1.1.2 Overlooked Treasure

Emacs has a powerful means of programmatically extending itself while it is running. Not many successful applications can boast of that, but I believe a powerful idea within Emacs has been overlooked as an Emacsism rather than an idea of general utility. Let me mention another idea that might have become a Lispism but has since seen widespread adoption.

The Lisp programming language introduced the term Read-Eval-Print-Loop (REPL, pronounced rep-pel), an interactive programming feature present in many dynamic languages: Python, Ruby, MATLAB, Mathematica, Lua to name a few. The pseudo code is given below.

<<Lisp REPL>>= (while #t (print (eval (read))))  The REPL interaction pattern is to enter one complete expression, hit the return key, and the result of that expression will be displayed. It might look like this:

> (+ 1 2)
3

The kernel of Emacs is conceptually similar to the REPL, but the level of interaction is more fine grained. A REPL assumes a command line interface. Emacs assumes a keyboard interface. I have not seen the kernel of Emacs exhibited in any other applications, but I think it is of similar utility to the REPL—and entirely separate from text editing. I’d like to name this the Key-Lookup-Execute-Command-Loop (KLECL, pronounced clec-cull).