Recursive edits in `save-excursion'

Recursive edits in `save-excursion'

[Stuart D. Herring]

I find that I get unexpected results from multiple recursive edits in
`save-excursion' forms in one command (e.g., evaluating

(list (save-excursion (recursive-edit) (current-buffer)) (save-excursion
(recursive-edit) (current-buffer)))

and, during each recursive edit, selecting some different buffer).  The
first time, `current-buffer' returns the buffer selected during the
recursive edit, but the second time the buffer that was current when the
evaluation began is returned.  Similar things happen with `point-marker'
in place of `current-buffer', which is how I found this.  Is this a bug,
or is it just some consequence of interaction between the (recursive)
command loop and `save-excursion' that I don't understand?

Thanks,
Davis

PS - The "Recent Input" below is everything from "emacs -Q", so can be
used as a precise test case.

In GNU Emacs 22.0.50.1 (i686-pc-linux-gnu, Motif Version 2.2.2)
 of 2006-09-24 on callahan
X server distributor `The XFree86 Project, Inc', version 11.0.40300000
configured using `configure '--with-x-toolkit=motif' '--with-xpm'
'--with-jpeg' '--with-tiff' '--with-gif' '--with-png' '--with-x''

Important settings:
  value of $LC_ALL: nil
  value of $LC_COLLATE: nil
  value of $LC_CTYPE: nil
  value of $LC_MESSAGES: nil
  value of $LC_MONETARY: nil
  value of $LC_NUMERIC: nil
  value of $LC_TIME: nil
  value of $LANG: en_US.UTF-8
  locale-coding-system: utf-8
  default-enable-multibyte-characters: t

Major mode: Fundamental

Minor modes in effect:
  tooltip-mode: t
  tool-bar-mode: t
  mouse-wheel-mode: t
  menu-bar-mode: t
  file-name-shadow-mode: t
  global-font-lock-mode: t
  blink-cursor-mode: t
  unify-8859-on-encoding-mode: t
  utf-translate-cjk-mode: t
  auto-compression-mode: t
  line-number-mode: t

Recent input:
C-x b a <return> C-x b b <return> C-x b * s c r <tab>
<return> ( l i s t SPC ( s a v e - e x c u r s i o
n SPC ( r e c u r s i v e - e d i t ) S-SPC ( c u r
r e n t - b u f f e r ) ) C-- C-M-k C-y SPC C-y ) C-x
C-e C-x b a <return> C-M-c C-x b b <return> C-M-c <help-echo>
<help-echo> <help-echo> <help-echo> <menu-bar> <help-menu>
<report-emacs-bug>

Recent messages:
(emacs -Q)
For information about the GNU Project and its goals, type C-h C-p.
Mark set [2 times]
(#<buffer a> #<buffer *scratch*>)
Loading emacsbug...
Loading regexp-opt...done
Loading emacsbug...done

-- 
This product is sold by volume, not by mass.  If it appears too dense or
too sparse, it is because mass-energy conversion has occurred during
shipping.

Re: Recursive edits in `save-excursion'

[Richard Stallman]

The explanation for this is that save-excursion saves the current
buffer, but what you set when you switch buffers is the selected
window's buffer.  save-excursion does not restore that.  The first
save-excursion restores the original current buffer but doesn't
switch to it.  The second recursive edit notices that the current
buffer isn't the same one you've switched to, so it arranges to
save and restore the current buffer.

I will document this.

Re: Recursive edits in `save-excursion'

[Stuart D. Herring]

> The explanation for this is that save-excursion saves the current
> buffer, but what you set when you switch buffers is the selected
> window's buffer.  save-excursion does not restore that.  The first
> save-excursion restores the original current buffer but doesn't
> switch to it.  The second recursive edit notices that the current
> buffer isn't the same one you've switched to, so it arranges to
> save and restore the current buffer.

So -- just to make sure that I understand correctly -- `recursive-edit'
acts like it was implemented as follows?

(defun recursive-edit ()
  "..."
  (let ((obuf (unless (eq (current-buffer) (window-buffer))
                (current-buffer)))
    (set-buffer (window-buffer))
    (unwind-protect
        (if (eq (catch 'exit (command-loop) nil) t) (signal 'quit nil))
      (if (obuf (set-buffer obuf))))))

In other words, it acts like it is a `save-excursion' embedded if and only
if the current buffer and current window's buffer differ?  So you could
defeat this by doing something like what follows?

(save-excursion (set-buffer (window-buffer)) (recursive-edit) ...)

My concern is not which buffer is current around this form, or in which
buffer the `recursive-edit' starts, but just that the `...' be evaluated
with the buffer current that was current when the recursive edit was
exited.

I also wonder if it would be profitable to use the original form, but
calling `window-buffer' instead of `current-buffer'?

(save-excursion (recursive-edit) (window-buffer))

A quick test with the 21.3 I have in front of me seems to support my
theory (both of the modified examples work as I would expect).  Assuming
I'm correct that they work reliably, is there a reason to prefer one or
the other?

Thanks,
Davis

-- 
This product is sold by volume, not by mass.  If it appears too dense or
too sparse, it is because mass-energy conversion has occurred during
shipping.

Re: Recursive edits in `save-excursion'

[Richard Stallman]

    So -- just to make sure that I understand correctly -- `recursive-edit'
    acts like it was implemented as follows?

Yes, I think so.