save-excursion doesn't restore point

save-excursion doesn't restore point

[Dominic Mitchell]

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In GNU Emacs 21.3.1 (i386--freebsd, X toolkit, Xaw3d scroll bars)
 of 2005-12-09 on ppe.happygiraffe.net
configured using `configure  --x-libraries=/usr/X11R6/lib --x-includes=/usr/X11R6/include --prefix=/usr/local i386--freebsd'
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: nil
  locale-coding-system: nil
  default-enable-multibyte-characters: t

Please describe exactly what actions triggered the bug
and the precise symptoms of the bug:

I have this bit of code in my .emacs file:

(defun perltidy ()
  "Run perltidy on the current region or buffer."
  (interactive)
  (save-excursion
    (unless mark-active (mark-defun))
    (shell-command-on-region (point) (mark) "perltidy -q" nil t)))

If I run it from inside a Perl function (in cperl-mode), then the
point ends up _above_ the Perl function, instead of remembering where
it was.

I would guess that this is because save-excursion uses markers, and
those markers then get invalidated by shell-command-on-region.  But
it's still a little weird.  Is this a bug?

Thanks,
-Dom

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Re: save-excursion doesn't restore point

[Richard M. Stallman]

    I would guess that this is because save-excursion uses markers, and
    those markers then get invalidated by shell-command-on-region.

I think so.

								    But
    it's still a little weird.  Is this a bug?
    
I think it is simply inevitable.  If all the text is replaced,
Emacs has no way of knowing what new position corresponds in any sense
to some old position.