Re: mode-line-format - local variableness

[Stefan Monnier]

>   (with-current-buffer (or buffer (current-buffer))
>     (let ((mode-line-format  text))
>       (force-mode-line-update) (sit-for 2))
>     (force-mode-line-update)))

Combining let-binding and buffer-local values is a difficult exercise.
The behavior of the above depends on whether mode-line-format is
currently buffer-local in the current buffer.  Some Elisp variables are
never buffer-local (e.g. the ones that are frame-local), others (most)
are by default global can be made buffer-local to a particular buffer by
calling `make-local-variable', others are buffer-local by default, which
means that they become buffer-local as soon as you use `setq' on them
(but not `let'), and finally yet others are simply always buffer-local
(e.g. default-directory).

It seems that mode-line-format was "always buffer-local" in Emacs-20 and
changed to "buffer-local by default" in Emacs-21.  I don't know what was
the reason for it, but in any case an explicit call to
`make-local-variable' before the let will clear up any ambiguity.


        Stefan