Re: use of (defvar <foo>)

[Dave Love <fx@gnu.org>]

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

> ,----[ http://article.gmane.org/gmane.emacs.devel/43027 ]
> | From: Richard M. Stallman
> | Subject: Re: defvars at compile time
> |
> | The only effect of a defvar with no initial value
> | is to silence the compiler.  So it is superfluous
> | to put it inside eval-when-compile.
> `----

That's certainly not true in Emacs 21:

(progn
  (with-temp-file "/tmp/x.el"
    (insert "(defvar foo)\n"))
  (byte-compile-file "/tmp/x.el")
  (with-temp-buffer 
    (insert-file-contents-literally "/tmp/x.elc")
    (goto-char (point-max))
    (delete-region 1 (line-beginning-position 0))
    (buffer-string)))
  => "(byte-code \"\\301\bB\x10\\301\\207\" [current-load-list foo] 2)
"

You can completely screw Emacs if you unload a feature that does that
on important variables.  It also causes confusion with find-function &
al.  I fixed a fair number of cases in the Emacs source that could
clobber things like `font-lock-keywords' and documented using
eval-when-compile in the Lisp manual.  I don't remember what the
justification was for the one-arg defvar modifying the load history at
the time; that sort of policy decision often gets reversed.