bytecomp.el: issue warning when `mapcar' is called for effect

bytecomp.el: issue warning when `mapcar' is called for effect

[Markus Triska]

This makes the byte-compiler mention `mapc' and `dolist' where one of
them is preferable over `mapcar'.


2007-06-19  Markus Triska  <markus.triska@gmx.at>

	* emacs-lisp/bytecomp.el (byte-compile-normal-call): Issue warning
	when `mapcar' is called for effect.

Index: bytecomp.el
===================================================================
RCS file: /sources/emacs/emacs/lisp/emacs-lisp/bytecomp.el,v
retrieving revision 2.202
diff -c -r2.202 bytecomp.el
*** bytecomp.el	14 Jun 2007 21:25:11 -0000	2.202
--- bytecomp.el	19 Jun 2007 16:44:36 -0000
***************
*** 2831,2836 ****
--- 2831,2840 ----
  (defun byte-compile-normal-call (form)
    (if byte-compile-generate-call-tree
        (byte-compile-annotate-call-tree form))
+   (when (and for-effect (eq (car form) 'mapcar))
+     (byte-compile-set-symbol-position 'mapcar)
+     (byte-compile-warn
+      "`mapcar' called for effect; use `mapc' or `dolist' instead"))
    (byte-compile-push-constant (car form))
    (mapc 'byte-compile-form (cdr form))	; wasteful, but faster.
    (byte-compile-out 'byte-call (length (cdr form))))

RE: bytecomp.el: issue warning when `mapcar' is called for effect

[Drew Adams]

> This makes the byte-compiler mention `mapc' and `dolist' where one of
> them is preferable over `mapcar'.
>
> +      "`mapcar' called for effect; use `mapc' or `dolist' instead"))

How about "consider using" instead of "use"? This is, after all, a warning,
not an error.

Some libraries aim for use with multiple Emacs versions, and some Emacs
versions don't have `mapc' or `dolist' (and a library might not have
required `cl' at compile time). When users see such warnings, they can freak
out, submit bug reports to the library maintainers, etc.

If the message length is a problem, drop "instead" -

 "`mapcar' called for effect; consider using `mapc' or `dolist'"
is not much longer than
 "`mapcar' called for effect; use `mapc' or `dolist' instead"

Re: bytecomp.el: issue warning when `mapcar' is called for effect

[Richard Stallman]

Would someone please install this, with whatever improvements seem
worth doing.