bug#47150: [External] : bug#47150: 28.0.50; Incorrect major-mode in minibuffer

[Alan Mackenzie <acm@muc.de>]

Hello, Stefan.

On Mon, Apr 12, 2021 at 16:46:34 -0400, Stefan Monnier wrote:

[ .... ]

> > Am I right in thinking that your main worry is the hook not getting
> > called at the end of every MB action?

> No.  My worries are:
> - not having the minibuffer-inactive-mode-map active when the minibuffer
>   is inactive.
> - running minibuffer-inactive-mode-hook at other times than when the
>   minibuffer becomes inactive.

> >> > The idea here is to avoid the proliferation of unneeded major modes.
> >> Major modes are cheap.  There is no problem with proliferation.
> > That's not true - the OP has found a problem, in that some minor modes
> > switch themselves on when (memq major-mode foo-mode-list).
> > The current situation, fundamental-mode (active),
> > minibuffer-inactive-mode (inactive) is causing problems with that
> > scheme, hence this bug.

> Their code was buggy/naive, will be broken no matter what we choose
> to do (except for sticking to what we had in Emacs<28), and is easy to
> fix in a backward compatible way using `minibufferp`.

This would make a special case of minibuffer_mode, which would require
minibufferp, when other modes would be matched with (memq major-mode
foo-list).

> So I don't think this matters very much.

This was the meat of the OP's bug report.  ;-)

> Most cases of (eq major-mode <foo>) are bugs waiting to bite you.

I don't see this at the moment, but I won't ask you to elaborate here
and now.

> > How about having just minibuffer-mode, and calling it at the end of
> > every MB action (as was previously done with
> > minibuffer-inactive-mode), but not at the start of a MB action?
> > This will call the mode hook at the same times as the
> > m-inactive-m-hook used to be called, and reset the MB's keymap to
> > the inactive map at the same time.

> IOW just renaming `minibuffer-inactive-mode` to `minibuffer-mode` and
> calling it one extra time at the very beginning?

Yes.  It's that "one extra time at the very beginning" which makes it
nasty.  :-(

> Technically, it won't break any of my uses, of course, but then it leads
> to rather counter-intuitive situations where "the keymap of
> `minibuffer-mode`" is almost never used (it's only active when the
> minibuffer is inactive), "the hook of `minibuffer-mode`" is run not when
> entering a minibuffer but when leaving it, ...

> Also, there's a natural desire to occasionally use other major modes in
> the minibuffer (e.g. for `M-:`), so it would be very natural to make
> them derived modes of `minibuffer-mode` except that it would then
> inherit from a keymap which makes no sense in an active minibuffer.

> It just doesn't seem right at all.

Yes, it has disadvantages.  You're right.

> What's wrong with just making a new mode

>     (define-derived-mode minibuffer-mode nil "Minibuffer"
>       "Mode used inside minibuffers.")

> and using that instead of `fundamental-mode`.

The scheme you propose, having two modes minibuffer-\(inactive-\)?mode
also has disadvantages, pointed out by Drew in his post in this thread
from Date: Mon, 22 Mar 2021 17:09:32 +0000 - minibuffer-mode would be a
useless mode, just a placeholder; it has a key map, a syntax table, an
abbreviation table which will never be used (OK, some of these can be
specified nil in define-derived-mode), a redundant mode hook (there
exist minibuffer-setup-hook and minibuffer-exit-hook).  These can surely
only cause trouble down the line.  So Drew is right, too.

The problem is, we're in an anomalous situation.  Major modes aren't
really the right tool for the minibuffer, but not using major modes
isn't any better.  Anything we do here is bad.  :-(

Over the past few days, I've come to the conclusion that having two
modes for the minibuffer is the lesser evil than having just one mode
whose mode function would be called at the end of a minibuffer use.  The
deciding criterion, which might seem minor, is that with the one mode,
we'd have to call it "one extra time at the very beginning" as discussed
above.

So I intend to leave minibuffer-inactive-mode more or less alone, and to
implement minibuffer-mode, which will get called at the start of each
minibuffer use, flushing out old stuff from any previous minibuffer
(non-)use.

>         Stefan

-- 
Alan Mackenzie (Nuremberg, Germany).