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Re: Switch buffers without modifying the buffer list ordering?

From: martin rudalics
Subject: Re: Switch buffers without modifying the buffer list ordering?
Date: Mon, 31 Jan 2022 09:42:10 +0100

> With the following code, I can verify that the NORECORD argument of
> `switch-to-buffer' does what it promises:
> ```
> (progn
>    (switch-to-buffer "A")
>    (switch-to-buffer "B" 'norecord)
>    (car (buffer-list)))
> ```
> Namely, this makes buffer B current, but returns #<buffer A>.
> Now, here's a catch: if next I type `M-x (car (buffer-list))',

... M-: (car (buffer-list)) I suppose ...

> I get
> #<buffer B> instead of #<buffer A>.  Why does that happen, and is there
> a way to switch to buffer B and *really* not modify the buffer list
> ordering?

It's restoring the window configuration after reading from the
minibuffer but before evaluating the expression you typed that gets in
the way.  Try with emacs -Q and evaluate the forms below step by step

(display-buffer "*Messages*")

(defun foo ()
   "%s - %s" (selected-window) (car (buffer-list))))

(add-hook 'buffer-list-update-hook 'foo)

  (switch-to-buffer "A")
  (switch-to-buffer "B" 'norecord)
  (car (buffer-list)))

and now type M-: (car (buffer-list)).  This gets me in the *Messages*

#<window 3 on A> - A [3 times]
#<buffer A>
#<window 3 on B> - A
#<window 4 on  *Minibuf-1*> -  *Minibuf-1* [4 times]
#<window 3 on B> - B [2 times]
#<buffer B>

Now try again the same scenario but first evaluate

(setq read-minibuffer-restore-windows nil)

which gets me here the

#<window 3 on A> - A [3 times]
#<buffer A>

you probably expected.

> I'm asking this because of an issue with the live preview feature of the
> Consult package.  Very succinctly, the idea here is to temporarily
> change the current buffer during a `completing-read' call, in order to
> display more information about the candidates.  When the
> `completing-read' ends, we would like to restore the original order of
> the buffer list.  There seems to be no simple way to achieve that, and
> some kludge like the following seems necessary.  Any better suggestions?
> ```
> (let ((buffers (buffer-list)))
>      (unwind-protect
>          <do the completing-read with live preview>
>        (save-window-excursion
>          ;; Restore the original buffer list ordering
>          (dolist (buffer buffers)
>            (when (buffer-live-p buffer)
>              (bury-buffer-internal buffer))))))

That's a different issue.  'buffer-list' is a function that returns a
list from the internal C variable Vbuffer_alist - a variable, Lisp code
is not allowed to modify directly.  In order to modify Vbuffer_alist, a
Lisp programmer has to bury buffers in precisely the same way as you do.

You could try coming up with a macro, say 'with-buffer-list-unmodified'
(where you probably should suspend calling 'buffer-list-update-hook'
while running it).


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