GNU bug report logs - #79412
30.2; `define-globalized-minor-mode' does not respect `:variable'

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From: Eli Zaretskii <eliz <at> gnu.org>
To: inet0 <at> qq.com, Stefan Monnier <monnier <at> iro.umontreal.ca>
Cc: 79412 <at> debbugs.gnu.org
Subject: bug#79412: 30.2; `define-globalized-minor-mode' does not respect `:variable'
Date: Tue, 09 Sep 2025 15:02:56 +0300

> Date: Tue, 09 Sep 2025 03:40:36 +0800
> From: inet0--- via "Bug reports for GNU Emacs,
>  the Swiss army knife of text editors" <bug-gnu-emacs <at> gnu.org>
> 
> I am using Emacs as a GUI application on Android (distributed by F-Droid
> if that matters) so `emacs -Q' is not an option, but the symptoms should
> be reproduced by
> 
> (1) evaluating the program segments below, then
> 
> (2) typing `C-c b' twice.
> 
> (define-minor-mode bidi-display-reordering-mode
>   nil
>   :variable bidi-display-reordering)
> 
> (define-globalized-minor-mode global-bidi-display-reordering-mode
>   bidi-display-reordering-mode
>   (lambda () (bidi-display-reordering-mode 1))
>   :variable bidi-display-reordering)
> 
> (keymap-global-set "C-c b" 'global-bidi-display-reordering-mode)
> 
> Buffer *Messages*:
> 
> Loading gnus...done
> For information about GNU Emacs and the GNU system, type C-h C-a.
> Global Bidi-Display-Reordering mode enabled
> if: Symbol’s value as variable is void: bidi-display-reordering-mode

The ELisp manual says about ':variable':

     ‘:variable PLACE’
          This replaces the default variable MODE, used to store the
          state of the mode.  If you specify this, the MODE variable is
          not defined, and any INIT-VALUE argument is unused.

What this means is that using :variable leaves the mode variable, in
this case bidi-display-reordering-mode, undefined.  Therefore, I think
you must define some non-trivial BODY for your minor mode, because the
default body will try to reference bidi-display-reordering-mode, and
will fail, as this backtrace shows:

> Buffer *Backtrace*:
> 
> Debugger entered--Lisp error: (void-variable bidi-display-reordering-mode)
                                 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
> With that in mind, I suspect that the buffer-local mode name is
> mistakenly referenced instead of the name specified by keyword
> `:variable', thus causing a void-variable error.

I don't think this is accurate.  A globalized minor mode works by
walking the list of all the buffers and turning on the local minor
mode in each one of them.

> I have found something suspicious at source code
> `lisp/emacs-lisp/easy-mmode.el:569:23':
> 
> 	 ;; Go through existing buffers.
> 	 (dolist (buf (buffer-list))
> 	   (with-current-buffer buf
>              (if ,global-mode (funcall ,turn-on-function)
>                (when ,mode (,mode -1)))))
> 
> It seems that `mode' at the predicate position of `when' should really
> be `MODE-variable'!

Adding Stefan in case he has some insights.

> As an aside, the docstring of `define-globalized-minor-mode' is a bit
> off the description in the ELisp manual. (Jumping between `C-h f' and
> `C-h i m el i' is annoying, isn't it?)
> 
> The manual says:
> 
>      By default, the buffer-local minor mode variable that says whether
>      the mode is switched on or off is the same as the name of the mode
>      itself.  Use ‘:variable VARIABLE’ if that’s not the case–some minor
>      modes use a different variable to store this state information.
> 
> While in docstring:
> 
> Other keywords have the same meaning as in ‘define-minor-mode’,
> which see.
> 
> Clearly, `:variable' in `define-globalized-minor-mode' does not have the
> same meaning as in `define-minor-mode', otherwise you would be
> specifying the mode variable of the global mode, which is not the case.

Thanks.

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