Re: puzzling byte-compile-file message: `=' called for effect

[Yevgeniy Makarov <emakarov@cs.indiana.edu>]

Jim Ottaway wrote:
> When byte-compiling a file, I get this message:
> 
> In nl-publish-markup-list:
> nested-lists.el:395:19:Warning: `=' called for effect
> 
> What does this mean?  I grepped for 'for effect' in Emacs's
> lisp/emacs-lisp subdirectory, and found the relevant parts of
> byte-opt.el, but I am no wiser.
> 
> The code with the '=' in it is:
> 
> (when (and nl-paras-fixed-p
>                      (save-excursion
>                        (goto-char nl-markup-start)
>                        (goto-char (muse-line-beginning-position))
>                        (= (forward-line -1) 0)
>                        (looking-at "^\\S-")))
>             (with-current-buffer nl-temp-buffer
>               (goto-char (point-min))
>               (insert "\n\n")))
> 
> Any ideas? Does it mean that the byte-compiler thinks that
> (forward-line -1) will always return 0 at that point? Or does 'for
> effect' mean something else?
> 

Again, I am not an expert, so I don't know if this is useful. But the 
fact is that save-excursion executes its arguments in turn and returns 
the value of the last one. Therefore, the value of (= (forward-line -1) 
0) is not used. This line produces a side effect, however, namely, 
(forward-line -1). But the wrapping of = around it does not make sense 
because its value is irrelevant for the rest of the program.

Yevgeniy