, and at most one decimal point, optionally followed by an exponent part denoted by the letter E or e, an optional sign and one or more optional exponent digits.

The num-str function converts a decimal notation to either an integer as if by a radix 10 application of int-str, or to a floating point value as if by flo-str. The floating point interpretation is chosen if the possibly empty initial sequence of digits (following any whitespace and optional sign) is followed by a period, or by e or E.

 

14.18.19 Functions int-flo and flo-int

Syntax:


  (int-flo
float)
  (flo-int
integer)

Description:

These functions perform numeric conversion between integer and floating point type. The int-flo function returns an integer by truncating toward zero. The flo-int function returns an exact floating point value corresponding to integer, if possible, otherwise an approximation using a nearby floating point value.

 

14.18.20 Functions tofloat and toint

Syntax:


  (tofloat
value)
  (toint
value [radix])

Description:

These convenience functions convert value to floating-point or integer, respectively.

If a floating-point value is passed into tofloat, or an integer value into toint, then the value is simply returned.

If value is a string, then it is converted by tofloat as if by the function flo-str, , and by toint as if by the function int-str.

If value is an integer, then it is converted by tofloat as if by the function flo-int.

If value is a floating-point number, then it is converted by toint as if by the function int-flo.

 

14.18.21 Variables *flo-min*, *flo-max*, and *flo-epsilon*

Description:

These variables hold, respectively: the smallest positive floating-point value; the largest positive floating-point value; and the difference between 1.0 and the smallest representable value greater than 1.0.

*flo-min* and *flo-max* define the floating-point range, which consists of three regions: values from (- *flo-max*) to (- *flo-min*); the value 0.0, and values from *flo-min* to *flo-max*.

 

14.18.22 Variable *flo-dig*

Description:

This variable holds an integer representing the number of decimal digits in a decimal floating-point number such that this number can be converted to a TXR floating-point number, and back to decimal, without a change in any of the digits. This holds regardless of the value of the number, provided that it does not exceed the floating-point range.

 

14.18.23 Variables *pi* and *e*

Description:

These variables hold an approximation of the mathematical constants π and e. To four digits of precision, π is 3.142 and e is 2.718. The *pi* and *e* approximations are accurate to *flo-dig* decimal digits.

 

14.19 Bit Operations

In TXR Lisp, similarly to Common Lisp, bit operations on integers are based on a concept that might be called "infinite two's-complement". Under infinite two's complement, a positive number is regarded as having a binary representation prefixed by an infinite stream of zero digits (for example 1 is ...00001). A negative number in infinite two's complement is the biwise negation of its positive counterpart, plus one: it carries an infinite prefix of 1 digits. So for instance the number -1 is represented by ...11111111: an infinite sequence of 1 bits. There is no specific sign bit; any operation which produces such an infinite sequence of 1 digits on the left gives rise to a negative number. For instance, consider the operation of computing the bitwise complement of the number 1. Since the number 1 is represented as ...0000001, its complement is ...11111110. Each one of the digits in the infinite sequence is replaced by 1, And this leading sequence means that the number is negative, in fact corresponding to the two's-complement representation of the value -2. Hence, the infinite digit concept corresponds to an arithmetic interpretation.

In fact TXR Lisp's bignum integers do not use a two's complement representation internally. Numbers are represented as an array which holds a pure binary number. A separate field indicates the sign, positive or non-negative. That negative numbers appear as two's-complement under the bit operations is merely a carefully maintained illusion (which makes bit operations on negative numbers more expensive).

The logtrunc function, as well as a feature of the lognot function, allow bit manipulation code to be written which works with positive numbers only, even if complements are required. The trade off is that the application has to manage a limit on the number of bits.

 

14.19.1 Functions logand, logior, and logxor

Syntax:


  (logand
integer*)
  (logior
integer*)
  (logxor
int1 int2)

Description:

These operations perform the familiar bitwise and, inclusive or, and exclusive or operations, respectively. Positive values inputs are treated as pure binary numbers. Negative inputs are treated as infinite-bit two's-complement.

For example (logand -2 7) produces 6. This is because -2 is ...111110 in infinite-bit two's-complement. And-ing this value with 7 (or ...000111) produces 110.

The logand and logior functions are variadic, and may be called with zero, one, two, or more input values. If logand is called with no arguments, it produces the value -1 (all bits 1). If logior is called with no arguments it produces zero. In the one-argument case, the functions just return their argument value.

 

14.19.2 Function logtest

Syntax:


  (logtest
int1 int2)

Description:

The logtest function returns true if int1 and int2 have bits in common. The following equivalence holds:


  (logtest a b) <--> (not (zerop (logand a b)))

 

14.19.3 Functions lognot and logtrunc

Syntax:


  (lognot > value [
bits])
  (logtrunc > value
bits)

Description:

The lognot function performs a bitwise complement of value. When the one-argument form of lognot is used, then if value is nonnegative, then the result is negative, and vice versa, according to the infinite-bit two's complement representation. For instance (lognot -2) is 1, and (lognot 1) is -2.

The two-argument form of lognot produces a truncated complement. Conceptually, a bitwise complement is first calculated, and then the resulting number is truncated to the number of bits given by bits, which must be a nonnegative integer. The following equivalence holds:


  (lognot a b) <--> (logtrunc (lognot a) b)

The logtrunc function truncates the integer value to the specified number of bits. If value is negative, then the two's-complement representation is truncated. The return value of logtrunc is always a non-negative integer.

 

14.19.4 Function ash

Syntax:


  (ash
value bits)

Description:

The ash function shifts value by the specified number of bits producing a new value. If bits is positive, then a left shift takes place. If bits is negative, then a right shift takes place. If bit is zero, then value is returned unaltered. For positive numbers, a left shift by n bits is equivalent to a multiplication by two to the power of n, or (expt 2 n). A right shift by n bits of a positive integer is equivalent to integer division by (expt 2 n), with truncation toward zero. For negative numbers, the bit shift is performed as if on the two's-complement representation. Under the infinite two's-complement representation, a right shift does not exhaust the infinite sequence of 1 digits which extends to the left. Thus if -4 is shifted right it becomes -2 because the bitwise representations of these values are ...111100 and ...11110.

 

14.19.5 Function bit

Syntax:


  (bit
value bit)

Description:

The bit function tests whether the integer value has a 1 in bit position bit. The bit argument must be a non-negative integer. A value of zero of bit indicates the least significant bit position of value.

The bit function has a boolean result, returning the symbol t if bit bit of value is set, otherwise nil.

If value is negative, it is treated as if it had an infinite-bit two's complement representation. For instance, if value is -2, then the bit function returns nil for a bit value of zero, and t for all other values, since the infinite bit two's complement representation of -2 is ...11110.

 

14.19.6 Function mask

Syntax:


  (mask
integer*)

Description:

The mask function takes zero or more integer arguments, and produces an integer value which corresponds a bitmask made up of the bit positions specified by the integer values.

If mask is called with no arguments, then the return value is zero.

If mask is called with a single argument integer then the return value is the same as that of the expression (ash 1 <integer>): the value 1 shifted left by integer bit positions. If integer is zero, then the result is 1; if integer is 1, the result is 2 and so forth. If value is negative, then the result is zero.

If mask is called with two or more arguments, then the result is a bitwise of the masks individually computed for each of the values.

In other words, the following equivalences hold:


  (mask) <--> 0
  (mask a) <--> (ash 1 a)
  (mask a b c ...) <--> (logior (mask a) (mask b) (mask c) ...)

 

14.20 Exceptions

 

14.20.1 Functions throw, throwf and error

Syntax:


  (throw
symbol arg*)
  (throwf
symbol format-string format-arg*)
  (error
format-string format-arg*)

Description:

These functions generate an exception. The throw and throwf functions generate an exception identified by symbol, whereas error throws an exception of type error. The call (error ...) can be regarded as a shorthand for (throwf 'error ...).

The throw function takes zero or more additional arguments. These arguments become the arguments of a catch handler which takes the exception. The handler will have to be capable of accepting that number of arguments.

The throwf and error functions generate an exception which has a single argument: a character string created by a formatted print to a string stream using the format string and additional arguments.

 

14.20.2 Operator catch

Syntax:


  (catch
try-expression
     {(
symbol (arg*) body-form*)}*)

Description:

The catch operator establishes an exception catching block around the try-expression . The try-expression is followed by zero or more catch clauses. Each catch clause consists of a symbol which denotes an exception type, an argument list, and zero or more body forms.

If try-expression terminates normally, then the catch clauses are ignored. The catch itself terminates, and its return value is that of the try-expression.

If try-expression throws an exception which is a subtype of one or more of the type symbols given in the exception clauses, then the first (leftmost) such clause becomes the exit point where the exception is handled. The exception is converted into arguments for the clause, and the clause body is executed. When the clause body terminates, the catch terminates, and the return value of the catch is that of the clause body.

If try-expression throws an exception which is not a subtype of any of the symbols given in the clauses, then the search for an exit point for the exception continues through the enclosing forms. The catch clauses are not involved in the handling of that exception.

When a clause catches an exception, the number of arguments in the catch must match the number of elements in the exception. A catch argument list resembles a function or lambda argument list, and may be dotted. For instance the clause (foo (a . b)) catches an exception subtyped from foo, with one or more elements. The first element binds to parameter a, and the rest, if any, bind to parameter b. If there is only one element, b takes on the value nil.

Also see: the unwind-protect operator, and the functions throw, throwf and error.

 

14.21 Regular Expression Library

 

14.21.1 Functions search-regex and range-regex

Syntax:


  (search-regex
string regex [start [from-end]])
  (range-regex
string regex [start [from-end]])

Description:

The search-regex function searches through string starting at position start for a match for regex. If start is omitted, the search starts at position 0. If from-end is specified and has a non-nil value, the search proceeds in reverse, from the last position in the string, toward start. This function returns nil if no match is found, otherwise it returns a cons, whose car indicates the position of the match, and whose cdr indicates the length of the match.

The range-regex function is similar to search-regex, except that when a match is found, it returns a position range, rather than a position and length. A cons is returned whose car indicates the position of the match, and whose cdr indicates the position one element past the last character of the match. If the match is empty, the two integers are equal.

 

14.21.2 Function match-regex

Syntax:


  (match-regex
string regex [position])

Description:

The match-regex function tests whether regex matches at position in string. If position is not specified, it is taken to be zero. If the regex matches, then the length of the match is returned. If it does not match, then nil is returned.

 

14.21.3 Function match-regex-right

Syntax:


  (match-regex-right
string regex [end-position])

Description:

The match-regex function tests whether string contains a match which ends precisely on the character just before end-position. If end-position is not specified, it defaults to the length of the string, and the function performs a right-anchored regex match.

If a match is found, then the length of the match is returned, and the matching substring is then returned.

The match must terminate just before end-position in the sense that additional characters at end-position and beyond can no longer satisfy the regular expression. More formally, the function searches, starting from position zero, for positions where there occurs a match for the regular expression, taking the longest possible match. The length of first such a match which terminates on the character just before end-position is returned. If no such a match is found, then nil is returned.

Examples:


  ;; Return matching portion rather than length thereof.


  (defun match-regex-right-substring (str reg : end-pos)
    (set end-pos (or end-pos (length str)))
    (let ((len (match-regex-right str reg end-pos)))
      (if len
        [str (- end-pos len)..end-pos]
        nil)))


  (match-regex-right-substring "abc" #/c/) -> ""


  (match-regex-right-substring "acc" #/c*/) -> "cc"


  ;; Regex matches starting at multiple positions, but all
  ;; the matches extend past the limit.
  (match-regex-right-substring "acc" #/c*/ 2) -> nil


  ;; If the above behavior is not wanted, then
  ;; we can extract the string up to the limiting
  ;; position and do the match on that.
  (match-regex-right-substring ["acc" 0..2] #/c*/) -> "c"


  ;; Equivalent of above call
  (match-regex-right-substring "ac" #/c*/) -> "c"

 

14.21.4 Function regsub

Syntax:


  (regsub
regex replacement string)

Description:

The regsub function searches string for multiple occurrences of non-overlapping matches for regex. A new string is constructed similar to string but in which each matching region is replaced with using replacement as follows.

The replacement object may be a character or a string, in which case it is simply taken to be the replacement for each match of the regular expression.

The replacement object may be a function of one argument, in which case for every match which is found, this function is invoked, with the matching piece of text as an argument. The function's return value is then taken to be the replacement text.

Examples:


  ;; match every lower case e or o, and replace by filtering
  ;; through the upcase-str function:


  [regsub #/[eo]/ upcase-str "Hello world!"] -> "HEllO wOrld!"


  ;; Replace Hello with Goodbye:
  (regsub #/Hello/ "Goodbye" "Hello world!") -> "Goodbye world!"

 

14.21.5 Function regexp

Syntax:


  (regexp
obj)

Description:

The regexp function returns t if obj is a compiled regular expression object. For any other object type, it returns nil.

 

14.21.6 Function regex-compile

Syntax:


  (regex-compile
form-or-string [error-stream])

Description:

The regex-compile function takes the source code of a regular expression, expressed as a Lisp data structure representing an abstract syntax tree, or else a regular expression specified as a character string, and compiles it to a regular expression object.

If form-or-string is a character string, it is parsed to an abstract syntax tree first, if by the regex-parse function. If the parse is successful (the result is not nil) then the resulting tree structure is compiled by a recursive call to regex-compile.

The optional error-stream argument is passed down to regex-parse as well as in the recursive call to regex-compile, if that call takes place.

If error-stream is specified, it must be a stream. Any error diagnostics are sent to that stream.

Examples:


  ;; the equivalent of #/[a-zA-Z0-9_/
  (regex-compile '(set (#\a . #\z) (#\A . #\Z)
                       (#\0 . #\9) #\_))


  ;; the equivalent of #/.*/ and #/.+/
  (regex-compile '(0+ wild))
  (regex-compile '(1+ wild))


  ;; #/a|b|c/
  (regex-compile '(or (or #\a #\b) #\c))


  ;; string
  (regex-compile "a|b|c")

 

14.21.7 Function regex-parse

Syntax:


  (regex-parse
string [error-stream])

Description:

The regex-parse function parses a character string which contains a regular expression (without any surrounding / characters) and turns it into a Lisp data structure (the abstract syntax tree representation of the regular expression).

The regular expression syntax #/RE/ produces the same structure, but as a literal which is processed at the time TXR source code is read; the regex-parse function performs this parsing at run-time.

If there are parse errors, the function returns nil.

The optional error-stream argument specifies a stream to which error messages are sent from the parser. By default, diagnostic output goes to the *stdnull* stream, which discards it. If error-stream is specified as t, then the diagnostic output goes to the *stdout* stream.

If regex-parse returns a non-nil value, that structure is then something which is suitable as input to regex-compile.

 

14.22 Hashing Library

 

14.22.1 Functions make-hash, and hash

Syntax:


  (make-hash
weak-keys weak-vals equal-based)
  (hash { :weak-keys | :weak-vals | :equal-based }*)

Description:

These functions construct a new hash table.

A hash table is an object which retains an association between pairs of objects. Each pair consists of a key and value. Given an object which is similar to a key in the hash table, it is possible to retrieve the corresponding value. Entries in a hash table are not ordered in any way, and lookup is facilitated by hashing: quickly mapping a key object to a numeric value which is then used to index into one of many buckets where the matching key will be found (if such a key is present in the hash table).

make-hash takes three mandatory boolean arguments. The weak-keys argument specifies whether the hash table shall have weak keys. The weak-vals argument specifies whether it shall have weak values, and equal-based specifies whether it is equal-based. The hash function defaults all three of these properties to false, and allows them to be overridden to true by the presence of keyword arguments.

It is an error to attempt to construct an equal-based hash table which has weak keys.

The hash function provides an alternative interface. It accepts optional arguments which are keyword symbols. Any combination of the three symbols :weak-keys, :weak-vals and :equal-based can be specified in any order to turn on the corresponding properties in the newly constructed hash table. If any of the keywords is not specified, the corresponding property defaults to nil.

If a hash table has weak keys, this means that from the point of view of garbage collection, that table holds only weak references to the keys stored in it. Similarly, if a hash table has weak values, it means that it holds a weak reference to each value stored. A weak reference is one which does not prevent the reclamation of an object by the garbage collector. That is to say, when the garbage collector discovers that the only references to some object are weak references, then that object is considered garbage, just as if it had no references to it. The object is reclaimed, and the weak references "lapse" in some way, which depends on what kind they are. Hash table weak references lapse by entry removal: if either a key or a value object is reclaimed, then the corresponding key-value entry is erased from the hash table.

Important to the operation of a hash table is the criterion by which keys are considered same. By default, this similarity follows the eql function. A hash table will search for a stored key which is eql to the given search key. A hash table constructed with the equal-based property compares keys using the equal function instead.

In addition to storing key-value pairs, a hash table can have a piece of information associated with it, called the user data.

 

14.22.2 Function hash-construct

Syntax:


  (hash-construct
hash-args key-val-pairs)

Description:

The hash-construct function constructs a populated hash in one step. The hash-args argument specifies a list suitable as an argument list in a call to the hash function. The key-val-pairs is a sequence of pairs, which are two-element lists representing key-value pairs.

A hash is constructed as if by a call to [apply hash hash-args], then populated with the specified pairs, and returned.

 

14.22.3 Function hash-update

Syntax:


  (hash-update
hash function)

Description:

The hash-update function replaces each values in hash, with the value of function applied to that value.

The return value is hash.

 

14.22.4 Function hash-update-1

Syntax:


  (hash-update-1
hash key function [init])

Description:

The hash-update-1 function operates on a single entry in the hash table.

If key exists in the hash table, then its corresponding value is passed into function, and the return value of function is then installed in place of the key's value. The value is then returned.

If key does not exist in the hash table, and no init argument is given, then hash-update-1 does nothing and returns nil.

If key does not exist in the hash table, and an init argument is given, then function is applied to init, and then key is inserted into hash with the value returned by function as the datum. This value is also returned.

 

14.22.5 Function group-by

Syntax:


  (group-by
func sequence option*)

Description:

The group-by function produces a hash table from sequence, which is a list or vector. Entries of the hash table are not elements of sequence, but lists of elements of sequence. The function func is applied to each element of sequence to compute a key. That key is used to determine which list the item is added to in the hash table.

The trailing arguments option* if any, consist of the same keywords that are understood by the hash function, and determine the properties of the hash.

Example:

Group the integers from 0 to 10 into three buckets keyed on 0, 1 and 2 according to the modulo 3 congruence:


  (group-by (op mod @1 3) (range 0 10)))


  -> #H(() (0 (0 3 6 9)) (1 (1 4 7 10)) (2 (2 5 8)))

 

14.22.6 Functions make-similar-hash and copy-hash

Syntax:


  (make-similar-hash
hash)
  (copy-hash
hash)

Description:

The make-similar-hash and copy-hash functions create a new hash object based on the existing hash object.

make-similar-hash produces an empty hash table which inherits all of the attributes of hash. It uses the same kind of key equality, the same configuration of weak keys and values, and has the same user data (see the set-hash-userdata function).

The copy-hash function is like make-similar-hash, except that instead of producing an empty hash table, it produces one which has all the same elements as hash: it contains the same key and value objects.

 

14.22.7 Function inhash

Syntax:


  (inhash
hash key [init])

Description:

The inhash function searches hash table hash for key. If key is found, then it return the hash table's cons cell which represents the association between hash and key. Otherwise, it returns nil.

If argument init is specified, then the function will create an entry for key in hash whose value is that of init. The cons cell representing that association is returned.

Note: for as long as the key continues to exist inside hash. modifying the car field of the returned cons has ramifications for the logical integrity of the hash. Modifying the cdr field has the effect of updating the association with a new value.

 

14.22.8 Function gethash

Syntax:


  (gethash
hash key [alt])

Description:

The gethash function searches hash table hash for key key. If the key is found then the associated value is returned. Otherwise, if the alt argument was specified, it is returned. If the alt argument was not specified, nil is returned.

 

14.22.9 Function sethash

Syntax:


  (sethash
hash key value)

Description:

The sethash function places a value into hash table under the given key. If a similar key already exists in the hash table, then that key's value is replaced by value. Otherwise, the key and value pair is newly inserted into hash.

 

14.22.10 Function pushhash

Syntax:


  (pushhash
hash key element)

Description:

The pushhash function is useful when the values stored in a hash table are lists. If the given key does not already exist in hash, then a list of length one is made which contains element, and stored in hash table under key. If the key already exists in the hash table, then the corresponding value must be a list. The element value is added to the front of that list, and the extended list then becomes the new value under key.

 

14.22.11 Function remhash

Syntax:


  (remhash
hash key)

Description:

The remhash function searches hash for a key similar to the key. If that key is found, then that key and its corresponding value are removed from the hash table.

 

14.22.12 Function hash-count

Syntax:


  (hash-count
hash)

Description:

The hash-count function returns an integer representing the number of key-value pairs stored in hash.

 

14.22.13 Function get-hash-userdata

Syntax:


  (get-hash-userdata
hash)

Description:

This function retrieves the user data object associated with hash. The user data object of a newly-created hash table is initialized to nil.

 

14.22.14 Function set-hash-userdata

Syntax:


  (set-hash-userdata
hash object)

Description:

The set-hash-userdata replaces, with the object, the user data object associated with hash.

 

14.22.15 Function hashp

Syntax:


  (hashp
object)

Description:

The hashp function returns t if the object is a hash table, otherwise it returns nil.

 

14.22.16 Function maphash

Syntax:


  (maphash
hash binary-function)

Description:

The maphash function successively invokes binary-function for each entry stored in hash. Each entry's key and value are passed as arguments to binary-function.

The function returns nil.

 

14.22.17 Functions hash-eql and hash-equal

Syntax:


  (hash-eql
object)
  (hash-equal
object)

Description:

These functions each compute an integer hash value from the internal representation of object, which satisfies the following properties. If two objects A and B are the same under the eql function, then (hash-eql A) and (hash-eql B) produce the same integer hash value. Similarly, if two objects A and B are the same under the equal function, then (hash-equal A) and (hash-equal B) each produce the same integer hash value. In all other circumstances, the hash values of two distinct objects are unrelated, and may or may not be the same.

 

14.22.18 Functions hash_keys, hash_values, hash_pairs and hash_alist

Syntax:


  (hash-keys
hash)
  (hash-values
hash)
  (hash-pairs
hash)
  (hash-alist
hash)

Description:

These functions retrieve the bulk key-value data of hash table hash in various ways. hash-keys retrieves a list of the keys. hash-values retrieves a list of the values. hash-pairs retrieves a list of pairs, which are two-element lists consisting of the key, followed by the value. Finally, hash-pairs retrieves the key-value pairs as a Lisp association list: a list of cons cells whose car fields are keys, and whose cdr fields are the values.

These functions all retrieve the keys and values in the same order. For example, if the keys are retrieved with hash-keys, and the values with hash-values, then the corresponding entries from each list pairwise correspond to the pairs in hash.

 

14.22.19 Operator dohash

Syntax:


  (dohash (
key-var value-var hash-form [result-form])
    
body-form*)

Description:

The dohash operator iterates over a hash table. The hash-form expression must evaluate to an object of hash table type. The key-var and value-var arguments must be symbols suitable for use as variable names. Bindings are established for these variables over the scope of the body-forms and the optional result-form.

For each element in the hash table, the key-var and value-var variables are set to the key and value of that entry, respectively, and each body-form, if there are any, is evaluated.

When all of the entries of the table are thus processed, the result-form is evaluated, and its return value becomes the return value of the dohash form. If there is no result-form, the return value is nil.

The result-form and body-forms are in the scope of an implicit anonymous block, which means that it is possible to terminate the execution of dohash early using (return value) or (return).

 

14.22.20 Functions hash-uni, hash-diff and hash-isec

Syntax:


  (hash-uni
hash1 hash2 [join-func])
  (hash-diff
hash1 hash2)
  (hash-isec
hash1 hash2 [join-func])

Description:

These functions perform basic set operations on hash tables in a nondestructive way, returning a new hash table without altering the inputs. The arguments hash1 and hash2 must be compatible hash tables. This means that their keys must use the same kind of equality.

The resulting hash table inherits attributes from hash1, as if created by the make-similar-hash function. If hash1 has userdata, the resulting hash table has the same userdata. If hash1 has weak keys, the resulting table has weak keys, and so forth.

The hash-uni function performs a set union. The resulting hash contains all of the keys from hash1 and all of the keys from hash2, and their corresponding values. If a key occurs both in hash1 and hash2, then it occurs only once in the resulting hash. In this case, if the join-func argument is not given, the value associated with this key is the one from hash1. If join-func is specified then it is called with two arguments: the respective data items from hash1 and hash2. The return value of this function is used as the value in the union hash.

The hash-diff function performs a set difference. First, a copy of hash1 is made as if by the copy-hash function. Then from this copy, all keys which occur in hash2 are deleted.

The hash-isec function performs a set intersection. The resulting hash contains only those keys which occur both in hash1 and hash2. If join-func is not specified, the values selected for these common keys are those from hash1. If join-func is specified, then for each key which occurs in both hash1 and hash2, it is called with two arguments: the respective data items. The return value is then used as the data item in the intersection hash.

 

14.23 Partial Evaluation and Combinators

 

14.23.1 Macros op and do

Syntax:


  (op
form+)
  (do
form+)

Description:

The op and do macro operators are similar.

Like the lambda operator, the op operator creates an anonymous function based on its syntax. The difference is that the arguments of the function are implicit, or optionally specified within the function body, rather than as a formal parameter list before the body.

Also, the form arguments of op are implicitly turned into a DWIM expression, which means that argument evaluation follows Lisp-1 rules. (See the dwim operator).

The. do operator is like the op operator with the following difference: the form arguments of op are not implicitly treated as a DWIM expression, but as an ordinary expression. In particular, this means that operator syntax is permitted. Note that the syntax (op @1) makes sense, since the argument can be a function, which will be invoked, but (do @1) doesn't make sense because it will produce a Lisp-2 form like (#:arg1 ...) referring to nonexistent function #:arg1. Because it accepts operators, do can be used with imperative constructs which are not functions, like set: like set: for instance (do set x) produces an anonymous function which, if called with one argument, stores that argument into x.

The argument forms are arbitrary expressions, within which a special convention is permitted:

@num
A number preceded by a @ is a metanumber. This is a special syntax which denotes an argument. For instance @2 means that the second argument of the anonymous function is to be substituted in place of the @2. op generates a function which has a number of required arguments equal to the highest value of num apearing in a construct in the body. For instance (op car @3) generates a three-argument function (which passes its third argument to car, returning the result, and ignores its first two arguments). There is no way to use op to generate functions which have optional arguments.
@rest
If the meta-symbol @rest appears in the op syntax, it explicitly denotes the list of trailing arguments, allowing them to be placed anywhere in the expression.

Functions generated by op are always variadic; they always take additional arguments after any required ones, whether or not the @rest syntax is used.

If the body does not contain any @num or @rest syntax, then @rest is implicitly inserted. What this means is that, for example, since the form (op foo) does not contain any numeric positional arguments like @1, and does not contain @rest, it is actually a shorthand for (op foo . @rest): a function which applies all of its arguments to foo.

The actions of op be understood by these examples, which show how op is rewritten to lambda. However, note that the real translator uses generated symbols for the arguments, which are not equal to any symbols in the program.


  (op) -> invalid


  (op +) -> (lambda rest [+ . rest])


  (op + foo) -> (lambda rest [+ foo . rest])


  (op @1 @2) -> (lambda (arg1 arg2 . rest) [arg1 arg2])


  (op @1 . @rest) -> (lambda (arg1 . rest) [arg1 . @rest])


  (op @1 @rest) -> (lambda (arg1 . rest) [arg1 @rest])


  (op @1 @2) -> (lambda (arg1 arg2 . rest) [arg1 arg2])


  (op foo @1 (@2) (bar @3)) -> (lambda (arg1 arg2 arg3 . rest)
                                  [foo arg1 (arg2) (bar arg3)])


  (op foo @rest @1) -> (lambda (arg1 . rest) [foo rest arg1])


  (do + foo) -> (lambda rest (+ foo . rest))


  (do @1 @2) -> (lambda (arg1 arg2 . rest) (arg1 arg2))


  (do foo @rest @1) -> (lambda (arg1 . rest) (foo rest arg1))

Note that if argument @n appears, it is not necessary for arguments @1 through @n-1 to appear. The function will have n arguments:


  (op @3) -> (lambda (arg1 arg2 arg3 . rest) [arg3])

The op and do operators can be nested, in any combination. This raises the question: if a metanumber like @1 or @rest occurs in an op that is nested within an op , what is themeaning?

A metanumber always belongs with the inner-most op or do operator. So for instance (op (op @1)) means that an (op @1) expression is nested within an op expression which itself contains no meta-syntax. The @1 belongs with the inner op.

There is a way for an inner op to refer to an outer op metanumber argument. This is expresed by adding an extra @ prefix for every level of escape. For example in (op (op @@1)) the @@1 belongs to the outer op: it is the same as @1 appearing in the outer op. That is to say, in the expression (op @1 (op @@1)), the @1 and @@1 are the same thing: both are parameter 1 of the lambda function generated by the outer op. By contrast, in the expression (op @1 (op @1)) there are two different parameters: the first @1 is argument of the outer function, and the second @1 is the first argument of the inner function. Of course, if there are three levels of nesting, then three @ meta-prefixes are needed to insert a parameter from the outermost op into the innermost op.

Examples:


  ;; Take a list of pairs and produce a list in which those pairs
  ;; are reversed.


  (mapcar (op list @2 @1) '((1 2) (a b)))   ->   ((2 1) (b a))

 

14.23.2 Macros ap, ip, ado and ido.

Syntax:


  (ap
form+)
  (ip
form+)
  (ado
form+)
  (ido
form+)

Description:

The ap macro is based on the op macro and has identical argument conventions.

The ap macro analyzes its arguments and produces a function f, in exactly the same same way as the op macro. However, instead of returning f, directly, it returns a different function g, which is a one-argument function which accepts a list, and then applies the list as arguments to f.

In other words, the following equivalence holds:


  (ap form ...) <--> (apf (op form ...))

The ap macro nests properly with op and do, in any combination, in regard to the ...@@n notation.

The macro is very similar to the ap macro, except that it is based on the semantics of the function iapply rather than apply, according to the following equivalence:


  (ip form ...) <--> (ipf (op form ...))

The ado and ido macros are related to do macro in the same way that ap and ip are related to op. They produce a one-argument function which works as if by applying its arguments to the function generated by do, according to the following equivalence:


  (ado form ...) <--> (apf (do form ...))


  (ido form ...) <--> (ipf (do form ...))

See also: the apf and ipf functions.

 

14.23.3 Macro ret

Syntax:


  (ret
form)

Description:

The ret macro's form argument is treated similarly to the second and subsequent arguments of the op operator.

The ret macro produces a function which takes any number of arguments, and returns the value specified by form.

form can contain op meta syntax like @n and @rest.

The following equivalence holds:


  (ret x) <--> (op identity x))

Thus the expression (ret @2) returns a function similar to (lambda (x y . z) y), and the expression (ret 42) returns a function similar to (lambda (. rest) 42).

 

14.23.4 Function chain

Syntax:


  (chain
func*)

Description:

The chain function accepts zero or more functions as arguments, and returns a single function, called the chained function, which represents the chained application of those functions, in left to right order.

If chain is given no arguments, then it returns a variadic function which ignores all of its arguments and returns nil.

Otherwise, the first function may accept any number of arguments. The second and subsequent functions, if any, must accept one argument.

The chained function can be called with an argument list which is acceptable to the first function. Those arguments are in fact passed to the first function. The return value of that call is then passed to the second function, and the return value of that call is passed to the third function and so on. The final return value is returned to the caller.

Example:


  (call [chain + (op * 2)] 3 4) -> 14

In this example, a two-element chain is formed from the + function and the function produced by (op * 2) which is a one-argument function that returns the value of its argument multiplied by two. (See the definition of the op operator).

The chained function is invoked using the call function, with the arguments 3 and 4. The chained evaluation begins by passing 3 and 4 to +, which yields 7. This 7 is then passed to the (op * 2) doubling function, resulting in 14.

A way to write the above example without the use of the DWIM brackets and the op operator is this:


  (call (chain (fun +) (lambda (x) (* 2 x))) 3 4)

 

14.23.5 Function juxt

Syntax:


  (juxt
func*)

Description:

The juxt function accepts a variable number of arguments which are functions. It combines these into a single function which, when invoked, passes its arguments to each of these functions, and collects the results into a list.

Note: the juxt function can be understood in terms of the following reference implementation:


  (defun juxt (funcs)
    (lambda (. args)
      (mapcar (lambda (fun)
                (apply fun args))
              funcs)))

Example:


   ;; separate list (1 2 3 4 5 6) into lists of evens and odds,
   ;; which end up juxtaposed in the output list:


   [(op [juxt keep-if remove-if] evenp)
    '(1 2 3 4 5 6)] -> ((2 4 6) (1 3 5))


   ;; call several functions on 1, collecting their results:
   [[juxt (op + 1) (op - 1) evenp sin cos] 1]'
   -> (2 0 nil 0.841470984807897 0.54030230586814)

 

14.23.6 Functions andf and orf

Syntax:


  (andf
func*)
  (orf
func*)

Description:

The andf and orf functions are the functional equivalent of the and and or operators. These functions accept multiple functions and return a new function which represents the logical combination of those functions.

The input functions should have the same arity. Failing that, there should exist some common argument arity with which each of these can be invoked. The resulting combined function is then callable with that many arguments.

The andf function returns a function which combines the input functions with a short-circuiting logical conjunction. The resulting function passes its arguments to the functions successively, in left to right order. As soon as any of the functions returns nil, then nil is returned immediately, and the remaining functions are not called. Otherwise, if none of the functions return nil, then the value returned by the last function is returned. If the list of functions is empty, then t is returned. That is, (andf) returns a function which accepts any arguments, and returns t.

The orf function combines the input functions with a short-circuiting logical disjunction. The function produced by orf passes its arguments down to the functions successively, in left to right order. As soon as any function returns a non-nil value, that value is returned and the remaining functions are not called. If all functions return nil, then nil is returned. The expression (orf) returns a function which accepts any arguments and returns nil.

 

14.23.7 Functions iff and iffi

Syntax:


  (iff
cond-func then-func [else-func])
  (iffi
cond-func then-func [else-func])

Description:

The iff function is the functional equivalent of the if operator. It accepts functional arguments and returns a function.

The resulting function takes its arguments and applies them to cond-func. If cond-func yields true, then the arguments are passed to then-func and the resulting value is returned. Otherwise the arguments are passed to else-func and the resulting value is returned.

If then-func needs to be called, but is nil, then nil is returned immediately. Likewise, if else-func needs to be called, but is omitted or nil, then nil is returned.

The iffi function differs from iff only in the defaulting behavior with respect to the else-func argument. The following equivalences hold:


  (iffi a b c)   <--> (iff a b c)


  (iffi a b)     <--> (iff a b identity)


  (iffi a b nil) <--> (iff a b identity)

The iffi function defaults to the identity function when else-func is omitted or nil, and therefore is useful in situations when one value is to be replaced with another one when the condition is true, otherwise preserved.

 

14.23.8 Functions tf and nilf

Syntax:


  (tf
arg*)
  (nilf
arg*)

Description:

The tf and nilf functions take zero or more arguments, and ignore them. The tf function returns t, and the nilf function returns nil.

Note: the following equivalences hold between these functions and the ret operator, and retf function.


  (fun tf) <--> (ret t) <--> (retf t)
  (fun nilf) <--> (ret nil) <--> (ret) <--> (retf nil)

In Lisp-1-style code, tf and nilf behave like constants which can replace uses of (ret t) and (ret nil):


  [mapcar (ret nil) list] <--> [mapcar nilf list]

Example:


  ;; tf and nilf are useful when functions are chained together.
  ;; test whether (trunc n 2) is odd.


  (defun trunc-n-2-odd (n)
    [[chain (op trunc @1 2) [iff oddp tf nilf]] n)

In this example, two functions are chained together, and n is passed through the chain such that it is first divided by two via the function denoted by (op trunc @1 2) and then the result is passed into the function denoted by [iff oddp tf nilf]. The iff function passes its argument into oddp, and if oddp yields true, it passes the same argument to tf. Here tf proves its utility by ignoring that value and returning t. If the argument (the divided value) passed into iff is even, then iff passes it into the nilf function, which ignores the value and returns nil.

 

14.23.9 Function retf

Syntax:


  (retf
value)

Description:

The retf function returns a function. That function can take zero or more arguments. When called, it ignores its arguments and returns value.

See also: the ret macro.

Example:


  ;; the function returned by (retf 42)
  ;; ignores 1 2 3 and returns 42.
  (call (retf 42) 1 2 3) -> 42

 

14.23.10 Functions apf and ipf

Syntax:


  (apf
function)
  (ipf
function)

Description:

The apf function returns a one-argument function which accepts a list. When the function is called, it treats the list as argument which are applied to function as if by apply. It returns whatever function returns.

The ipf function is similar to apf, except that the returned function applies arguments as if by iapply rather than apply.

See also: the ap macro.

Example:


  ;; Function returned by [apf +] accepts the
  ;; (1 2 3) list and applies it to +, as
  ;; if (+ 1 2 3) were called.


  (call [apf +] '(1 2 3)) -> 6

 

14.24 Input and Output (Streams)

TXR Lisp supports input and output streams of various kinds, with generic operations that work across the stream types.

In general, I/O errors are usually turned into exceptions. When the description of error reporting is omitted from the description of a function, it can be assumed that it throws an error.

 

14.24.1 Special variables *stdout*, *stddebug*, *stdin*, *stderr* and *stdnull*

Description:

These variables hold predefined stream objects. The *stdin*, *stdout* and *stderr* streams closely correspond to the underlying operating system streams. Various I/O functions require stream objects as arguments.

The *stddebug* stream goes to the same destination as *stdout*, but is a separate object which can be redirected independently, allowing debugging output to be separated from normal output.

The *stdnull* stream is a special kind of stream called a null stream. This stream is not connected to any device or file. It is similar to the /dev/null device on Unix, but does not involve the operating system.

 

14.24.2 Function format

Syntax:


  (format
stream-designator format-string format-arg*)

Description:

The format function performs output to a stream given by stream-designator, by interpreting the actions implicit in a format-string, incorporating material pulled from additional arguments given by format-arg*. Though the function is simple to invoke, there is complexity in format string language, which is documented below.

The stream-designator argument can be a stream object, or one of the values t or nil. The value t serves as a shorthand for *stdout*. The value nil means that the function will send output into a newly instantiated string output stream, and then return the resulting string.

Format string syntax:

Within format-string, most characters represent themselves. Those characters are simply output. The character ~ (tilde) introduces formatting directives, which are denoted by a single character, usually a letter.

The special sequence ~~ (tilde-tilde) encodes a single tilde. Nothing is permitted between the two tildes.

The syntax of a directive is generally as follows:


  ~[ [
width ] [ ,precision ] ] letter

The letter is a single alphabetic character which determines the general action of the directive. The optional width and precision can be numeric digits, or special codes documented below.

width
The width specifier consists of an optional < (left angle bracket) character, followed by an optional width specification.

If the leading < character is present, then the printing will be left-adjusted within this field. Otherwise it will be right-adjusted by default.

The width can be specified as a decimal integer, or as the character *. The * notation means that instead of digits, the value of the next argument is consumed, and expected to be an integer which specifies the width. If that integer value is negative, then the field will be left-adjusted. If the value is positive, but the < character is present in the width specifier, then the field is left adjusted.

precision
The precision specifier is introduced by a leading comma. If this comma appaers immediately after the directive's ~ character, then it means that width is being omitted; there is only a precision field.

The precision specifier may begin with these optional characters:

0
(the "leading zero flag"),
+
(print a sign for positive values")
space
(print a space in place of a positive sign).

The precision specifier itself is either a decimal integer that does not begin with a zero digit, or the * character.

The precision field's components have a meaning which depends on the type of object printed and the conversion specifier.

For integer arguments, the precision value specifies the minimum number of digits to print. If the precision field has a leading zero flag, then the integer is padded with zeros to the required number of digits, otherwise the number is padded with spaces instead of zeros. If zero or space padding is present, and a leading positive or negative sign must be printed, then it is placed before leading zeros, or after leading spaces, as the case may be.

For floating-point values, the meaning of the precision value depends on which specific conversion specifier (f, e, a or s) is used. The details are documented in the description of each of these, below. The leading zero flag is ignored for floating-point values regardless of the conversion specifier.

For integer or floating-point arguments, if the precision specifier has a + sign among the special characters, then a + sign is printed for positive numbers. If the precision specifier has a leading space instead of a + sign, then the sign is rendered as a space for positive numbers. If there is no leading space or +, then a sign character is omitted for positive numbers. Negative numbers are unconditionally prefixed with a - sign.

For all other objects, the precision specifies the maximum number of characters to print. The object's printed representation is crudely truncated at that number of characters.

Format directives:

Format directives are case sensitive, so that for example ~x and ~X have a different effect, and ~A doesn't exist whereas ~a does. They are:

a
Prints any object in an aesthetic way, as if by the pprint function. The aesthetic notation violates read-print consistency: this notation is not necessarily readable if it is implanted in TXR source code. The field width specifier is honored, including the left-right adjustment semantics.

When this specifier is used for floating-point values, the precision specifies the maximum number of total significant figures, which do not include any digits in the exponent, if one is printed. Numbers are printed in exponential notation if their magnitude is small, or else if their exponent exceeds their precision. (If the precision is not specified, then it defaults to the system-dependent number of digits in a floating point value, derived from the C language DBL_DIG constant.) Floating point values which are integers are printed without a trailing .0 (point zero).

s
Prints any object in a standard way, as if by the print function. Objects for which read-print consistency is possible are printed in a way such that if their notation is implanted in TXR source, they are readable. The field width specifier is honored, including the left-right adjustment semantics. The precision field is treated very similarly to the ~a format directive, except that non-exponentiated floating point numbers that would be mistaken for integers include a trailing .0 for the sake of read-print consistency. Objects truncated by precision may not have read-print consistency. For instance, if a string object is truncated, it loses its trailing closing quote, so that the resulting representation is no longer a properly formed string object.

x
Requires an argument of character or integer type. The integer value or character code is printed in hexadecimal, using lower-case letters for the digits a through f. Width and precision semantics are as described for the a format directive, for integers.

X
Like the x directive, but the hexadecimal digits a through f are rendered in upper case.

o
Like the x directive, but octal is used instead of hexadecimal.

f
The f directive prints numbers in a fixed point decimal notation, with a fixed number of digits after the decimal point. It requires a numeric argument. (Unlike x, X and o, it does not allow an argument of character type). The precision specifier gives the number of digits past the decimal point. The number is rounded off to the specified precision, if necessary. Furthermore, that many digits are always printed, regardless of the actual precision of the number or its type. If it is omitted, then the default value is three: three digits past the decimal point. A precision of zero means no digits pas the decimal point, and in this case the decimal point is suppressed (regardless of whether the numeric argument is floating-point or integer).

e
The e directive prints numbers in exponential notation. It requires a numeric argument. (Unlike x, X and o, it does not allow an argument of character type). The precision specifier gives the number of digits past the decimal point printed in the exponential notation, not counting the digits in the exponent. Exactly that many digits are printed, regardless of the precision of the number. If the precision is omitted, then the number of digits after the decimal point is three. If the precision is zero, then a decimal portion is truncated off entirely, including the decimal point.

 

14.24.3 Functions print, pprint, prinl, pprinl, tostring and tostringp

Syntax:


  (print
obj [stream])
  (pprint
obj [stream])
  (prinl
obj [stream])
  (pprinl
obj [stream])
  (tostring
obj)
  (tostringp
obj)

Description:

The print and pprint functions render a printed character representation of the obj argument into stream. If a stream argument is not supplied, then the destination is the stream currently stored in the *stdout* variable. The print function renders in a way which strives for read-print consistency: an object is printed in a notation which is recognized as a similar object of the same kind when it appears in TXR source code. The pprint function ("pretty print") does not strive for read-print consistency. For instance it prints a string object simply by dumping its characters, rather than by adding the surrounding quotes and rendering escape syntax for special characters. Both functions return obj.

The prinl and pprinl functions are like print and pprint, except that they issue a newline character after printing the object. These functions also return obj.

The tostring and tostringp functions are like print and pprint, but they do not accept a stream argument. Instead they print to a freshly instantiated string stream, and return the resulting string.

The following equivalences hold between calls to the format function and calls to the above functions:


  (format stream "~s" obj)  <-->  (print obj stream)
  (format t "~s" obj)       <-->  (print obj)
  (format t "~s\n" obj)     <-->  (prinl obj)
  (format nil "~s" obj)     <-->  (tostring obj)

For pprint, tostringp and pprinl, the equivalence is produced by using ~a in format rather than ~s.

Note: for characters, the print function behaves as follows: most control characters in the Unicode C0 and C1 range are rendered using the #\x notation, using two hex digits. Codes in the range D800 to DFFF, and the codes FFFE and FFFF are printed in the #\xNNNN with four hexadecimal digits, and character above this range are printed using the same notation, but with six hexadecimal digits. Certain characters in the C0 range are printed using their names such as #\nul and #\return, which are documented in the Character Literals section. The DC00 character is printed as #\pnul. All other characters are printed as #\char where char is the actual character.

Caution: read-print consistency is affected by trailing material. If additional digits are printed immediately after a number without intervening whitespace, they extend that number. If hex digits are printed after the character x, which is rendered as #\x, they look like a hex character code.

 

14.24.4 Function streamp

Syntax:


  (streamp
obj)

Description:

The streamp function returns t if obj is any type of stream. Otherwise it returns nil.

 

14.24.5 Function real-time-stream-p

Syntax:


  (real-time-stream-p
obj)

Description:

The real-time-streamp-p function returns t if obj is a stream marked as "real-time". If obj is not a stream, or not a stream marked as "real-time", then it returns nil.

Only certain kinds of streams accept the real-time attribute: file streams and tail streams. This attribute controls the semantics of the application of lazy-stream-cons to the stream. For a real-time stream, lazy-stream-cons returns a stream with "naive" semantics which returns data as soon as it is available, at the cost of generating spurious nil item when the stream terminates. The application has to recognize and discard that nil item. The ordinary lazy streams read ahead by one line and suppress this extra item, so their representation is more accurate.

Streams connected to TTY devices (devices which for which the isatty POSIX function reports true) are automatically marked as real-time. This is only supported on platforms that have an isatty function.

 

14.24.6 Function make-string-input-stream

Syntax:


  (make-string-input-stream
string)

Description:

This function produces an input stream object. Character read operations on the stream object read successive characters from string. Output operations and byte operations are not supported.

 

14.24.7 Function make-string-byte-input-stream

Syntax:


  (make-string-byte-input-stream
string)

Description:

This function produces an input stream object. Byte read operations on this stream object read successive byte values obtained by encoding string into UTF-8. Character read operations are not supported, and neither are output operations.

 

14.24.8 Function make-string-output-stream

Syntax:


  (make-string-output-stream)

Description:

This function, which takes no arguments, creates a string output stream. Data sent to this stream is accumulated into a string object. String output streams supports both character and byte output operations. Bytes are assumed to represent a UTF-8 encoding, and are decoded in order to form characters which are stored into the string.

If an incomplete UTF-8 code is output, and a character output operation then takes place, that code is assumed to be terminated and is decoded as invalid bytes. The UTF-8 decoding machine is reset and ready for the start of a new code.

The get-string-from-stream function is used to retrieve the accumulated string.

If the null character is written to a string output stream, either via a character output operation or as a byte operation, the resulting string will appear to be prematurely terminated. TXR strings cannot contain null bytes.

 

14.24.9 Function get-string-from-stream

Syntax:


  (get-string-from-stream
stream)

Description:

The stream argument must be a string output stream. This function finalizes the data sent to the stream and retrieves the accumulated character string.

If a partial UTF-8 code has been written to stream, and then this function is called, the byte stream is considered complete and the partial code is decoded as invalid bytes.

After this function is called, further output on the stream is not possible.

 

14.24.10 Function make-strlist-output-stream

Syntax:


  (make-strlist-output-stream)

Description:

This function is very similar to make-string-output-stream. However, the stream object produced by this function does not produce a string, but a list of strings. The data is broken into multiple strings by newline characters written to the stream. Newline characters do not appear in the string list. Also, byte output operations are not supported.

 

14.24.11 Function get-list-from-stream

Syntax:


  (get-list-from-stream
stream)

Description:

This function returns the string list which has accumulated inside a string output stream given by stream. The string output stream is finalized, so that further output is no longer possible.

 

14.24.12 Function close-stream

Syntax:


  (close-stream
stream [throw-on-error-p])

Description:

The close-stream function performs a close operation on stream, whose meaning is depends on the type of the stream. For some types of streams, such as string streams, it does nothing. For streams which are connected to operating system files or devices, will perform a close of the underlying file descriptor, and dissociate that descriptor from the stream. Any buffered data is flushed first.

close-stream returns a boolean true value if the close has occurred without errors, otherwise nil.

For most streams, "without errors" means that any buffered output data is flushed successfully.

For command and process pipes (see open-command and open-process), success also means that the process terminates normally, with a successful error code, or an unsuccessful one. An abnormal termination is considered an error, as as is the inability to retrieve the termination status, as well as the situation that the process continues running in spite of the close attempt. Detecting these situations is platform specific.

If the throw-on-error-p argument is specified, and isn't nil, then the function throws an exception if an error occurs during the close operation instead of returning nil.

 

14.24.13 Functions get-line, get-char and get-byte

Syntax:


  (get-line [
stream])
  (get-char [
stream])
  (get-byte [
stream])

Description:

These fundamental stream functions perform input. The stream argument is optional. If it is specified, it should be an input stream which supports the given operation. If it is not specified, then the *stdin* stream is used.

The get-char function pulls a character from a stream which supports character input. Streams which support character input also support the get-line function which extracts a line of text delimited by the end of the stream or a newline character and returns it as a string. (The newline character does not appear in the string which is returned).

Character input from streams based on bytes requires UTF-8 decoding, so that get-char actually may read several bytes from the underlying low level operating system stream.

The get-byte function bypasses UTF-8 decoding and reads raw bytes from any stream which supports byte input. Bytes are represented as integer values in the range 0 to 255.

Note that if a stream supports both byte input and character input, then mixing the two operations will interfere with the UTF-8 decoding.

These functions return nil when the end of data is reached. Errors are represented as exceptions.

See also: get-lines

 

14.24.14 Function get-string

Syntax:


  (get-string [
stream [count]])

Description:

The get-string function reads characters from a stream, and assembles them into a string, which is returned. If the stream argument is omitted, then the *stdin* stream is used.

If the count argument is missing, then all of the characters from the stream are read and assembled into a string.

If present, the count argument should be a positive integer indicating a limit on how many characters to read. The returned string will be no longer than count, but may be shorter.

 

14.24.15 Functions unget-char and unget-byte

Syntax:


  (unget-char
char [stream])
  (unget-byte
byte [stream])

Description:

These functions put back, into a stream, a character or byte which was previously read. The character or byte must match the one which was most recently read. If the stream argument is omitted, then the *stdin* stream is used.

If the operation succeeds, the byte or character value is returned. A nil return indicates that the operation is unsupported.

Some streams do not support these operations; some support only one of them. In general, if a stream supports get-char, it supports unget-char, and likewise for get-byte and unget-byte.

Space is available for only one character or byte of pushback.

Pushing both a byte and a character, in either order, is also unsupported. Pushing a byte and then reading a character, or pushing a character and reading a byte, are unsupported mixtures of operations.

If the stream is binary, then pushing back a byte decrements its position, except if the position is already zero. At that point, the position becomes indeterminate.

 

14.24.16 Functions put-string, put-line, put-char and put-byte

Syntax:


  (put-string
string [stream])
  (put-line [
string [stream]])
  (put-char
char [stream])
  (put-byte
byte [stream])

Description:

These functions perform output on an output stream. The stream argument must be an output stream which supports the given operation. If it is omitted, then *stdout* is used.

The put-char function writes a character given by char to a stream. If the stream is based on bytes, then the character is encoded into UTF-8 and multiple bytes are written. Streams which support put-char also support put-line, and put-string.

The put-string function writes the characters of a string out to the stream as if by multiple calls to put-char. The string argument may be a symbol, in which case its name is used as the string.

The put-line function is like put-string, but also writes an additional newline character. The string is optional in put-line, and defaults to the empty string.

The put-byte function writes a raw byte given by the byte argument to stream, if stream supports a byte write operation. The byte value is specified as an integer value in the range 0 to 255.

 

14.24.17 Functions put-strings and put-lines

Syntax:


  (put-strings
sequence [stream]])
  (put-lines
sequence [stream]])

Description:

These functions assume sequence to be a sequence of strings, or of symbols, or a mixture thereof. These strings are sent to the stream. The stream argument must be an output stream. If it is omitted, then *stdout* is used.

The put-strings function iterates over sequence and writes each element to the stream as if using the put-string function.

The put-lines function iterates over sequence and writes each element to the stream as if using the put-line function.

Both functions return t.

 

14.24.18 Function flush-stream

Syntax:


  (flush-stream
stream)

Description:

This function is meaningful for output streams which accumulate data which is passed on to the operating system in larger transfer units. Calling flush-stream causes all accumulated data inside stream to be passed to the operating system. If called on streams for which this function is not meaningful, it does nothing.

 

14.24.19 Function seek-stream

Syntax:


  (seek-stream
stream offset whence)

Description:

The seek-stream function is meaningful for file streams. It changes the current read/write position within stream. It can also be used to determine the current position: see the notes about the return value below.

The offset argument is a positive or negative integer which gives a displacement that is measured from the point identified by the whence argument.

Note that for text files, there isn't necessarily a 1:1 correspondence between characters and positions due to line-ending conversions and conversions to and from UTF-8.

The whence argument is one of three keywords: :from-start, :from-current and :from-end. These denote the start of the file, the current position and the end of the file.

If offset is zero, and whence is :from-current, then seek-stream returns the current absolute position within the stream, if it can successfully obtain it. Otherwise, it returns t if it is successful.

If a character has been successfully put back into a text stream with unget-char and is still pending, then the position value is unspecified. If a byte has been put back into a binary stream with unget-byte, and the previous position wasn't zero, then the position is decremented by one.

On failure, it throws an exception of type stream-error.

 

14.24.20 Functions stream-get-prop and stream-set-prop

Syntax:


  (stream-get-prop
stream indicator)
  (stream-set-prop
stream indicator value)

Description:

These functions get and set properties on a stream. Only certain properties are meaningful with certain kinds of streams, and the meaning depends on the stream. If two or more stream types support a property of the same name, it is expected that the property has the same or very similar meaning for both streams to the maximum extent that similarity is possible.

The stream-set-prop function sets a property on a stream. The indicator argument is a symbol, usually a keyword symbol, denoting the property, and value is the property value. If the stream understands and accepts the property, the function returns t. Otherwise it returns nil.

The stream-get-prop function inquires about the value of a property on a stream. If the stream understands the property, then it returns its current value. If the stream does not understand a property, nil is returned, which is also returned if the property exists, but its value happens to be nil.

Properties are currently used for marking certain streams as "real-time" (see the stream-real-time-p function above), and also for setting the priority at which messages are reported to syslog by the *stdlog* stream (see *stdlog* in the UNIX SYSLOG section).

If stream is a catenated stream (see the function make-catenated-stream) then these functions transparently operate on the current head stream of the catenation.

 

14.24.21 Function make-catenated-stream

Syntax:


  (make-catenated-stream
stream*)

Description:

The make-catenated-stream function takes zero or more arguments which are input streams of the same type.

A catenated stream does not support seeking operations or output, regardless of the capabilities of the streams in the list.

If the stream list is not empty, then the leftmost element of the list is called the head stream.

The get-char, get-byte, get-line, unget-char and unget-byte functions delegate to the corresponding operations on the head stream, if it exists. If the stream list is empty, they return nil to the caller.

If the get-char, get-byte or get-line operation on the head stream yields nil, and there are more lists in the stream, then the stream is removed from the list, and the next stream, if any, becomes the head list. The operation is then tried again. If any of these operations fail on the last list, it is not removed from the list, so that a stream remains in place which can take the unget-char or unget-byte operations.

In this manner, the catenated streams appear to be a single stream.

Note that the operations can fail due to being unsupported. It is the caller's responsibility to make sure all of the streams in the list are compatible with the intended operations.

If the stream list is empty then an empty catenated stream is produced. Input operations on this stream yield nil, and the unget-char and unget-byte operations throw an exception.

 

14.24.22 Functions open-files and open-files*

Syntax:


  (open-files
path-list [alternative-stream])
  (open-files*
path-list [alternative-stream])

Description:

The open-files and open-files* functions create a list of streams by invoking the open-file function on each element of path-list. These streams are turned into a catenated stream as if applied as arguments to make-catenated-stream.

The effect is that multiple files appear to be catenated together into a single input stream.

If the optional alternative-stream argument is supplied, then if path-list is empty, alternative-stream is returned instead of an empty catenated stream.

The difference between open-files and open-files* is that open-files creates all of the streams up-front. So if any of the paths cannot be opened, the operation throws. The open-files* variant is lazy: it creates a lazy list of streams out of the path list. The streams are opened as needed: before the second stream is opened, the program has to read the first stream to the end, and so on.

Example:

Collect lines from all files that are given as arguments on the command line. If there are no files, then read from standard input:


   @(next @(open-files *args* *stdin*))
   @(collect)
   @line
   @(end)

 

14.24.23 Function abs-path-p

Syntax:


  (abs-path-p
path)

Description:

The abs-path-function tests whether the argument path is an absolute path, returning a t or nil indication.

An absolute path is a string which either begins with a slash or backslash character, or which begins with an alphanumeric word, followed by a colon, followed by a slash or backslash.

Examples of absolute paths:


  /etc
  c:/tmp
  ftp://user@server
  disk0:/home
  Z:\Users

Examples of strings which are not absolute paths.


  
(the empty string)
  .
  abc
  foo:bar/x
  $:\abc

 

14.24.24 Function read

Syntax:


  (read [
source [error-stream [error-return-value]]])

Description:

The read function converts text denoting TXR Lisp structure, into the corresponding data structure. The source argument may be either a character string, or a stream. If it is omitted, then *stdin* is used as the stream.

The source must provide the text representation of one complete TXR Lisp object.

Multiple calls to read on the same stream will extract successive objects from the stream. To parse successive objects from a string, it is necessary to convert it to a string stream.

The optional error-stream argument can be used to specify a stream to which parse errors diagnostics are sent. If absent, the diagnostics are suppressed.

If there are no parse errors, the function returns the parsed data structure. If there are parse errors, and the error-return-value parameter is present, its value is returned. If the error-return-value parameter is not present, then an exception of type syntax-error is thrown.

 

14.25 Filesystem Access

 

14.25.1 Function stat

Syntax:


  (stat
path)

Description:

The stat function inquires the filesystem about the existence of an object denoted by the string path. If the object is not found or cannot be accessed, an exception is thrown.

Otherwise, information is retrieved about the object. The information takes the form of a property list in which keyword symbols denote numerous properties. An example such property list is:


  (:dev 2049 :ino 669944 :mode 16832 :nlink 23
   :uid 500 :gid 500 :rdev 0
   :size 12288 :blksize 4096 :blocks 24
   :atime 1347933533 :mtime 1347933534 :ctime 1347933534)

These properties correspond to the similarly-named entries of the struct stat structure in POSIX. For instance, the :dev property has the same value as the st_dev field.

 

14.25.2 Special variables s-ifmt, s-iflnk, s-ifreg, s-ifblk, ... , s-ixoth

The following variables exist, having integer values. These are bitmasks which can be applied against the value given by the :mode property in the property list returned by the function stat: s-ifmt, s-ifsock, s-iflnk, s-ifreg, s-ifblk, s-ifdir, s-ifchr, s-ififo, s-isuid, s-isgid, s-isvtx, s-irwxu, s-irusr, s-iwusr, s-ixusr, s-irwxg, s-irgrp, s-iwgrp, s-ixgrp, s-irwxo, s-iroth, s-iwoth and s-ixoth.

These variables correspond to the C language constants from POSIX: S_IFMT, S_IFLNK, S_IFREG and so forth.

The logtest function can be used to test these against values of mode. For example (logtest mode s-irgrp) tests for the group read permission.

 

14.25.3 Function open-directory

Syntax:


  (open-directory
path)

Description:

The open-directory function tries to create a stream which reads the directory given by the string argument path. If a filesystem object exists under the path, is accessible, and is a directory, then the function returns a stream. Otherwise, a file error exception is thrown.

The resulting stream supports the get-line operation. Each call to the get-line operation retrieves a string representing the next directory entry. The value nil is returned when there are no more directory entries. The . and .. entries in Unix filesystems are not skipped.

 

14.25.4 Function open-file

Syntax:


  (open-file
path [mode-string])

Description:

The open-file function creates a stream connected to the file which is located at the given path, which is a string.

The mode-string argument is a string which uses the same conventions as the mode argument of the C language fopen function. The mode string determines whether the stream is an input stream or output stream. Note that the "b" mode is not supported. Whether a stream is text or binary depends on which operations are invoked on it.

If the mode-string argument is omitted, mode "r" is used.

 

14.25.5 Function open-tail

Syntax:


  (open-tail
path [[mode-string] seek-to-end-p])

Description:

The open-tail function creates a tail stream connected to the file which is located at the given path. The mode-string argument is a string which uses the same conventions as the mode argument of the C language fopen function. If it is missing, it defaults to "r". Note that the "b" mode is not supported. Whether a stream is text or binary depends on which operations are invoked on it.

The seek-to-end-p argument is a boolean which determines whether the initial read/write position is at the start of the file, or just past the end. It defaults to nil. This argument only makes a difference if the file exists at the time open-tail is called. If the file does not exist, and is later created, then the tail stream will follow that file from the beginning. In other words, seek-to-end-p controls whether the tail stream reads all the existing data in the file, if any, or whether it reads only newly added data from approximately the time the stream is created.

A tail stream has special semantics with regard to reading at the end of file. A tail stream never reports an end-of-file condition; instead it polls the file until more data is added. Furthermore, if the file is truncated, or replaced with a smaller file, the tail stream follows this change: it automatically opens the smaller file and starts reading from the beginning (the seek-to-end-p flag only applies to the initial open). In this manner, a tail stream can dynamically growing rotating log files.

Caveat: since a tail stream can re-open a new file which has the same name as the original file, it behave incorrectly if the program changes the current working directory, and the path name is relative.

 

14.25.6 Function remove-path

Syntax:


  (remove-path
path)

Description:

The remove-path function tries to remove the filesystem object named by path, which may be a file, directory or something else.

If successful, it returns t.

A failure to remove the object results in an exception of type file-error.

 

14.25.7 Function rename-path

Syntax:


  (rename-path
from-path to-path)

Description:

The remove-path function tries to rename filesystem path from-path, which may refer to a file, directory or something else, to the path to-path.

If successful, it returns t.

A failure results in an exception of type file-error.

 

14.26 Coprocesses

 

14.26.1 Functions open-command and open-process

Syntax:


  (open-command
system-command [mode-string])
  (open-process
program mode-string [argument-string*])

Description:

These functions spawn external programs which execute concurrently with the TXR program. Both functions return a unidirectional stream for communicating with these programs: either an output stream, or an input stream, depending on the contents of mode-string.

In open-command, the mode-string argument is optional, defaulting to the value "r" if it is missing.

The open-command function accepts, via the system-command string parameter, a system command, which is in a system-dependent syntax. On a POSIX system, this would be in the POSIX Shell Command Language.

The open-process function specifies a program to invoke via the command argument. This is subject to the operating system's search strategy. On POSIX systems, if it is an absolute or relative path, it is treated as such, but if it is a simple base name, then it is subject to searching via the components of the PATH environment variable. If open-process is not able to find program, or is otherwise unable to execute the program, the child process will exit, using the value of the C variable errno as its exit status. This value can be retrieved via close-stream.

The mode-string argument follows the convention used by the POSIX popen function.

The argument-strings argument is a list of strings which specifies additional optional arguments to be passed passed to the program. The program argument becomes the first argument, and argument-string become the second and subsequent arguments. If argument-strings is omitted, it defaults to empty.

If a coprocess is open for writing (mode-string is specified as "w"), then writing on the returned stream feeds input to that program's standard input file descriptor. Indicating the end of input is performed by closing the stream.

If a coprocess is open for reading (mode-string is specified as "r"), then the program's output can be gathered by reading from the returned stream. When the program finishes output, it will close the stream, which can be detected as normal end of data.

If a coprocess terminates abnormally or unsuccessfully, an exception is raised.

 

14.27 Symbols and Packages

A package is an object which serves as a container of symbols.

A symbol which exists inside a package is said to be interned in that package. A symbol can be interned in at most one package at a time.

Eacy symbol has a name, which is a string. It is not necessarily unique: two distinct symbols can have the sama name. However, a symbol name is unique within a package, because it serves as the key which associates the symbol with the package. Two symbols cannot be in the same package if they have the same name. Moreover, a symbol cannot exist in more than one package at at time, although it can be relocated from one package to another. A symbols exist which is not entered into any package: such a symbol is called "uninterned".

Packages are held in a global list which can be used to search for a package by name. The find-package function performs this lookup. A package may be deleted from the list with the delete-package function, but it continues to exist until the program loses the last reference to that package.

 

14.27.1 Special variables *user-package*, *keyword-package*, and *system-package*

Description:

These variables hold predefined packages. The *user-package* is the one in which symbols are read when a TXR program is being scanned. The *keyword-package* holds keyword symbols, which are printed with a leading colon. The *system-package* is for internal symbols, helping the implementation avoid name clashes with user code in some situations.

 

14.27.2 Function make-sym

Syntax:


  (make-sym
name)

Description:

The make-sym function creates and returns a new symbol object. The argument name, which must be a string, specifies the name of the symbol. The symbol does not belong to any package (it is said to be "uninterned").

Note: an uninterned symbol can be interned into a package with the rehome-sym function. Also see the intern function.

 

14.27.3 Function gensym

Syntax:


  (gensym [
prefix])

Description:

The gensym function is similar to make-sym. It creates and returns a new symbol object. If the prefix argument is omitted, it defaults to "g". Otherwise it must be a string.

The difference between gensym and make-sym is that gensym creates the name by combining the prefix with a numeric suffix.

The numeric suffix is a decimal digit string, taken from the value of the variable *gensym-counter*, after incrementing it.

Note: the variation in name is not the basis of the uniqueness assurance offered by make-sym and gensym; the basis is that the returned symbol is a freshly instantiated object. make-sym still returns unique symbols even if repeatedly called with the same string.

 

14.27.4 Special variable *gensym-counter*

Description:

This variable is initialized to 0. Each time the gensym function is called, it is incremented. The incremented value forms the basis of the numeric suffix which gensym uses to form the name of the new symbol.

 

14.27.5 Function make-package

Syntax:


  (make-package
name)

Description:

The make-package function creates and returns a package named name, where name is a string. It is an error if a package by that name exists already.

 

14.27.6 Function packagep

Syntax:


  (packagep
obj)

Description:

The packagep function returns if obj is a package, otherwise it returns nil.

 

14.27.7 Function find-package

Syntax:


  (find-package
name)

Description:

The argument name should be a string. If a package called name exists, then it is returned. Otherwise nil is returned.

 

14.27.8 Function intern

Syntax:


  (intern
name [package])

Description:

The argument name should be a symbol. The optional argument package should be a package. If package is not supplied, then the value taken is that of *user-package*.

The intern function searches package for a symbol called name. If that symbol is found, it is returned. If that symbol is not found, then a new symbol called name is created and inserted into package, and that symbol is returned. In this case, the package becomes the symbol's home package.

 

14.27.9 Function rehome-sym

Syntax:


  (rehome-sym
symbol [package])

Description:

The arguments symbol and package must be a symbol and package object, respectively. If package is not given, then it defaults to the value of *user-package*.

The rehome-sym function moves symbol into package. If symbol is already in a package, it is first removed from that package. If a symbol of the same name exists in package that symbol is first removed from package.

 

14.27.10 Function symbolp

Syntax:


  (symbolp
obj)

Description:

The symbolp function returns t if obj is a symbol, otherwise it returns nil.

 

14.27.11 Function symbol-name

Syntax:


  (symbol-name
symbol)

Description:

The symbol-name function returns the name of symbol.

 

14.27.12 Function symbol-package

Syntax:


  (symbol-package
symbol)

Description:

The symbol-package function returns the home package of symbol.

 

14.27.13 Function packagep

Syntax:


  (packagep
obj)

Description:

The packagep function returns t if obj is a package, otherwise it returns nil.

 

14.27.14 Function keywordp

Syntax:


  (keywordp
obj)

Description:

The keywordp function returns t if obj is a keyword symbol, otherwise it returns nil.

 

14.28 Pseudo-random Numbers

 

14.28.1 Special variable *random-state*

Description:

The *random-state* variable holds an object which encapsulates the state of a pseudo-random number generator. This variable is the default argument value for the random-fixnum and random functions, for the convenience of writing programs which are not concerned about the management of random state.

On the other hand, programs can create and manage random states, making it possible to obtain repeatable sequences of pseudo-random numbers which do not interfere with each other. For instance objects or modules in a program can have their own independent streams of random numbers which are repeatable, independently of other modules making calls to the random number functions.

When TXR starts up, the *random-state* variable is initialized with a newly created random state object, which is produced as if by the call (make-random-state 42).

 

14.28.2 Function make-random-state

Syntax:


  (make-random-state [
seed])

Description:

The make-random-state function creates and returns a new random state, an object of the same kind as what is stored in the *random-state* variable.

The seed, if specified, must be an integer value.

Note that the sign of the seed is ignored, so that negative seed values are equivalent to their additive inverses.

If the seed is not specified, then make-random-state produces a seed based on some information in the process environment, such as current time of day. It is not guaranteed that two calls to (make-random-state nil) that are separated by less than some minimum increment of real time produce different seeds. The minimum time increment depends on the platform.

On a platform with a millisecond-resolution real-time clock, the minimum time increment is a millisecond. Calls to make-random-state less than a millisecond apart may predictably produce the same seed.

 

14.28.3 Function random-state-p

Syntax:


  (random-state-p
obj)

Description:

The random-state-p function returns t if obj is a random state, otherwise it returns nil.

 

14.28.4 Functions random-fixnum, random and rand

Syntax:


  (random-fixnum [
random-state])
  (random
random-state modulus)
  (rand
modulus [random-state])

Description:

All three functions produce pseudo-random numbers, which are positive integers.

The numbers are obtained from a WELLS 512 pseudo-random number generator, whose state is stored in the random state object.

The random-fixnum function produces a random fixnum integer: a reduced range integer which fits into a value that does not have to be heap-allocated.

The random and rand functions produce a value in the range [0, modulus). They differ only in the order of arguments. In the rand function, the random state object is the second argument and is optional. If it is omitted, the global *random-state* object is used.

 

14.29 Time

 

14.29.1 Functions time and time-usec

Syntax:


  (time)
  (time-usec)

Description:

The time function returns the number of seconds that have elapsed since midnight, January 1, 1970, in the UTC timezone.

The time-usec function returns a cons cell whose car field holds the seconds measured in the same way, and whose cdr field extends the precision by giving number of microseconds as an integer value between 0 and 999999.

 

14.29.2 Functions time-string-local and time-string-utc

Syntax:


  (time-string-local
time format)
  (time-string-utc
time format)

Description:

These functions take the numeric time returned by the time function, and convert it to a textual representation in a flexible way, according to the contents of the format string.

The time-string-local function converts the time to the local timezone of the host system. The time-string-utc function produces time in UTC.

The format argument is a string, and follows exactly the same conventions as the format string of the C library function strftime.

The time argument is an integer representing seconds obtained from the time function or from the car field of the cons returned by the time-usec function.

 

14.29.3 Functions time-fields-local and time-fields-utc

Syntax:


  (time-fields-local
time)
  (time-fields-utc
time)

Description:

These functions take the numeric time returned by the time function, and convert it to a list of seven fields.

The time-string-local function converts the time to the local timezone of the host system. The time-string-utc function produces time in UTC.

The fields returned as a list consist of six integers, and a boolean value. The six integers represent the year, month, day, hour, minute and second. The boolean value indicates whether daylight savings time is in effect (always nil in the case of time-fields-utc).

The time argument is an integer representing seconds obtained from the time function or from the time-usec function.

 

14.29.4 Functions make-time and make-time-utc

Syntax:


  (make-time
year month day hour minute second dst-advice)
  (make-time-utc
year month day hour minute second dst-advice)

Description:

The make-time function returns a time value, similar to the one returned by the time function. The time value is constructed not from the system clock, but from a date and time specified as arguments. The year argument is a calendar year, like 2014. The month argument ranges from 1 to 12. The hour argument is a 24-hour time, ranging from 0 to 23. These arguments represent a local time, in the current time zone.

The dst-advice argument specifies whether the time is expressed in daylight savings time (DST). It takes on three possible values: nil, the keyword :auto, or else the symbol t. Any other value has the same interpretation as t.

If dst-advice is t, then the time is assumed to be expressed in DST. If the argument is nil, then the time is assumed not to be in DST. If dst-advice is :auto, then the function tries to determine whether DST is in effect in the current time zone for the specified date and time.

The make-time-utc function is similar to make-time, except that it treats the time as UTC rather than in the local time zone. The dst-advice argument is supported by make-time-utc for function call compatibility with make-time. It may or may not have any effect on the output (since the UTC zone by definition doesn't have daylight savings time).

 

14.30 Environment Variables and Command Line

 

14.30.1 Special variables *args* and *args-full*

Description:

The *args* variable holds a list of strings representing the remaining arguments which follow any options processed by the TXR executable, and the script name.

The *args-full* variable holds the original, complete list of arguments passed from the operating system.

Note: the *args* variable is nil during the processing of the command line, so TXR Lisp expressions invoked using the -p or -e option cannot use it.

 

14.30.2 Function env

Syntax:


  (env)

Description:

The env function retrieves the list of environment variables. Each variable is represented by a single entry in the list: a string which contains an = (equal) character somewhere, separating the variable name from its value.

See also: the env-hash function.

 

14.30.3 Function env-hash

Syntax:


  (env-hash)

Description:

The env-hash function constructs and returns an :equal-based hash. The hash is populated with the environment variables, represented as key-value pairs.

 

14.31 Unix Programming

 

14.31.1 Function errno

Syntax:


  (errno [
new-errno])

Description:

The errno function retrieves the current value of the C library error variable errno. If the argument new-errno is present and is not nil, then it specifies a value which is stored into errno. The value returned is the prior value.

 

14.31.2 Function exit

Syntax:


  (exit
status)

Description:

The exit function terminates the entire process (running TXR image), specifying the termination status to the operating system. Values of status may be nil, t, or an integer value. The value nil corresponds to the C constant EXIT_FAILURE, and t corresponds to EXIT_SUCCESS. These are platform-independent indicators of failed or successful termination. The numeric value 0 also indicates success.

 

14.31.3 Function usleep

Syntax:


  (usleep
usec)

Description:

The usleep function suspends the execution of the program for at least usec microseconds.

The return value is t if the sleep was successfully executed. A nil value indicates premature wakeup or complete failure.

Note: the actual sleep resolution is not guaranteed, and depends on granularity of the system timer. Actual sleep times may be rounded up to the nearest 10 millisecond multiple on a system where timed suspensions are triggered by a 100 Hz tick.

 

14.31.4 Function mkdir

Syntax:


  (mkdir
path [mode])

Description:

mkdir tries to create the directory named path using the POSIX mkdir function. An exception of type file-error is thrown if the function fails. Returns t on success.

The mode argument specifies the request numeric permissions for the newly created directory. If omitted, the requested permissions are #o777 (511): readable and writable to everyone. The requested permissions are subject to the system umask.

 

14.31.5 Function chdir

Syntax:


  (chdir
path)

Description:

chdir changes the current working directory to path, and returns t, or else throws an exception of type file-error.

 

14.31.6 Function pwd

Syntax:


  (pwd)

Description:

The pwd function retrieves the current working directory. If the underlying getcwd C library function fails with an errno other than ERANGE, an exception will be thrown.

 

14.31.7 Functions getpid and getppid

Syntax:


  (getpid)
  (getppid)

Description:

These functions retrieve the current proces ID and the parent process ID respectively. They are wrappers for the POSIX functions getpid and getppid.

 

14.31.8 Function daemon

Syntax:


  (daemon
nochdir-p noclose-p)

Description:

This is a wrapper for the function daemon which originated in BSD Unix.

It returns t if successful, nil otherwise, and the errno variable is set in that case.

 

14.31.9 Functions makedev, minor and major

Syntax:


  (makedev
minor major)
  (minor
dev)
  (major
dev)

Description:

The parameters minor, major and dev are all integers. The makedev function constructs a combined device number from a minor and major pair (by calling the Unix makedev function). This device number is suitable as an argument to the mknod function (see below). Device numbers also appear the :dev property returned by the stat function.

The minor and major functions extract the minor and major device number from a combined device number.

 

14.31.10 Function mknod

Syntax:


  (mknod
path mode [dev])

Description:

The mknod function tries to create an entry in the filesystem: a file, FIFO, or a device special file, under the name path. If it is successful, it returns t, otherwise it throws an exception of type file-error.

The mode argument is a bitwise or combination of the requested permissions, and the type of object to create: one of the constants s-ifreg, s-ififo, s-ifchr, s-ifblk or s-ifsock. The permissions are subject to the system umask.

If a block or character special device (s-ifchr or s-ifblk) is being created, then the dev argument specifies the major and minor numbers of the device. A suitable value can be constructed from a major and minor pair using the makedev function.

Example:


   ;; make a character device (8, 3) called /dev/foo
   ;; requesting rwx------ permissions


   (mknod "dev/foo" (logior #o700 s-ifchr) (makedev 8 3))

 

14.31.11 Functions symlink and link

Syntax:


  (symlink
target path)
  (link
target path)

Description:

The symlink function creates a symbolic link called path whose contents are the absolute or relative path target. target does not actually have to exist.

The link function creates a hard link. The object at target is installed into the filesystem at path also.

If these functions succeed, they return t. Otherwise they throw an exception of type file-error.

 

14.31.12 Function readlink

Syntax:


  (readlink
path)

Description:

If path names a filesystem object which is a symbolic link, the readlink function reads the contents of that symbolic link and returns it as a string. Otherwise, it fails by throwing an exception of type file-error.

 

14.31.13 Functions sh and run

Syntax:


  (sh
system-command)
  (run
program [argument-list])

Description:

The sh function executes system-command using the system command interpreter. The run function spawns a program, searching for it using the system PATH. Using either method, the executed process receives environment variables from the parent.

TXR blocks until the process finishes executing. If the program terminates normally, then its integer exit status is returned. The value zero indicates successful termination.

The return value nil indicates an abnormal termination, or the inability to run the process at all.

In the case of the run function, if the child process is created successfully but the program cannot be executed, then the exit status will be an errno value from the failed exec attempt.

 

14.32 Unix Signal Handling

On platforms where certain advanced features of POSIX signal handling are available at the C API level, TXR exposes signal-handling functionality.

A TXR program can install a TXR Lisp function (such as an anonymous. lambda, or the function object associated with a named function) as the handler for a signal.

When that signal is delivered, TXR will intercept it with its own safe, internal handler, mark the signal as deferred (in a TXR sense) and then dispatch the registered function at a convenient time.

Handlers currently are not permitted to interrupt the execution of most TXR internal code. Immediate, asynchronous execution of handlers is currently enabled only while TXR is blocked on I/O operations or sleeping. Additionally, the sig-check function can be used to dispatch and clear deferred signals. These handlers are then safely called if they were subroutines of sig-check, and not asynchronous interrupts.

 

14.32.1 Special variables sig-hup, sig-int, sig-quit, sig-ill, sig-trap, sig-abrt, sig-bus, sig-fpe, sig-kill, sig-usr1, sig-segv, sig-usr2, sig-pipe, sig-alrm, sig-term, sig-chld, sig-cont, sig-stop, sig-tstp, sig-ttin, sig-ttou, sig-urg, sig-xcpu, sig-xfsz, sig-vtalrm, sig-prof, sig-poll, sig-sys, sig-winch, sig-iot, sig-stkflt, sig-io, sig-lost and sig-pwr

Description:

These variables correspond to the C signal constants SIGHUP, SIGINT and so forth. The variables sig-winch, sig-iot, sig-stkflt, sig-io, sig-lost and sig-pwr may not be available since a system may lack the corresponding signal constants. See notes for the function log-authpriv.

The highest signal number is 31.

 

14.32.2 Functions set-sig-handler and get-sig-handler

Syntax:


  (set-sig-handler
signal-number handling-spec)
  (get-sig-handler
signal-number)

Description:

The set-sig-handler function is used to specify the handling for a signal, such as the installation of a handler function. It updates the signal handling for a signal whose number is signal-number (usually one of the constants like sig-hup, sig-int and so forth), and returns the previous value. The get-sig-handler function returns the current value.

The signal-number must be an integer the range 1 to 31.

Initially, all 31 signal handling specifications are set to the value t.

The handling-spec parameter may be a function. If a function is specified, then the signal is enabled and connected to that function until another call to set-sig-handler changes the handling for that signal.

If handling-spec is the symbol nil, then the function previously associated with the signal, if any, is removed, and the signal is disabled. For a signal to be disabled means that the signal is set to the SIG_IGN disposition (refer to the C API).

If handling-spec is the symbol t, then the function previously associated with the signal, if any, is removed, and the signal is set to its default disposition. This means that it is set to SIG_DFL (refer to the C API). Some signals terminate the process if they are generated while the handling is configured to the default disposition.

Note that the certain signals like sig-quit and sig-kill cannot be ignored or handled. Please observe the signal documentation in the IEEE POSIX standard, and your platform.

A signal handling function must take two arguments. It is of the form:


  (lambda (
signal async-p) ...)

The signal argument is an integer indicating the signal number for which the handler is being invoked. The asyncp-p argument is a boolean value. If it is t, it indicates that the handler is being invoked asynchronously—directly in a signal handling context. If it is nil, then it is a deferred call. Handlers may do more things in a deferred call, such as terminate by throwing exceptions, and perform I/O.

The return value of a handler is normally ignored. However if it invoked asynchronously (the async-p argument is true), then if the handler returns a non-nil value, it is understood that the handler requesting that it be deferred. This means that the signal will be marked as deferred, and the handler will be called again at some later time in a deferred context, whereby async-p is nil. This is not guaranteed, however; it's possible that another signal will arrive before that happens, possibly resulting in another async call, so the handler must be prepared to deal with an async call at any time.

If a handler is invoked synchronously, then its return value is ignored.

In the current implementation, signals do not queue. If a signal is delivered to the process again, while it is marked as deferred, it simply stays deferred; there is no counter associated with a signal, only a boolean flag.

 

14.32.3 The sig-check function

Syntax:


  (sig-check)

Description:

The sig-check function tests whether any signals are deferred, and for each deferred signal in turn, it executes the corresponding handler. For a signal to be deferred means that the signal was caught by an internal handler in TXR and the event was recorded by a flag. If a handler function is removed while a signal is deferred, the deferred flag is cleared for that signal.

Calls to the sig-check function may be inserted into CPU-intensive code that has no opportunity to be interrupted by signals, because it doesn't invoke any I/O functions.

 

14.32.4 The kill function

Syntax:


  (kill
process-id [signal])

Description:

The kill function is used for sending a signal to a process group or process. It is a wrapper for the POSIX kill function.

If the signal argument is omitted, it defaults to the same value as sig-term.

 

14.33 Unix Itimers

Itimers ("interval timers") can be used in combination with signal handling to execute asynchronous actions. Itimers deliver delayed, one-time signals, and also periodically recurring signals. For more information, consult the POSIX specification.

 

14.33.1 Variables itimer-real, itimer-virtual, and itimer-prof

Description:

These variables correspond to the POSIX constants ITIMER_REAL, ITIMER_VIRTUAL and ITIMER_PROF. Their values are suitable as the timer argument of the getitimer and setitimer functions.

 

14.33.2 Functions getitimer and setitimer

Syntax:


  (getitimer
timer)
  (setitimer
timer interval value)

Description:

The getitimer function returns the current value of the specified timer, which must be itimer-real, itimer-virtual or itimer-prof.

The current value consists of a list of two integer values, which represents microseconds. The first value is the timer interval, and the second value is the timer's current value.

Like getitimer, the setitimer function also retrieves the specified timer. In addition, it stores a new value in the timer, which is given by the two arguments, expressed in microseconds.

 

14.34 Unix Syslog

On platforms where a Unix-like syslog API is available, TXR exports this interface. TXR programs can configure logging via the openlog function, control the logging mask via setlogmask and generate logs via syslog, or using special syslog streams.

 

14.34.1 Special variables log-pid, log-cons, log-ndelay, log-odelay, log-nowait and log-perror

Description:

These variables take on the values of the corresponding C preprocessor constants from the <syslog.h> header: LOG_PID, LOG_CON S,etc. These integer values represent logging options used in the option argument to the openlog function.

Note: LOG_PERROR is not in POSIX, and so log-perror might not be available. See notes about LOG_AUTHPRIV in the documentation for log-authpriv.

 

14.34.2 Special variables log-user, log-daemon, log-auth and log-authpriv

Description:

These variables take on the values of the corresponding C preprocessor constants from the <syslog.h> header: LOG_USER, LOG_DAEMON, LOG_AUTH and LOG_AUTHPRIV. These are the integer facility codes specified in the openlog function.

Note: LOG_AUTHPRIV is not in POSIX, and so log-authpriv might not be available. For portability use code like (or (symbol-value 'log-authpriv) 0) to evaluate to 0 if log-authpriv doesn't exist, or else check for its existence using (boundp 'log-authpriv).

 

14.34.3 Special variables log-emerg, log-alert, log-crit, log-err, log-warning, log-notice, log-info and log-debug

These variables take on the values of the corresponding C preprocessor constants from the <syslog.h> header: LOG_EMERG, LOG_ALERT, etc. These are the integer priority codes specified in the syslog function.

 

14.34.4 The *stdlog* special variable

Description:

The *stdlog* variable holds a special kind of stream: a syslog stream. Each newline-terminated line of text sent to this stream becomes a log message.

The stream internally maintains a priority value that is applied when it generates messages. By default, this value is that of log-info. The stream holds the priority as the value of the :prio stream property, which may be changed with the stream-set-prop function.

The latest priority value which has been configured on the stream is used at the time the newline character is processed and the log message is generated, not necessarily the value which was in effect at the time the accumulation of a line began to take place.

Messages sent to *stdlog* are delimited by newline characters. That is to say, each line of text written to the stream is a new log.

 

14.34.5 The openlog function

Syntax:


  (openlog
id-string [options [facility]])

Description:

The openlog function is a wrapper for the openlog C function, and the arguments have the same semantics. It is not necessary to use openlog in order to call the syslog function or to write data to *stdlog*. The call is necessary in order to override the default identifying string, to set options, such as having the PID (process ID) recorded in log messages, and to specify the facility.

The id-string argument is mandatory.

The option argument is a bitwise mask (see the logior function) of option values such as log-pid and log-cons. If it is missing, then a value of 0 is used, specifying the absence of any options.

The facility argument is one of the values log-user, log-daemon or log-auth. If it is missing, then log-user is assumed.

 

14.34.6 The closelog function

Syntax:


  (closelog)

Description:

The closelog function is a wrapper for the C function closelog.

 

14.34.7 The setlogmask function

Syntax:


  (setlogmask
bitmask-integer)

Description:

The setlogmask function interfaces to the corresponding C function, and has the same argument and return value semantics. The bitmask-integer argument is a mask of priority values to enable. The return value is the prior value. Note that if the argument is zero, then the function doesn't set the mask to zero; it only returns the current value of the mask.

Note that the priority values like log-emerg and log-debug are integer enumerations, not bitmasks. These values cannot be combined directly to create a bitmask. Rather, the mask function should be used on these values.

Example:


  ;; Enable LOG_EMERG and LOG_ALERT messages,
  ;; suppressing all others
  (setlogmask (mask log-emerg log-alert))

 

14.34.8 The syslog function

Syntax:


  (syslog
priority format format-arg*)

Description:

This function is the interface to the syslog C function. The printf formatting capabilities of the function are not used; the format argument follows the conventions of the TXR Lisp format function instead. Note in particular that the %m convention for interpolating the value of strerror(errno) which is available in some versions of the syslog C function is currently not supported.

Note that syslog messages are not newline-terminated.

 

14.35 Web Programming Support

 

14.35.1 Functions url-encode and url-decode

Syntax:


  (url-encode
string [space-plus-p])
  (url-decode
string [space-plus-p])

Description:

These functions convert character strings to and from a form which is suitable for embedding into the request portions of URL syntax.

Encoding a string for URL use means identifying in it certain characters that might have a special meaning in the URL syntax and representing it using "percent encoding": the percent character, followed by the ASCII value of the character. Spaces and control characters are also encoded, as are all byte values greater than or equal to 127 (7F hex). The printable ASCII characters which are percent-encoded consist of this set:


  :/?#[]@!$&'()*+,;=%

More generally, strings can consists of Unicode characters, but the URL encoding consists only of printable ASCII characters. Unicode characters in the original string are encoded by expanding into UTF-8, and applying percent-encoding the UTF-8 bytes, which are all in the range \xx80-\xxFF.

Decoding is the reverse process: reconstituting the UTF-8 byte sequence specified by the URL-encoding, and then decoding the UTF-8 sequence into the string of Unicode characters.

There is an additional complication: whether or not to encode spaces as plus, and to decode plus characters to spaces. In encoding, if spaces are not encoded to the plus character, then they are encoded as %20, since spaces are reserved characters that must be encoded. In decoding, if plus characters are not decoded to spaces, then they are left alone: they become plus characters in the decoded string.

The url-encode function performs the encoding process. If the space-plus-p argument is omitted or specified as nil, then spaces are encoded as %20. If the argument is a value other than nil, then spaces are encoded as the character + (plus).

The url-decode function performs the decoding process. If the space-plus-p argument is omitted or specified as nil, then + (plus) characters in the encoded data are retained as + characters in the decoded strings. Otherwise, plus characters are converted to spaces.

 

14.35.2 Functions html-encode and html-decode

Syntax:


  (html-encode
text-string)
  (html-decode
html-string)

Description:

The html-encode and html-decode functions convert between an HTML and raw representation of of text.

The html-encode function returns a string which is based on the content of text-string, but in which all characters which have special meaning in HTML have been replaced by HTML codes for representing those characters literally. The returned string is the HTML-encoded verbatim representation of text-string.

The html-decode function converts html-string, which may contain HTML character encodings, into a string which contains the actual characters represented by those encodings.

The function composition (html-decode (html-encode text)) returns a string which is equal to text.

The reverse composition (html-encode (html-decode html)) does not necessarily return a string equal to html.

For instance if html is the string "<p>Hello, world&#33;</p>", then html-decode produces "<p>Hello, world!</p>". From this, html-encode produces "&lt;p&gt;Hello, world!&lt;/p&gt;".

 

14.36 Filter Module

The filter module provides a trie (pronounced "try") data structure, which is suitable for representing dictionaries for efficient filtering. Dictionaries are unordered collections of keys, which are strings, which have associated values, which are also strings. A trie can be used to filter text, such that keys appearing in the text are replaced by the corresponding values. A trie supports this filtering operation by providing an efficient prefix-based lookup method which only looks at each input character ones, and which does not require knowledge of the length of the key in advance.

 

14.36.1 Function make-trie

Syntax:


  (make-trie)

Description:

The make-trie function creates an empty trie. There is no special data type for a trie; a trie is some existing type such as a hash table.

 

14.36.2 Function trie-add

Syntax:


  (trie-add
trie key value)

Description:

The trie-add function adds the string key to the trie, associating it with value. If key already exists in trie, then the value is updated with value.

The trie must not have been compressed with trie-compress.

A trie can contain keys which are prefixes of other keys. For instance it can contain "dog" and "dogma". When a trie is used for matching and substitution, the longest match is used. If the input presents the text "doggy", then the match is "dog". If the input is "dogmatic", then "dogma" matches.

 

14.36.3 Function trie-compress

Syntax:


  (trie-compress
trie)

Description:

The trie-compress function changes the representation of trie to a representation which occupies less space and supports faster lookups. The new representation is returned.

The compressed representation of a trie does not support the trie-add function.

This function destructively manipulates trie, and may return an object that is the same object as trie, or it may return a different object, while at the same time still modifying the internals of trie. Consequently, the program should not retain the input object trie, but use the returned object in its place.

 

14.36.4 Function trie-lookup-begin

Syntax:


  (trie-lookup-begin
trie)

Description:

The trie-lookup-begin function returns a context object for performing an open-coded lookup traversal of a trie. The tri argument is expected to be a trie that was created by the make-trie function.

 

14.36.5 Function trie-lookup-feed-char

Syntax:


  (trie-lookup-feed-char
trie-context char)

Description:

The trie-lookup-feed-char function performs a one character step in a trie lookup. The trie-context argument must be a trie contxt returned by trie-lookup-begin, or by some previous call to trie-lookup-feed-char. The char argument is the next character to match.

If the lookup is successful (the match through the trie can continue with the given character) then a new trie context object is returned. The old trie context remains valid.

If the lookup is unsuccessful, nil is returned.

Note: determining whether a given string is stored in a trie can be performed looking up every character of the string successively with trie-lookup-feed-char, using the newly returned context for each successive operation. If every character is found, it means that either that exact string is found in the trie, or a prefix. The ambiguity can be resolved by testing whether the trie has a value at the last node using tree-value-at. For instance, if "catalog" is inserted into an empty trie with value "foo", then "cat" will look up successfully, being a prefix of "catalog"; however, the value at "cat" is nil, indicating that "cat" is only a prefix of one or more entries in the trie.

 

14.36.6 Function tree-value-at

Syntax:


  (trie-value-at
trie-context)

Description:

The trie-value-at function returns the value stored at the node in in the trie given by trie-context. Nodes which have not been given a value hold the value nil.

 

14.36.7 Function filter-string-tree

Syntax:


  (filter-string-tree
filter obj)

Description:

The filter-string-tree a tree structure similar to obj, in which all of the string atoms have been filtered through filter.

The obj argument is a string tree structure: either the symbol nil, denoting an empty structure; a string; or a list of tree structures. If obj is nil, then filter-string-tree returns nil.

The filter argument is a filter: it is either a trie, a function, or nil. If filter is nil, then filter-string-trie just returns obj.

If filter is a function, it must be a function that can be called with one argument. The strings of the string tree are filtered by passing each one into the function and substituting the return value into the corresponding place in the returned structure.

Otherwise if filter is a trie, then this trie is used for filtering, the string elements similarly to a function. For each string, a new string is returned in which occurrences of the keys in the trie are replaced by the values in the trie.

 

14.36.8 Function filter-equal

Syntax:


  (filter-equal
filter-1 filter-2 obj-1 obj-2)

Description:

The filter-equal function tests whether two string trees are equal under the given filters.

The precise semantics can be given by this expression:


  (equal (filter-string-tree
filter-1 obj-1)
         (filter-string-tree
filter-2 obj-2))

The string tree obj-1 is filtered through filter-1, as if by the filter-string-tree function, and similarly, obj-2 is filtered through filter-2. The resulting structures are compared using equal, and the result of that is returned.

 

14.37 Access To TXR Pattern Language From Lisp

 

14.37.1 Function match-fun

Syntax:


  (match-fun
name args input files)

Description:

The match-fun function invokes a TXR pattern function whose name is given by name, which must be a symbol.

The args argument is a list of expressions. The expressions may be symbols which will be interpreted as pattern variables, and may be bound or unbound. If they are not symbols, then they are treated as expressions (of the pattern language, not TXR Lisp) and evaluated accordingly.

The input argument is a list of strings, which may be lazy. It represents the lines of the text stream to be processed.

The file argument is a list of filename specifications, which follow the same conventions as files given on the TXR command line. If the pattern function uses the @(next) directive, it can process these additional files.

The match-fun function's return value falls into three cases. If there is a match failure, it returns nil. Otherwise it returns a cons cell. The car field of the cons cell holds the list of captured bindings. The cdr of the cons cell is one of two values. If the entire input was processed, the cdr field holds the symbol t. Otherwise it holds another cons cell whose car is the remainder of the list of lines which were not matched, and whose cdr is the line number.

Example:


  @(define foo (x y))
  @x:@y
  @line
  @(end)
  @(do
     (format t "~s\n"
               (match-fun 'foo '(a b)
                          '("alpha:beta" "gamma" "omega") nil)))


  Output:
  (((a . "alpha") (b . "beta")) ("omega") . 3)

In the above example, the pattern function foo is called with arguments (a b). These are unbound variables, so they correspond to parameters x and y of the function. If x and y get bound, those values propagate to a and b. The data being matched consists of the lines "alpha:beta", "gamma" and "omega". Inside foo, x and y bind to "alpha" and "beta", and then the line variable binds to "gamma". The input stream is left with "omega".

Hence, the return value consists of the bindings of x and y transferred to a and b, and the second cons cell which gives information about the rest of the stream: it is the part starting at "omega", which is line 3. Note that the binding for the line variable does not propagate out of the pattern function foo; it is local inside it.

 

14.38 Quote/Quasiquote Operator Syntax

 

14.38.1 Operator quote

Syntax:


  (quote
form)

Description:

The quote operator, when evaluated, suppresses the evaluation of form, and instead returns form itself as an object. For example, if form is a symbol, then form is not evaluated to the symbol's value; rather the symbol itself is returned.

Note: the quote syntax '<form> is translated to (quote form).

Example:


  (quote a) ;; yields a


  (quote (+ 2 2)) ;; yields (+ 2 2), not 4.

 

14.38.2 Macro qquote

Syntax:


  (qquote
form)

Description:

The qquote (quasi-quote) macro operator implements a notation for convenient list construction. If form is an atom, or a list structure which does not contain any unquote or splice operators, then (qquote form) is equivalent to (qquote form).

If form, however, is a list structure which contains unquote or splice operators, then the substitutions implied by those operators are performed on form, and the qquote operator returns the resulting structure.

Note: how the qquote operator actually works is that it is compiled into code. It becomes a Lisp expression which, when evaluated, computes the resulting structure.

A qquote can contain another qquote. If an unquote or splice operator occurs within a nested qquote, it belongs to that qquote, and not to the outer one.

However, an unquote operator which occurs inside another one belongs one level higher. For instance in


  (qquote (qquote (unquote (unquote x))))

the leftmost qquote belongs with the rightmost unquote, and the inner qquote and unquote belong together. When the outer qquote is evaluated, it will insert the value of x, resulting in the object (qquote (unquote [value-of-x])). If this resulting qquote value is evaluated again as Lisp syntax, then it will yield [value-of-value-of-x], the value of [value-of-x] when treated as a Lisp expresion and evaluated.

Examples:


  (qquote a) -> a


  (qquote (a b c)) -> (a b c)


  (qquote (1 2 3 (unquote (+ 2 2)) (+ 2 3))) -> (1 2 3 4 (+ 2 3))


  (qquote (unquote (+ 2 2))) -> 4

In the second-to-last example, the 1 2 3 and the (+ 2 3) are quoted verbatim. Whereas the (unquote (+ 2 2)) operator caused the evaluation of (+ 2 2) and the substitution of the resulting value.

The last example shows that form can itself (the entire argument of qquote) can be an unquote operator. However, note: (quote (splice form)) is not valid.

Note: a way to understand the nesting behavior is a via a possible model of quasi-quote expansion which recursively compiles any nested quasi quotes first, and then treats the result of their expansion. For instance, in the processing of


  (qquote (qquote (unquote (unquote x))))

the qquote operator first encounters the embedded (qquote ...) and compiles it to code. During that recursive compilation, the syntax (unquote (unquote x)) is encountered. The inner quote processes the outer unquote which belongs to it, and the inner (unquote x) becomes material that is embedded verbatim in the compilation, which will then be found when the recursion pops back to the outer quasiquote, which will then traverse the result of the inner compilation and find the (unquote x).

Dialect note:

In Lisp dialects which have a published quasiquoting operator syntax, there is the expectation that the quasiquote read syntax corresponds to it. That is to say, that for instance the read syntax ^(a b ,c) is expected translated to (qquote b (unquote c)).

In TXR Lisp, this is not true! Although ^(b b ,c) is translated to a quasiquoting macro, it is an internal one, not based on the public qquote, unquote and splice symbols being documentd here.

This idea exists for hygiene. The quasiquote read syntax is not confused by the presence of the symbols qquote, unquote or splice in the template, since it doesn't treat them specially.

This also allows programmers to use the quasiquote read syntax to construct quasiquote macros. For instance


  ^(qquote (unquote ,x)) ;; does not mean ^^,x 

To the quasiquote reader, the qquote and unquote symbols mean nothing special, and so this syntax simply means that if the value of x is foo, the result will be (qquote (unquote foo)).

The form's expansion is actually this:


  (sys:qquote (qquote (unquote (sys:unquote x))))

the sys:qquote macro recognizes sys:unquote embedded in the form, and the other symbols not in the sys: package are just static template material.

The sys:quote macro and its associated sys:unquote and sys:splice operators work exactly like their ordinary counterparts. So in effect, TXR has two nearly identical, independent quasi-quote implementations, one of which is tied to the read syntax, and one of which isn't. This is useful for writing quasiquotes which write quasiquotes.

 

14.38.3 Operator unquote

Syntax:


  (qquote (... (unquote
form) ...))
  (qquote (unquote
form))

Description:

The unquote operator is not an operator per se. The unquote symbol has no binding in the global environment. It is a special syntax that is recognized within a qquote form, to indicate forms within the quasiquote which are to be evaluated and inserted into the resulting structure.

The syntax (qquote (unquote form)) is equivalent to form: the qquote and unquote "cancel out".

 

14.38.4 Operator splice

Syntax:


  (qquote (... (splice
form) ...))

Description:

The splice operator is not an operator per se. The splice symbol has no binding in the global environment. It is a special syntax that is recognized within a qquote form, to indicate forms within the quasiquote which are to be evaluated and inserted into the resulting structure.

The syntax (qquote (splice form)) is not permitted and raises an exception if evaluated. The splice syntax must occur within a list, and not in the dotted position.

The splice form differs from unquote in that (splice form) requires that form must evaluate to a list. That list is integrated into the surrounding list.

 

14.39 Macros

TXR Lisp supports structural macros. TXR's model of macroexpansion is that TXR Lisp code is processed in two phases: the expansion phase and the evaluation phase. The expansion phase is invoked on Lisp code early during the processing of source code. For instance when a TXR file containing a @(do ...) directive is loaded, expansion of the Lisp forms are its arguments takes place during the parsing of the entire source file, and is complete before any of the code in that file is executed. If the @(do ...) form is later executed, the expanded forms are then evaluated.

TXR Lisp also supports symbol macros, which are symbolic forms that stand for forms, with which they are replaced at macro expansion time.

When Lisp data is processed as code by the eval function, it is first expanded, and so processed in its entirety in the expansion phase. Then it is processed in the evaluation phase.

 

14.39.1 Macro parameter lists

TXR macros support destructuring, similarly to Common Lisp macros. This means that macro parameter lists are like function argument lists, but support nesting. A macro parameter list can specify a nested parameter list in every place where a function argument list allows only a parameter name. For instance, consider this macro parameter list:


  ((a (b c)) : (c frm) ((d e) frm2 de-p) . g)

The top level of this list has four elements: the mandatory parameter (a (b c)), the optional parameter c (with default init form frm), the optional parameter (d e) (with default init form frm2 and presence-indicating variable de-p), and the dot-position parameter g which captures trailing arguments.

Note that some of the parameters are compounds: (a (b c)) and (d e). These compounds express nested macro parameter lists.

Macro parameter lists match a similar tree structure to their own. For instance a mandatory parameter (a (b c)) matches a structure like (1 (2 3)), such that the parameters a, b and c will end up bound to 1, 2 and 3, respectively.

The binding strictness is relaxed for optional parameters. If (a (b c)) is optional, and the argument is, say, (1), then a gets 1, and b and c receive nil.

Macro parameter lists also support two special keywords, namely :env and :whole.

The parameter list (:whole x :envy) will bind parameter x to the entire macro form, and bind parameter y to the macro environment. The :whole and :env notation can occur anywhere in a macro parameter list.

 

14.39.2 Operator macro-time

Syntax:


  (macro-time
form*)

Description:

The macro-time operator has a syntax similar to the progn operator. Each form is evaluated from left to right, and the resulting value is that of the last form.

The special behavior of macro-time is that the evaluation takes place during the expansion phase, rather than during the evaluation phase.

During the expansion phase, all macro-time expressions which occur in a context that calls for evaluation are evaluated, and replaced by their quoted values. For instance (macro-time (list 1 2 3)) evaluates (list 1 2 3) to the object (1 2 3) and the entire macro-time form is replaced by that value, quoted: '(1 2 3). If the form is evaluated again at evaluation-time, the resulting value will be that of the quote, in this case (1 2 3).

macro-time forms do not see the surrounding lexical environment; the see only global function and variable bindings and macros.

Note 1: macro-time is intended for defining helper functions and variables that are used by macros. A macro cannot "see" a defun function or defvar variable because defun and defvar forms are evaluated at evaluation time, which occurs after expansion time. The macro-time operator hoists the evaluation of these forms to macro-expansion time.

Note 2: defmacro forms are implicitly in macro-time; they do not have to be wrapped with the macro-time operator. The macro-time operator doesn't have to be used in programs whose macros do not make references to variables or functions.

 

14.39.3 Operator defmacro

Syntax:


  (defmacro
name (param* [: opt-param* ] [. rest-param ])
    
body-form*)

Description:

The defmacro operator is evaluated at expansion time. It defines a macro-expander function under the name name, effectively creating a new operator.

Note that the parameter list is a macro parameter list, and not a function parameter list. This means that each param and opt-param can be not only a symbol, but it can itself be a parameter list. The corresponding argument is then treated as a structure which matches that parameter list. This nesting of parameter lists can be carried to an arbitrary depth.

A macro is called like any other operator, and resembles a function. Unlike in a function call, the macro receives the argument expressions themselves, rather than their values. Therefore it operates on syntax rather than on values. Also, unlike a function call, a macro call occurs in the expansion phase, rather than the evaluation phase.

The return value of the macro is the macro expansion. It is substituted in place of the entire macro call form. That form is then expanded again; it may itself be another macro call, or contain more macro calls.

Examples:


  ;; A macro to swap two places, such as variables.
  ;; (swap x y) will now exchange the contents
  ;; of x and y.


  (defmacro swap (a b)
    (let ((temp (gensym)))
      ^(let ((,temp ,a))
         (set ,a ,b)
         (set ,b ,temp))))


  ;; dolist macro similar to Common Lisp's:
  ;;
  ;; The following will print 1, 2 and 3
  ;; on separate lines:
  ;; and return 42.
  ;;
  ;; (dolist (x '(1 2 3) 42)
  ;;   (format t "~s\n"))


  (defmacro dolist ((var list : result) . body)
    (let ((i (my-gensym)))
      ^(for ((i ,list)) (i ,result) ((set i (cdr i)))
         (let ((,var (car i)))
           ,*body))))

 

14.39.4 Operator macrolet

Syntax:


  (macrolet ({(
name macro-style-params macro-body-form*)}*)
    
body-form*)

Description:

The macrolet binding operator extends the macro-time lexical environment by making zero or more new local macros visible.

The macrolet symbol is followed by a list of macro definitions. Each definition is a form which begins with a name, followed by macro-style-params which is a macro parameter list, and zero or more macro-body-forms. These macro definitions are similar to those globally defined by the defmacro operator, except that they are in a local environment.

The macro definitions are followed by optional body-forms. The macros specified in the definitions are visible to these forms.

Forms inside the macro definitions such as the macro-body-forms, and initializer forms appearing in the macro-style-params are subject to macro-expansion in a scope in which none of the new macros being defined are yet visible. Once the macro definitions are themselves macro-expanded, they are placed into a new macro environment, which is then used for macro expanding the body-forms.

A macrolet form is fully processed in the expansion phase of a form, and is effectively replaced by progn form which contains expanded versions of body-formss. This expanded structure shows no evidence that any macrolet forms ever existed in it. Therefore, it is impossible for the code evaluated in the bodies and parameter lists of macrolet macros to have any visibility to any surrounding lexical variable bindings, which are only instantiated in the evaluation phase, after expansion is done and macros no longer exist.

 

14.39.5 Function macro-form-p

Syntax:


  (macro-form-p
obj [env])

Description:

The macro-form-p function returns t if obj represents the syntax of a form which is a macro form: either a compound macro or a symbol macro. Otherwise it returns nil.

A macro form is one that will transform under macroexpand-1 or macroexpand; an object which isn't a macro form will not undergo expansion.

The optional env parameter is a macroexpansion environment. A macroexpansion environment is passed down to macros and can be received via their special :env parameter.

env is used by macro-form-p to determine whether obj is a macro in a lexical macro environment.

If env is not specified or is nil, then macro-form-p only recognizes global macros.

Example:


  ;; macro which translates to 'yes if its
  ;; argument is a macro from, or otherwise
  ;; transforms to the form 'no.


  (defmacro ismacro (:env menv form)
    (if (macro-form-p form menv)
     ''yes ''no))


  (macrolet ((local ()))
    (ismacro (local)))    ;; yields yes


  (ismacro (local))       ;; yields no


  (ismacro (ismacro foo)) ;; yields yes

During macro expansion, the global macro ismacro is handed the macro-expansion environment via :env menv.

When the macro is invoked within the macrolet, this environment includes the macro-time lexical scope in which the local macro is defined. So when global checks whether the argument form (local) is a macro, the conclusion is yes: the (local) form is a macro call in that environment: macro-form-p yields t.

When (global (local)) is invoked outside of the macrolet, no local macro is visible is there, and so macro-form-p yields nil.

 

14.39.6 Functions macroexpand-1 and macroexpand

Syntax:


  (macroexpand-1
obj [env])
  (macroexpand
obj [env])

Description:

If obj is a macro form (an object for which macro-form-p returns t), these functions expand the macro form and return the expanded form. Otherwise, they return obj.

macroexpand-1 performs a single expansion, expanding just the macro that is referenced by the symbol in the first position of obj, and returns the expansion. That expansion may itself be a macro form.

macroexpand performs an expansion similar to macroexpand-1. If the result is a macro form, then it expands that form, and keeps repeating this process until the expansion yields a non-macro-form. That non-macro-form is then returned.

The optional env parameter is a macroexpansion environment. A macroexpansion environment is passed down to macros and can be received via their special :env parameter. The environment they receive is their lexically apparent macro-time environment in which local macros may be visible. A macro can use this environment to "manually" expand some form in the context of that environment.

Example:


  ;; (foo x) expands x, and if x begins with a number,
  ;; it removes  the number and returns the resulting
  ;; form. Otherwise, it returns the entire form.


  (defmacro rem-num (:env menv some-form)
    (let ((expanded (macroexpand some-form menv)))
      (if (numberp (car expanded))
        (cdr expanded)
        some-form)))


  ;; The following yields (42 a).


  (macrolet ((foo () '(1 list 42))
             (bar () '(list 'a)))
    (list (rem-num (foo)) (rem-num (bar)))))

The rem-num macro is able to expand the (foo) and (bar) forms it receives as the some-form argument, even though these forms use local macro that are only visible in their local scope. This is thanks to the macro environment passed to rem-num. It is correctly able to work with the expansions (1 list 42) and (list 'a) to produce (list 42) and (list 'a) which evaluate to 42 and a respectively.

 

14.39.7 Operator defsymacro

Syntax:


  (defsymacro
sym form)

Description:

A defsymacro form introduces a symbol macro: a symbol macro is a parameterless substitution keyed to a symbol. In contexts where a symbol macro definition of sym is visible, if the form sym appears such that its evaluation is called for, it is subject to replacement by form. After replacement takes place, form itself is then processed for further replacement of macros and symbol macros.

Symbol macros are also recognized in contexts where sym denotes a place which is the target of an assignment operation like set and similar.

A defsymacro form is implicitly executed at expansion time, and thus need not be wrapped in a macro-time form, just like defmacro.

Note: if a symbol macro expands to itself directly, expansion stops. However, if a symbol macro expands to itself through a chain of expansions, an infinite expansion time loop results.

 

14.39.8 Operator symacrolet

Syntax:


  (symacrolet ({(
sym form)}*) body-form*)

Description:

The symacrolet operator binds local, lexically scoped macros that are similar to the global symbol macros introduced by defsymacro.

Each sym in the bindings list is bound to its corresponding form, creating a new extension of the expansion-time lexical macro environment.

Each body-form is subsequently macro-expanded in this new environment in which the new symbol macros are visible.

Note: ordinary lexical bindings such as those introduced by let or by function parameters lists shadow symbol macros. If a symbol x is bound by nested instances of macrolet and a let, then the scope enclosed by both constructs will see whichever of the two bindings is more inner, even though the bindings are active in completely separate phases of processing.

 

14.39.9 Operator tree-bind

Syntax:


  (tree-bind
macro-style-params expr form*)

Description:

The tree-bind operator evaluates expr, and then uses the resulting value as a counterpart to a macro-style parameter list. If the value has a tree structure which matches the parameters, then those parameters are established as bindings, and the forms, if any, are evaluated in the scope of those bindings. The value of the last form is returned. If there are no forms, nil is returned.

Note: this operator throws an exception if there is a structural mismatch between the parameters and the value of expr.

One way to avoid this exception is to use tree-case.

 

14.39.10 Operator tree-case

Syntax:


  (tree-case
expr {(macro-style-params << form *)}*)

Description:

The tree-case operator evaluates expr and matches it against a succession of zero or more cases. Each case defines a pattern match, expressed as a macro style parameter list macro-style-params.

If the object produced by expr matches macro-style-params, then the parameters are bound, becoming local variables, and the forms, if any, are evaluated in order in the environment in which those variables are visible. If there are forms, the value of the last form becomes the result value of the case, otherwise the result value of the case is nil.

If the result value of a case is the object : (the colon symbol), then processing continues with the next case. Otherwise the evaluation of tree-case terminates, returning the result value.

If the value of expr does not match the macro-style-params parameter list of a case, processing continues with the next case.

If no cases match, then tree-case terminates, returning nil.

Example:


  ;; reverse function implemented using tree-case


  (defun tb-reverse (obj)
    (tree-case obj
      (() ())      ;; the empty list is just returned
      ((a) obj)    ;; one-element list returned (unnecessary case)
      ((a . b) ^(,*(tb-reverse b) ,a)) ;; car/cdr recursion
      (a a)))     ;; atom is just returned

Note that in this example, the atom case is placed last, because an argument list which consists of a symbol is a "catch all" match that matches any object. We know that it matches an atom, because the previous (a . b) case matches conses. In general, the order of the cases in tree-case is important: even more so than the order of cases in a cond or caseql.

 

14.40 Debugging Functions

 

14.40.1 Functions source-loc and source-loc-str

Syntax:


  (source-loc
form)
  (source-loc-str
form)

Description:

These functions map an expression in a TXR program to the file name and line number of the source code where that form came from.

The source-loc function returns the raw information as a cons cell whose car/cdr consist of the line number, and file name.

The source-loc-str function formats the information as a string.

If form is not a piece of the program source code that was constructed by the TXR parser, then source-loc returns nil, and source-loc-str returns a string whose text says that source location is not available.

 

14.40.2 Function rlcp

Syntax:


  (rlcp
dest-form source-form)

Description:

The rlcp function copies the source code location info ("rl" means "read location") from the source-form object to the dest-form object. These objects are pieces of list-based syntax.

Note: the function is intended to be used in macros. If a macro transforms source-form to dest-form, this function can be used to propagate the source code location info also, so that when the TXR Lisp evaluator encounters errors in transformed code, it can give diagnostics which refer to the original untransformed source code.

 

14.41 Profiling

 

14.41.1 Operator prof

Syntax:


  (prof
form*)

Description:

The prof operator evaluates the enclosed forms from left to right similarly to progn, while determining the memory allocation requests and time consumed by the evaluation of the forms.

If there are no forms, the prof operator measures the smallest measurable operation of evaluating nothing and producing nil.

If the evaluation terminates normally (not abruptly by a non-local control transfer), then prof yields a list consisting of:


  (
value malloc-bytes gc-bytes milliseconds)

where value is the value returned by the rightmost form, or nil if there are no forms, malloc-bytes is the total number of bytes of all memroy allocation requests (or at least those known to the TXR runtime, such as those of all internal objects), gc-bytes is the total number of bytes drawn from the garbage-collected heaps, and milliseconds is the total processor time consumed over the execution of those forms.

Notes:

The bytes allocated by the garbage collector from the C function malloc to create heap areas are not counted as malloc-bytes. malloc-bytes includes storage such as the space used for dynamic strings, vectors and bignums (in addition to their gc-heap allocated nodes), and the various structures used by the cobj type objects such as streams and hashes. Objects in external libraries that use uninstrumented allocators are not counted: for instance the C FILE * streams.

 

14.41.2 Macro pprof

Syntax:


  (pprof
form*)

Description:

The pprof (pretty-printing prof) macro is similar to progn. It evaluates forms, and returns the rightmost one, or nil if there are no forms.

Over the evaluation of forms, it counts memory allocations, and measures CPU time. If forms terminate normally, then just prior to returning, pprof prints these statistics in a concise report on the *std-output*.

The pprof macro relies on the prof operator.

 

14.42 Garbage Collection

 

14.42.1 Function sys:gc

Syntax:


  (sys:gc)

Description:

The gc function triggers garbage collection. Garbage collection means that unreachable objects are identified and reclaimed, so that their storage can be re-used.

 

14.42.2 Function sys:gc-set-delta

Syntax:


  (sys:gc-set-delta
bytes)

Description:

The gc-set-delta function sets the GC delta parameter.

Note: This function may disappear in a future release of TXR or suffer a backward-incompatible change in its syntax or behavior.

When the amount of new dynamic memory allocated since the last garbage collection equals or exceeds the GC delta, a garbage collection pass is triggered. From that point, a new delta begins to be accumulated.

Dynamic memory is used for allocating heaps of small garbage-collected objects such as cons cells, as well as the satellite data attached to some objects: like the storage arrays of vectors, strings or bignum integers. Most garbage collector behaviors are based on counting objects in the heaps.

Sometimes a program works with a small number of objects which are very large, frequently allocating new, large objects and turning old ones into garbage. For instance a single large integer could be many megabytes long. In such a situation, a small number of heap objects therefore control a large amount of memory. This requires garbage collection to be triggered much more often than when working with small objects, such as conses, to prevent runaway allocation of memory. It is for this reason that the garbage collector uses the GC delta.

There is a default GC delta of 64 megabytes. This may be overridden in special builds of TXR for small systems.

 

14.43 Modularization

 

14.43.1 Special variable *self-path*

Description:

This variable holds the invocation path name of the TXR program.

 

14.44 Debugger

TXR has a simple, crude, built-in debugger. The debugger is invoked by adding the -d command line option to an invocation of TXR. In this debugger it is possible to step through code, set breakpoints, and examine the variable binding environment.

Prior to executing any code, the debugger waits at the txr> prompt, allowing for the opportunity to set breakpoints.

Help can be obtained with the h or ? command.

Whenever the program stops at the debugger, it prints the Lisp-ified piece of syntax tree that is about to be interpreted. It also shows the context of the input being matched.

The s command can be used to step into a form; n to step over. Sometimes the behavior seems counter-intuitive. For instance stepping over a @(next) directive actually means skipping everything which follows it. This is because the query material after a @(next) is actually child nodes in the abstract syntax tree node of the next directive, whereas the surface syntax appears flat.

 

14.44.1 Sample Session

Here is an example of the debugger being applied to a web scraping program which connects to a US NAVY clock server to retrieve a dynamically-generated web page, from which the current time is extracted, in various time zones. The handling of the web request is done by the wget command; the TXR query opens a wget command as and scans the body of the HTTP response containing HTML. This is the code, saved in a file called navytime.txr:


  @(next `!wget -c http://tycho.usno.navy.mil/cgi-bin/timer.pl -O - 2> /dev/null`)
  <!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 3.2 Final"//EN>
  <html>
  <body>
  <TITLE>What time is it?</TITLE>
  <H2> US Naval Observatory Master Clock Time</H2> <H3><PRE>
  @(collect :vars (MO DD HH MM SS (PM "  ") TZ TZNAME))
  <BR>@MO. @DD, @HH:@MM:@SS @(maybe)@{PM /PM/} @(end)@TZ@/\t+/@TZNAME
  @  (until)
  </PRE>@/.*/
  @(end)
  </PRE></H3><P><A HREF="http://www.usno.navy.mil"> US Naval Observatory</A>


  </body></html>
  @(output)
  @  (repeat)
  @MO-@DD @HH:@MM:@SS @PM @TZ
  @  (end)
  @(end)

This is the debug session:


  $ txr -d navytime.txr
  stopped at line 1 of navytime.txr
  form: (next (sys:quasi "!wget -c http://tycho.usno.navy.mil/cgi-bin/timer.pl -O - 2> /dev/null"))
  depth: 1
  data (nil):
  nil

The user types s to step into the (next ...) form.


  txr> s
  stopped at line 2 of navytime.txr
  form: (sys:text "<!DOCTYPE" (#<sys:regex: 95e4590> 1+ #\space) "HTML" (#<sys:regex: 95e4618> 1+ #\space) "PUBLIC" (#<sys:regex: 95e46a8> 1+ #\space) "\"-//W3C//DTD" (#<sys:regex: 95e4750> 1+ #\space) "HTML" (#<sys:regex: 95e47d8> 1+ #\space) "3.2" (#<sys:regex: 95e4860> 1+ #\space) "Final\"//EN>")
  depth: 2
  data (1):
  "<!DOCTYPE HTML PUBLIC \"-//W3C//DTD HTML 3.2 Final\"//EN>"
  txr> s

The current form now is a sys:text form which is an internal representation of a block of horizontal material. The pattern matching is in vertical mode at this point, and so the line of data is printed without an indication of character position.


  stopped at line 2 of navytime.txr
  form: (sys:text "<!DOCTYPE" (#<sys:regex: 95e4590> 1+ #\space) "HTML" (#<sys:regex: 95e4618> 1+ #\space) "PUBLIC" (#<sys:regex: 95e46a8> 1+ #\space) "\"-//W3C//DTD" (#<sys:regex: 95e4750> 1+ #\space) "HTML" (#<sys:regex: 95e47d8> 1+ #\space) "3.2" (#<sys:regex: 95e4860> 1+ #\space) "Final\"//EN>")
  depth: 3
  data (1:0):
  "" . "<!DOCTYPE HTML PUBLIC \"-//W3C//DTD HTML 3.2 Final\"//EN>"

The user types s to step in.


  txr> s
  stopped at line 2 of navytime.txr
  form: "<!DOCTYPE"
  depth: 4
  data (1:0):
  "" . "<!DOCTYPE HTML PUBLIC \"-//W3C//DTD HTML 3.2 Final\"//EN>"

Now, the form about to be processed is the first item of the (sys:text ...), the string "<!DOCTYPE".

The input is shown broken into two quoted strings with a dot in between. The dot indicates the current position. The left string is empty, meaning that this is the leftmost position. The programmer steps:


  txr> s
  stopped at line 2 of navytime.txr
  form: (#<sys:regex: 95e4590> 1+ #\space)
  depth: 4
  data (1:9):
  "<!DOCTYPE" . " HTML PUBLIC \"-//W3C//DTD HTML 3.2 Final\"//EN>"

Control has now passed to the second element of the (sys:text ...), a regular expression which matches one or more spaces, generated by a single space in the source code according to the language rules.

The input context shows that "<!DOCTYPE" was matched in the input, and the position moved past it.


  txr> s
  stopped at line 2 of navytime.txr
  form: "HTML"
  depth: 4
  data (1:10):
  "<!DOCTYPE " . "HTML PUBLIC \"-//W3C//DTD HTML 3.2 Final\"//EN>"

Now, the regular expression has matched and moved the position past the space; the facing input is now "HTML ...".

The programmer then repeats the s command by hitting Enter.


  txr>
  stopped at line 2 of navytime.txr
  form: (#<sys:regex: 95e4618> 1+ #\space)
  depth: 4
  data (1:14):
  "<!DOCTYPE HTML" . " PUBLIC \"-//W3C//DTD HTML 3.2 Final\"//EN>"
  txr>
  stopped at line 2 of navytime.txr
  form: "PUBLIC"
  depth: 4
  data (1:15):
  "<!DOCTYPE HTML " . "PUBLIC \"-//W3C//DTD HTML 3.2 Final\"//EN>"
  txr>
  stopped at line 2 of navytime.txr
  form: (#<sys:regex: 95e46a8> 1+ #\space)
  depth: 4
  data (1:21):
  "<!DOCTYPE HTML PUBLIC" . " \"-//W3C//DTD HTML 3.2 Final\"//EN>"
  txr>
  stopped at line 2 of navytime.txr
  form: "\"-//W3C//DTD"
  depth: 4
  data (1:22):
  "<!DOCTYPE HTML PUBLIC " . "\"-//W3C//DTD HTML 3.2 Final\"//EN>"
  txr>
  stopped at line 2 of navytime.txr
  form: (#<sys:regex: 95e4750> 1+ #\space)
  depth: 4
  data (1:34):
  "<!DOCTYPE HTML PUBLIC \"-//W3C//DTD" . " HTML 3.2 Final\"//EN>"

It is not evident from the session transcript, but during interactive use, the input context appears to be animated. Whenever the programmer hits Enter, the new context is printed and the dot appears to advance.

Eventually the programmer becomes bored and place a breakpoint on line 15, where the @(output) block begins, and invokes the c command to continue the execution:


  txr> b 15
  txr> c
  stopped at line 15 of navytime.txr
  form: (output (((repeat nil (((sys:var MO nil nil) "-" (sys:var DD nil nil) " " (sys:var HH nil nil) ":" (sys:var MM nil nil) ":" (sys:var SS nil nil) " " (sys:var PM nil nil) " " (sys:var TZ nil nil))) nil nil nil nil nil nil))))
  depth: 2
  data (16):
  ""

The programmer issues a v command to take a look at the variable bindings, which indicate that the @(collect) has produced some lists:


  txr> v
  bindings:
  0: ((PM "  " "PM" "PM" "PM" "PM" "PM" "PM") (TZNAME "Universal Time" "Eastern Time" "Central Time" "Mountain Time" "Pacific Time" "Alaska Time" "Hawaii-Aleutian Time") (TZ "UTC" "EDT" "CDT" "MDT" "PDT" "AKDT" "HAST") (SS "35" "35" "35" "35" "35" "35" "35") (MM "32" "32" "32" "32" "32" "32" "32") (HH "23" "07" "06" "05" "04" "03" "01") (DD "30" "30" "30" "30" "30" "30" "30") (MO "Mar" "Mar" "Mar" "Mar" "Mar" "Mar" "Mar"))

Then a continue command, which finishes the program, whose output appears:


  txr> c
  Mar-30 23:22:52    UTC
  Mar-30 07:22:52 PM EDT
  Mar-30 06:22:52 PM CDT
  Mar-30 05:22:52 PM MDT
  Mar-30 04:22:52 PM PDT
  Mar-30 03:22:52 PM AKDT
  Mar-30 01:22:52 PM HAST

 

14.45 Compatibility

New TXR versions are usually intended to be backward-compatible with prior releases in the sense that documented features will continue to work in the same way. Due to new features, new versions of TXR will supply new behaviors where old versions of TXR would have produced an error, such as a syntax error. Though, strictly speaking, this means that something is working differently in a new version, replacing an error situation with functionality is usually not considered a deviation from backward-compatibility.

When a change is introduced which is not backward compatible, TXR's -C option can be used to request emulation of old behavior.

The option was introduced in TXR 98, and so the oldest TXR version which can be emulated is TXR 97.

Here are values which have a special meaning as arguments to the -C option, along with a description of what behaviors are affected:

97
Up to TXR 97, the error exception symbols such as file-error were named with underscores, as in file_error. These error symbols existed: type_error, internal_error, numeric_error, range_error, query_error, file_error and process_error.

 

15 Appendix

 

15.1 A. NOTES ON EXOTIC REGULAR EXPRESSIONS

Users familiar with regular expressions may not be familiar with the complement and intersection operators, which are often absent from text processing tools that support regular expressions. The following remarks are offered in hope that they are of some use.

Equivalence to Sets

Regexp intersection is not essential; it may be obtained from complement and union as follows, since De Morgan's law applies to regular expression algebra: (R1)&(R2) = ~(~(R1)|~(R2)). (The complement of the union of the complements of R1 and R2 constitutes the intersection.) This law works because the regular expression operators denote set operations in a straightforward way. A regular expression denotes a set of strings (a potentially infinite one) in a condensed way. The union of two regular expressions R1|R2 denotes the union of the set of texts denoted by R1 and that denoted by R2. Similarly R1&R2 denotes a set intersection, and ~R denotes a set complement. Thus algebraic laws that apply to set operations apply to regular expressions. It's useful to keep in mind this relationship between regular expressions and sets in understanding intersection and complement.

Given a finite set of strings, like the set { "abc", "def" } which corresponds to the regular expression (abc|def), the complement is the set which contains an infinite number of strings: it consists of all possible strings except "abc" and "def". It includes the empty string, all strings of length 1, all strings of length 2, all strings of length 3 other than "abc" and "def", all strings of length 4, etc. This means that a "harmless looking" expression like ~(abc|def) can actually match arbitrarily long inputs.

Set Difference

How about matching only three-character-long strings other than "abc" or "def"? To express this, regex intersection can be used: these strings are the intersection of the set of all three-character strings, and the set of all strings which are not "abc" or "def". The straightforward set-based reasoning leads us to this: ...&~(abc|def). This A&~B idiom is also called set difference, sometimes notated with a minus sign: A-B (which is not supported in TXR regular expression syntax). Elements which are in the set A, but not B, are those elements which are in the intersection of A with the complement of B. This is similar to the arithmetic rule A - B = A + -B: subtraction is equivalent to addition of the additive inverse. Set difference is a useful tool: it enables us to write a positive match which captures a more general set than what is intended (but one whose regular expression is far simpler than a positive match for the exact set we want), then we can intersect this over-generalized set with the complemented set of another regular expression which matches the particulars that we wish excluded.

Expressivity versus Power

It turns out that regular expressions which do not make use of the complement or intersection operators are just as powerful as expressions that do. That is to say, with or without these operators, regular expressions can match the same sets of strings (all regular languages). This means that for a given regular expression which uses intersection and complement, it is possible to find a regular expression which doesn't use these operators, yet matches the same set of strings. But, though they exist, such equivalent regular expressions are often much more complicated, which makes them difficult to design. Such expressions do not necessarily express what it is they match; they merely capture the equivalent set. They perform a job, without making it obvious what it is they do. The use of complement and intersection leads to natural ways of expressing many kinds of matching sets, which not only demonstrate the power to carry out an operation, but also easily express the concept.

Example: Matching C Language Comments

For instance, using complement, we can write a straightforward regular expression which matches C language comments. A C language comment is the digraph /*, followed by any string which does not contain the closing sequence */, followed by that closing sequence. Examples of valid comments are /**/, /* abc */ or /***/. But C comments do not nest (cannot contain comments), so that /* /* nested */ */ actually consists of the comment /* /* nested */, which is followed by the trailing junk */. Our simple characterization of interior part of a C comment as a string which does not contain the terminating digraph makes use of the complement, and can be expressed using the complemented regular expression like this: (~.*[*][/].*). That is to say, strings which contain */ are matched by the expression .*[*][/].*: zero or more arbitrary characters, followed by */, followed by zero or more arbitrary characters. Therefore, the complement of this expression matches all other strings: those which do not contain */. These strings make up the inside of a C comment between the /* and */.

The equivalent simple regex is quite a bit more complicated. Without complement, we must somehow write a positive match for all strings such that we avoid matching */. Obviously, sequences of characters other than * are included: [^*]*. Occurrences of * are also allowed, but only if followed by something other than a slash, so let's include this via union:


  ([^*]|[*][^/])*.

Alas, already, we have a bug in this expression. The subexpression [*][^/] can match **, since a * is not a /. If the next character in the input is /, we missed a comment close. To fix the problem we revise to this:


  ([^*]|[*][^*/])*

(The interior of a C language comment is a any mixture of zero or more non-asterisks, or digraphs consisting of an asterisk followed by something other than a slash or another asterisk). Oops, now we have a problem again. What if two asterisks occur in a comment? They are not matched by [^*], and they are not matched by [*][^*/]. Actually, our regex must not simply match asterisk-non-asterisk digraphs, but rather sequences of one or more asterisks followed by a non-asterisk:


  ([^*]|[*]*[^*/])*

This is still not right, because, for instance, it fails to match the interior of a comment which is terminated by asterisks, including the simple test cases where the comment interior is nothing but asterisks. We have no provision in our expression for this case; the expression requires all runs of asterisks to be followed by something which is not a slash or asterisk. The way to fix this is to add on a subexpression which optionally matches a run of zero or more interior asterisks before the comment close:


  ([^*]|[*]*[^*/])*[*]*

Thus our the semi-final regular expression is


  [/][*]([^*]|[*]*[^*/])*[*]*[*][/]

(Interpretation: a C comment is an interior string enclosed in /* */, where this interior part consists of a mixture of non-asterisk characters, as well as runs of asterisk characters which are terminated by a character other than a slash, except for possibly one rightmost run of asterisks which extends to the end of the interior, touching the comment close. Phew!) One final simplification is possible: the tail part [*]*[*][/] can be reduced to [*]+[/] such that the final run of asterisks is regarded as part of an extended comment terminator which consists of one or more asterisks followed by a slash. The regular expression works, but it's cryptic; to someone who has not developed it, it isn't obvious what it is intended to match. Working out complemented matching without complement support from the language is not impossible, but it may be difficult and error-prone, possibly requiring multiple iterations of trial-and-error development involving numerous test cases, resulting in an expression that doesn't have a straightforward relationship to the original idea.

The Non-Greedy Operator

The non-greedy operator % is actually defined in terms of a set difference, which is in turn based on intersection and complement. The uninteresting case (R%) where the right operand is empty reduces to (R*): if there is no trailing context, the non-greedy operator matches R as far as possible, possibly to the end of the input, exactly like the greedy operator. The interesting case (R%T) is defined as a "syntactic sugar" which expands to the expression ((R*)&(~.*(T&.+).*))T which means: match the longest string which is matched by R*, but which does not contain a non-empty match for T; then, match T. This is a useful and expressive notation. With it, we can write the regular expression for matching C language comments simply like this: [/][*].%[*][/] (match the opening sequence /*, then match a sequence of zero or more characters non-greedily, and then the closing sequence */. With the non-greedy operator, we don't have to think about the interior of the comment as set of strings which excludes */. Though the non-greedy operator appears expressive, its apparent simplicity may be deceptive. It looks as if it works "magically" by itself; "somehow" this .% part "knows" only to consume enough characters so that it doesn't swallow an occurrence of the trailing context. Care must be taken that the trailing context passed to the operator really is the correct text that should be excluded by the non-greedy match. For instance, take the expression .%abc. If you intend the trailing context to be merely a, you must be careful to write (.%a)bc. Otherwise, the trailing context is abc, and this means that the .% match will consume the longest string that does not contain abc, when in fact what was intended was to consume the longest string that does not contain a. The change in behavior of the % operator upon modifying the trailing context is not as intuitive as that of the * operator, because the trailing context is deeply involved in its logic.

On a related note, for single-character trailing contexts, it may be a good idea to use a complemented character class instead. That is to say, rather than (.%a)bc, consider [^a]*abc. The set of strings which don't contain the character a is adequately expressed by [^a]*.