This is Mixp, an XML Parser for Guile, written as an interface to James Clark’s expat library. The documentation is, of course, incomplete, and the interface is subject to change. However, it should be sufficient to get started. This documentation was last updated on 3 February 2020 and covers Mixp version 0.8.
Mixp is a Scheme interface to James Clark’s expat library1. It may be used to parse XML documents with Guile.
If you do not know expat, first have a look at the sample program
See Sample programs. Typically, you will create a parser object
parser-create, then associate one or more handlers to it
hset!, then parse the document with
The handlers work by side-effect, so unless the algorithm is pure
input/output, most likely you will also need to retrieve the state
they maintain, after the parse. See Expat interface.
If you happen to know expat already, you will find easily what you
are looking for by taking a C expat function name, removing the
XML_, using hyphens instead of capital letters to separate the
words, and searching it in the reference documentation. In most
cases, the prototype is the same, modulo the differences between C
The following sample program reads an XML file (provided with the
Mixp distribution), and displays its start and end tags. You can
launch a Guile shell from the test/ directory of the
distribution, and execute this code. Your
variable should contain the directory in which you installed Mixp
(that is, the directory which contains the mixp/ subdirectory).
(use-modules ((mixp expat) #:prefix E:) ((mixp utils) #:prefix U:)) (define (trace prefix) (lambda (name . ignored) (display prefix) (display name) (newline))) ;; Create the parser object. (let ((parser (E:parser-create))) ;; Specify handlers. (E:hset! parser 'element-start (trace "start ") 'element-end (trace "end ")) ;; Parse the file. (call-with-input-file "REC-xml-19980210.xml" (lambda (port) (U:parse-data port parser))))
For more information about the Expat interface and handlers, See Expat interface.
The following sample program builds a hierarchical tree structure from an XML document which resides in a string. This tree structure should be easy to use with traditional Scheme procedures.
(use-modules ((mixp utils) #:prefix U:)) (let ((xml-doc "<foo name='Paul'><bar>Some text</bar><void/></foo>")) (call-with-input-string xml-doc U:xml->tree)) ⇒ ((element ("foo" (("name" . "Paul"))) (element ("bar" ()) (character-data "Some text")) (element ("void" ()))))
For more information about this interface, See High-level extensions.
From the Guile shell or from a Guile script, you should type the following commands before using the Mixp API:
(use-modules (mixp expat)) (use-modules (mixp utils))
Actually, you may load just
(mixp expat) if you intend to use
only the raw expat interface (see Expat interface).
(mixp utils) if you want to use the extension
procedures (see High-level extensions).
Mixp contains two Scheme modules:
(mixp expat)is the low-level interface to expat. It doesn’t stay as close as possible to the expat API, but someone who already knows expat will be able to note gross similarities.
(mixp utils)contains additional procedures that might be useful. For example, if you need to parse an XML file, you can use
(mixp expat)). This module may evolve into a higher-level interface, for example an object-based interface.
From another point of view, Mixp contains two files in a directory
mixp, which in turn lives in a directory somewhere along your
This section describes a few common tasks which may be solved with Mixp.
(mixp utils) without specifying a
parser. A default one will be created, and it will do nothing
interesting except raise errors if there is any error in the document:
(call-with-input-string "<doc><elem></elem>" parse-data)
Suppose you want to retrieve the text contained between an opening tag
and the matching closing tag. You may do that by using an
element-handler and a character-data-handler together. The following
code will retrieve the text between
an XML document:
(use-modules ((mixp expat) #:select E:) ((mixp utils) #:select U:)) (let ((parser (E:parser-create)) (in-title? #f) ; becomes #t inside the tag (title "")) ; will contain the result (define (toggle sense) (lambda (name . ignored) (and (string=? "title" name) (set! in-title? sense)))) (define (handle-character-data value) (and in-title? (set! title (string-append title value)))) (E:hset! parser 'element-start (toggle #t) 'element-end (toggle #f) 'character-data handle-character-data) (call-with-input-string "<doc><title>Hello</title></doc>" (lambda (port) (U:parse-data port parser))) (display title) (newline))
(use-modules ((mixp utils) #:prefix U:)) (call-with-input-file "file.xml" U:xml->tree)
The following program will read an XML document in foo.xml,
parse the DTD which may be referenced in the
declaration, and expand the entities.
(use-modules ((mixp expat) #:prefix E:) ((mixp utils) #:prefix U:)) (define (fso s . args) (apply simple-format #t s args)) ;; Create the parser object. (let ((parser (E:parser-create))) (define (xref-h context base system-id public-id) (fso "Ref to external entity: ~A.~%" system-id) (open-input-file system-id)) (E:set-param-entity-parsing parser 'XML_PARAM_ENTITY_PARSING_ALWAYS) ;; Specify callback functions. (E:hset! parser 'character-data (lambda (value) (fso "Char: ~A.~%" value)) 'external-entity-ref xref-h) ;; Parse the file. (call-with-input-file "foo.xml" (lambda (port) (U:parse-data port parser))))
You may want to define handlers to be called when Mixp parses the DTD and finds an element declaration or an attribute list declaration. Unfortunately, this is not possible, due to limitations in expat.
However, you may try to use the default handler (see Expat handlers). If DTD reading is enabled (see the previous item), then the default handler will be called repeatedly while reading the DTD, and will receive each time a part of the DTD. However, there is no guarantee about what part of the DTD it will receive each time. Building a representation of the DTD would be possible with the default handler, but not easy.
Please send bug reports to <email@example.com>. We always appreciate feedback about Mixp, and suggestions about what could be improved in the interface.
This chapter describes the libexpat interface, i.e., the
(mixp expat) module. The interface has been modified
to be more “Schemey”; it does not correspond one-to-one with
libexpat. Notably, things are more symbolic and “condensed”.
Return a list containing version info of this expat.
The list has the form:
(major minor micro string)
where major, minor, micro are integers;
and string is a string that begins w/ "expat_" and
ends with the major, minor, micro numbers in dotted notation.
(expat-version) ⇒ (2 2 6 "expat_2.2.6")
Return an alist describing the features of the underlying libexpat. The alist keys are strings, one or more of:
XML_UNICODE XML_UNICODE_WCHAR_T XML_DTD XML_CONTEXT_BYTES XML_MIN_SIZE sizeof(XML_Char) sizeof(XML_LChar) XML_NS XML_LARGE_SIZE XML_ATTR_INFO
The values are absent if the feature is a simple one, otherwise some feature-specific positive integer.
Additionally, the first pair has key
and value a number which (in hex) represents the version
numbers of the underlying libexpat. For example:
131585 ⇒ #x020201 ⇒ version 2.2.1
Expat uses C
enums to operate symbolically.
(mixp expat), we use Scheme symbols directly.
Several procedures return a symbolic status, one of the set:
XML_STATUS_ERROR XML_STATUS_OK XML_STATUS_SUSPENDED
Here are all the symbolic error codes (see Doing a parse), presented without the ‘XML_ERROR_’ prefix.
;; since the beginning FEATURE_REQUIRES_XML_DTD NONE CANT_CHANGE_FEATURE_ONCE_PARSING NO_MEMORY SYNTAX ;; added in 1.95.7 NO_ELEMENTS UNBOUND_PREFIX INVALID_TOKEN UNCLOSED_TOKEN ;; added in 1.95.8 PARTIAL_CHAR UNDECLARING_PREFIX TAG_MISMATCH INCOMPLETE_PE DUPLICATE_ATTRIBUTE XML_DECL JUNK_AFTER_DOC_ELEMENT TEXT_DECL PARAM_ENTITY_REF PUBLICID UNDEFINED_ENTITY SUSPENDED RECURSIVE_ENTITY_REF NOT_SUSPENDED ASYNC_ENTITY ABORTED BAD_CHAR_REF FINISHED BINARY_ENTITY_REF SUSPEND_PE ATTRIBUTE_EXTERNAL_ENTITY_REF MISPLACED_XML_PI ;; added in 2.0 UNKNOWN_ENCODING RESERVED_PREFIX_XML INCORRECT_ENCODING RESERVED_PREFIX_XMLNS UNCLOSED_CDATA_SECTION RESERVED_NAMESPACE_URI EXTERNAL_ENTITY_HANDLING NOT_STANDALONE ;; added in 2.2.1 UNEXPECTED_STATE INVALID_ARGUMENT ENTITY_DECLARED_IN_PE
Everything revolves around the parser object. This section describes procedures to create and query such objects.
Return a new parser object. Optional arg encoding is a string specifying the encoding to use (for example, "UTF-8").
Optional arg encoding is a string specifying the encoding to use.
Second optional arg namespace-separator is a character used to
separate the namespace part from the local part (e.g.,
Note: Using this proc (instead of
parser-create) enables dispatch
#t if obj is an XML-Parser object,
Return “current” locus information for parser as a vector of four elements (all non-negative integers):
#(LINE COLUMN BYTE-COUNT BYTE-INDEX)
Optional arg stash specifies a vector to fill in rather
than constructing a new one. If an element in stash is
#f, the respective slot is skipped (it remains
You must specify a set of handlers, or callback procedures, for the parser to call when it encounters specific situations while processing its input. The handler name is a symbol. Unlike libexpat, there is one centralized procedure for setting and one for getting the set.
Set handlers for parser as specified in plist,
a list of alternating handler names (symbols) and values.
Valid values are a procedure,
() (the empty list) or
Note, however, that no arity checks are done on the procedures.
As a special backward-compatible case, if the first key is a full alist, use that instead to specify the handlers to set, and ignore the rest of the args. NB: Support for this calling convention WILL BE REMOVED by the 1.0 release of Mixp.
Return the procedure set as parser’s handler (a symbol).
If none is set, return
NB: The following procedure WILL BE REMOVED by the 1.0 release of Mixp.
Return an alist representing the handler set of parser.
If a particular handler is not specified, that pair’s CDR
#f. The alist keys are handler names.
In the following description, the handler’s name is followed by the arguments that it will be called with. These are normally string values unless otherwise noted.
This handler is called when expat sees an element start. attributes is an alist whose keys and values are all strings.
<foo a="1" b="2"> name ⇒ "foo" attributes ⇒ (("a" . "1") ("b" . "2"))
Likewise, for element end.
</foo> name ⇒ "foo"
This handler is called for normal text (outside ‘<>’ tags). data should never be the empty string. It is encoded in UTF-8.
This handler is called for every processing instruction
<? ... ?>).
<?a b c d e f ?> target ⇒ "a" pi-data ⇒ "b c d e f " ;; Note the trailing whitespace.
This handler is called for comments (
<!-- ... -->).
<!-- This is a comment. --> comment ⇒ " This is a comment. " ;; Note the surrounding whitespace.
This handler is called for CDATA sections
<![CDATA[ ... ]]>).
default-expand specify the default
handler. The difference regards processing of internal entities.
defaultinhibits expansion of internal entities; they are passed, instead, to the handler.
default-expanddoes not inhibit their expansion; they are not passed to the handler.
The default handler is called for any characters in the XML document for which there is no applicable handler. This includes both characters that are part of markup which is of a kind that is not reported (comments, markup declarations), or characters that are part of a construct which could be reported but for which no handler has been supplied. The characters are passed exactly as they were in the XML document except that they will be encoded in UTF-8.
Line boundaries are not normalized. Note that a byte order mark character is not passed to the default handler. There are no guarantees about how characters are divided between calls to the default handler: for example, a comment might be split between multiple calls.
This handler is called for the start of the
declaration, before any DTD or internal subset is parsed.
This handler is called for the start of the
declaration when the closing ‘>’ is encountered,
but after processing any external subset.
This handler is called for entity declarations.
The arg is-parameter-entity? will be
if the entity is a parameter entity,
For internal entities
(<!ENTITY foo "bar">), value will
#f and system-id, public-id, and
notation-name will be
Since it is legal to have zero-length values, do not use
this argument to test for internal entities.
For external entities, value will be
system-id will be non-
The public-id argument will be
#f unless a public
identifier was provided.
The notation-name argument will have a non-
#f value only
for unparsed entity declarations.
NB: This handler is obsolete; use
It WILL BE REMOVED by the 1.0 release of Mixp.
This handler is called for unparsed entity declarations
The entity-name, system-id and notation-name arguments
will never be
#f. The other arguments may be.
The base argument is whatever was set by
(see Expat misc).
<!ENTITY Antarctica SYSTEM 'http://www.antarctica.net' NDATA vrml> entity-name ⇒ "Antarctica" system-id ⇒ "http://www.antarctica.net" public-id ⇒ #f notation-name ⇒ "vrml"
This handler is called for notation declarations (
Except notation-name, some of the args may be
The base argument is whatever was set by
(see Expat misc).
<!NOTATION vrml PUBLIC 'VRML 2'> notation-name ⇒ "vrml" system-id ⇒ #f public-id ⇒ "VRML 2"
When namespace processing is enabled (i.e., the parser was created with
parser-create-ns), these are called once for each namespace
declaration. The call to the start and end element handlers occur
between the calls to the start and end namespace declaration
handlers. prefix may be
<html xmlns="http://www.w3.org/1999/xhtml" xml:lang="en" lang="en"> prefix ⇒ #f uri ⇒ "http://www.w3.org/1999/xhtml"
This handler is called if the document is not standalone (it has an
external subset or a reference to a parameter entity, but does not
have ‘standalone="yes"’). If this handler returns
then processing will not continue, and the parser will return a
Some of the args may be
The base argument is whatever was set by
(see Expat misc).
This handler is called when the parser finds a reference to an
external entity in the document. For example, the
...> declaration contains an external entity reference when it
specifies an external DTD. In that case, you should also call
set-param-entity-parsing (see Expat misc), because you
probably want the parser to expand the references to entities
declared in your DTD. For an example, See How to....
The external entity reference handler should return an open port to
the external entity. For example, assuming that
refers to a relative file path, you may define the handler as follows:
(lambda (context base system-id public-id) (open-input-file system-id))
The system identifier (system-id) is defined by the XML specification as a URI. Therefore, the example above will only work if you know that the system id is actually a filename. You may need to use, for example, some kind of http client library if you want to support URIs which start with ‘http://’.
Note that the behaviour of this handler is very different in expat.
This handler is called in two situations:
default-handlerhas been used.
Note: Skipped parameter entities in declarations and skipped general entities in attribute values cannot be reported, because the event would be out of sync with the reporting of the declarations or attribute values.
This handler is called when the parser does not recognize the
declared encoding of a document.
It should use
make-xml-encoding (see Expat misc),
if it thinks that it can teach the parser to decode name.
NB: Unknown encoding handlers have not been really tested, so they probably don’t work for now.
Expat supports the following encodings: UTF-8, UTF-16, ISO-8859-1, US-ASCII.
The encoding is usually indicated in the first line of an XML file
<?xml... ?> declaration). But every data you will receive
in your handlers (tag names, attributes, character data...), will be
encoded in UTF-8, whatever the original encoding was. UTF-8
represents ASCII characters with no modification, but represents other
characters with multi-byte characters. ISO-8859-1 has better
support in standard editors, but is too euro-centric.
The encoding features of expat are not completely supported in Mixp.
Using unknown encoding handlers will not work, or at least I have not
tested that feature. However, XML documents which encoding (as
specified in the
<?xml... ?> declaration) is supported by expat
should be parsed correctly. For example, you should get an error if
you parse a document which claims to be US-ASCII but contains 8-bit
Set the encoding for parser to encoding.
XML_STATUS_OK for success.
This is like calling
parser-create (see Parser)
with encoding as the first arg.
parse-buffer has no effect and returns
Declare that parser is the default current parser.
Set the parser’s hash salt to salt, an unsigned integer.
This number is used for internal hash calculations.
Setting it helps prevent DoS attacks
based on predicting hash function behavior.
#t if successful (called before parsing has started),
Set base for parser to base (a string). Return a symbolic status.
Return the base (a string) of parser.
If none is set, return
Get the specified attribute count for parser.
Return a 2-D array describing the attributes of the current element. Each element of the array is an unsigned integer representing the byte offset of the attribute and value start and end positions. The end positions are “one past the last byte”. The array has the form:
attr1-attr-start attr1-attr-end attr1-val-start attr1-val-end attr2-attr-start attr2-attr-end attr2-val-start attr2-val-end ... ... ... ... attrN-attr-start attrN-attr-end attrN-val-start attrN-val-end
The attribute count (number n) is half the value that
If the underlying libexpat is prior to 2.1.0, or if it doesn’t
XML_GetAttributeInfo, return simply
You can determine this from the absence of
the return value of
get-feature-list (see Expat interface).
Set entity parsing for parser to code (a symbol). Valid values for code are:
XML_PARAM_ENTITY_PARSING_NEVER XML_PARAM_ENTITY_PARSING_UNLESS_STANDALONE XML_PARAM_ENTITY_PARSING_ALWAYS
This controls parsing of parameter entities (including the external DTD subset). See /usr/include/expat.h for more information.
Return a new XML-Encoding object.
map is a vector of length 256. Each element is an integer specifying how many bytes are required to decode a multibyte “character” whose first byte is that element’s index.
convert is a proc that takes one arg, a unibyte string. It should return the "Unicode scalar value" of the string, or -1 if the byte sequence is malformed.
release is a thunk the parser calls when done all conversion work.
After all the set up (see Expat handlers), you will want to apply the parser to some input. This section describes two procedures to do that, as well as two procedures to help you understand things better when All Does Not Go Well.
Use parser to parse string s.
Optional third arg finalp, if non-
this call is the last parsing to be done on s.
Return a symbolic status.
Use parser to parse len bytes of the internal buffer.
Optional third arg finalp, if non-
means this call is the last parsing to be done.
Return a symbolic status.
Stop parser. Return a symbolic status. If resumable
#f, parsing is suspended. If
#f, parsing is
This should be called from a handler (e.g.,
Note that some handlers will continue to be called before fully
Resume parser. Return a symbolic status, the same as
parse-buffer, with the addition of
This should not be called from a handler. It should be called first on the most deeply nested child parser, then successively on the parent parser(s).
(status final-buffer?), where status
is the symbolic status of parser with respect to being initialized,
parsing, finished, or suspended; and final-buffer is non-
processing is occurring for the final buffer.
Return a symbol corresponding to the error code for parser.
Return a string representing the error code (a symbol).
If code is not recognized, return
Here is an example that uses the latter two procedures. See Symbols.
(define BAD-XML "<doc>dfssfd</do>") ;; NB: not same! ^^^ ^^ (use-modules ((mixp expat) #:prefix E:)) (define PARSER (E:parser-create)) (define RES (E:parse PARSER BAD-XML #t)) RES ⇒ XML_STATUS_ERROR (define ERR (E:error-symbol PARSER)) ERR ⇒ XML_ERROR_TAG_MISMATCH (E:error-string ERR) ⇒ "mismatched tag"
The following functions are part of the expat interface, but are not exposed to Scheme.
Customized memory management interaction is out of scope for Mixp. (Sorry.) That said, maybe the next maintainer will be more bold. Only time can tell…
This kind of integration awaits widespread (and stable) Guile “array leasing” facilities.
This chapter describes the
(mixp utils) module,
which provides high-level extensions to the raw expat interface.
Read all bytes from port (until it yields the EOF object), and throw an error if the input does not represent a valid XML document.
The next three procedures can operate on a source (first arg
identically named from) of input. Portability Note:
When from is a string, Guile 2 and later use
open-input-string with the fluid
set to “binary” input (i.e.,
Convert the byte list from from UTF-8 to Latin-1. Throw
invalid-utf8 if from is not a valid UTF-8 stream, and
no-latin1 if one of the characters is a multi-byte character
(and thus cannot be a Latin-1 character). If from is a string,
return a string. If from is a list, return a list.
Convert a UTF-8 string, such as those returned by the parser, to a
UCS-2 list. from may be a string or a list. Return a
list whose elements are sub-lists with length two, each encoding a
character from the original stream. Throw a
no-ucs2 error if
one of the characters decoded from the UTF-8 string is not a UCS-2
Convert a UTF-8 string, such as those returned by the parser, to a UCS-4 stream. from may be a string or a list. Return a list whose elements are sub-lists with length four, each encoding a character from the original stream.
Build a tree data structure from the XML document read from
port. Each XML element produces a new branch in the tree.
Optional arg parser specifies another parser to use.
The internal parser uses element start (and end),
For example, consider this sample XML document:
<foo name='Paul'><bar>Some text</bar><void/></foo>
Here is the data structure produced by
(element ("foo" (("name" . "Paul"))) (element ("bar" ()) (character-data "Some text")) (element ("void" ())))
SXML over SSAX is the usual way to go about things, but we don’t
mind being unusual on occasion. This chapter describes the
(mixp simit) module, which provides “SXML over Expat”,
more or less. (The level of imitation is low while we figure
out what the heck is going on. Later, things should align more,
and weasel words like “SXML-ish” should go away…)
Parse an XML document from port with some namespaces,
a list of elements each of the form
where nick is a symbol, and uri is a string.
The XML namespace is built-in:
If parsing is successful, return an SXML-ish tree.
parse-error with two args, the symbolic
reason (suitable for passing to
the location of the error as returned by
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An MMC is “eligible for relicensing” if it is licensed under this License, and if all works that were first published under this License somewhere other than this MMC, and subsequently incorporated in whole or in part into the MMC, (1) had no cover texts or invariant sections, and (2) were thus incorporated prior to November 1, 2008.
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To use this License in a document you have written, include a copy of the License in the document and put the following copyright and license notices just after the title page:
Copyright (C) year your name. Permission is granted to copy, distribute and/or modify this document under the terms of the GNU Free Documentation License, Version 1.3 or any later version published by the Free Software Foundation; with no Invariant Sections, no Front-Cover Texts, and no Back-Cover Texts. A copy of the license is included in the section entitled ``GNU Free Documentation License''.
If you have Invariant Sections, Front-Cover Texts and Back-Cover Texts, replace the “with…Texts.” line with this:
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If you have Invariant Sections without Cover Texts, or some other combination of the three, merge those two alternatives to suit the situation.
If your document contains nontrivial examples of program code, we recommend releasing these examples in parallel under your choice of free software license, such as the GNU General Public License, to permit their use in free software.
|Jump to:||C D E F G H L M N P R S U X|
|error code, symbolic:||Symbols|
|Doing a parse|
|Doing a parse|
|Doing a parse|
|leftovers, libexpat:||Not implemented|
|libexpat leftovers:||Not implemented|
|loading Mixp:||Loading Mixp|
|miscellaneous procedures, ||Expat misc|
|mixp components:||Mixp components|
|Doing a parse|
|Doing a parse|
|parser, application to input:||Doing a parse|
|Doing a parse|
|sample programs:||Sample programs|
|Doing a parse|
|symbolic error code:||Symbols|
|Jump to:||C D E F G H L M N P R S U X|