Re: Warning: `mapcar' called for effect; use `mapc' or `dolist'

[Dan Nicolaescu <dann@ics.uci.edu>]

Reiner Steib <reinersteib+gmane@imap.cc> writes:

  > On Mon, Oct 22 2007, Katsumi Yamaoka wrote:
  > 
  > >>>>>> Dan Nicolaescu wrote:
  > >> The subject is about a new warning produced by the byte
  > >> compiler. There's a lot of those for gnus.
  > >> Assuming that all emacs versions that gnus wants to support have
  > >> either mapc or dolist, can somebody please take care of these
  > >> warnings?
  > >
  > >> Thanks
  > >
  > > With No Gnus v0.7 I did `cd lisp; make warn' and tried replacing
  > > of `mapcar' with `mapc', `dolist', or `while' loops for a while,
  > > and realized it's not easy and not safe.  Ones who do it will
  > > need to study what things using `mapcar' do in all cases.  For
  > > instance, `mapcar' used in `gnus-group-highlight-line' cannot be
  > > replaced with `mapc' even if the compiler warns it.  
  > 
  > Could the byte-compiler be improved not to warn about such cases?
  > Here's the relevant code:
  > 
  > (defun gnus-group-highlight-line ()
  >   "Highlight the current line according to `gnus-group-highlight'."
  >   (let* ([...]
  > 	 (mailp (apply 'append
  > 		       (mapcar
  > 			(lambda (x)
  > 			  (memq x (assoc (symbol-name
  > 					  (car (or method gnus-select-method)))
  > 					 gnus-valid-select-methods)))
  > 			'(mail post-mail))))
  > 	 (level (or (gnus-info-level info) gnus-level-killed))

It seems that `mailp' is not used in that function. Adding a use for
it makes the warning go away...