Emacs-lisp (elisp) is a nice language to play around with code and try things as
you develop them – explorative programming. I often use the *scratch*
buffer for that, but sometimes it's nice to use a so-called 'REPL' (
Read-Eval-Print-Loop) instead. A REPL is a sort-of command-line interface
where your expressions are evaluated as soon as they are considered 'complete'
and you press Enter.
So, enter Emacs's built-in repl: IELM. You can activate it with M-x ielm,
and the interaction looks something like the following:
*** Welcome to IELM *** Type (describe-mode) for help.
ELISP> 123
123
ELISP> (+ 1 2)
3
ELISP> ;; comment
ELISP> (defun fac (n)
(if (= 0 n)
1
(* n (fac (- n 1)))))
fac
ELISP> (fac 5)
120
ELISP>
By default, IELM evaluates complete expressions automatically as soon you as you press Enter. So one thing to remember is that if you want to have multi-line expression (like above), you must make sure that after each line the expression is not complete (i.e., the brackets are not balanced) -- otherwise the expression will be evaluated too early. That makes modes like autopair or paredit a bit inconvenient for this.
If you don't like that behavior, you can do:
(setq ielm-dynamic-return nil)
which will allow you to Enter as much as you want and only evaluate things when
you press C-j. But then you might as well use *scratch* I
suppose. Personally, I use IELM mostly as a calculator.
7 comments:
If you're using IELM as a calculator, you really need to read up on Emacs calc! It's 100x better for that purpose.
@Anonymous: well, I know calc, but I've never been totally happy with the interaction model, even when using algebraic model. It's also a bit of overkill. Anyway, maybe I should try to give it another chance.
I was just coming here to suggest calc myself.
Try out C-x * q
It's kinda like M-x calculator.
I didn't know emacs lisp had a repl. Cool! As for doing calculations... When I need to calculate something, I fire Pari/GP in a terminal and then discard it when I'm finished. Usually Pari is overkill, but sometimes I just leave it open and start coding a little (Pari is used for high precision computation and number theoretic stuff).
Cheers,
Ruben
IELM is also immensely useful when developing and testing minor and major modes, due to the following feature: "The current working buffer may be changed (with a call to `set-buffer',
or with C-c C-b), and its value is preserved between successive
evaluations. In this way, expressions may be evaluated in a different
buffer than the *ielm* buffer. By default, its name is shown on the
mode line; you can always display it with C-c C-v, or the buffer itself
with C-c C-f."
isnt't eshell also an elisp repl, and much more? i use eshell all the time for this -- or for shorter things, eval-expression (M-:).
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