GNU bug report logs - #13122
transpose-subr defeats save-excursion

Previous Next

To add a comment to this bug, you must first unarchive it, by sending
a message to control AT debbugs.gnu.org, with unarchive 13122 in the body.
You can then email your comments to 13122 AT debbugs.gnu.org in the normal way.

Toggle the display of automated, internal messages from the tracker.

Message #5 received at submit <at> debbugs.gnu.org (full text, mbox):

From: Jyothis V <jyothisv <at> gmail.com>
To: bug-gnu-emacs <at> gnu.org
Subject: save-excursion not saving point
Date: Sat, 8 Dec 2012 11:50:13 +0530

[Message part 1 (text/plain, inline)]

Hi,

I wrote the following version of transpose-chars which is supposed to
always transform the last two chars without moving the point.
(defun gosmacs-tranpose-chars (n)
  (interactive "p")
  (save-excursion
    (forward-char (- n))
    (transpose-chars 1)))

The trouble is, even though I have used save-excursion, the point does
move 2 characters backwards when n=1! For other values of n, this seems to
be working fine.

Regards,
Jyothis V

In GNU Emacs 24.2.1 (x86_64-unknown-linux-gnu, GTK+ Version 3.6.2)
 of 2012-11-18 on eric
Windowing system distributor `The X.Org Foundation', version 11.0.11300000
Configured using:
 `configure '--prefix=/usr' '--sysconfdir=/etc' '--libexecdir=/usr/lib'
 '--localstatedir=/var' '--with-x-toolkit=gtk3' '--with-xft'
 'CFLAGS=-march=x86-64 -mtune=generic -O2 -pipe -fstack-protector
 --param=ssp-buffer-size=4 -D_FORTIFY_SOURCE=2'
 'LDFLAGS=-Wl,-O1,--sort-common,--as-needed,-z,relro''

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
  value of $XMODIFIERS: nil
  locale-coding-system: utf-8-unix
  default enable-multibyte-characters: t

Major mode: Lisp Interaction

Minor modes in effect:
  diff-auto-refine-mode: t
  global-auto-complete-mode: t
  auto-complete-mode: t
  icomplete-mode: t
  ido-everywhere: t
  shell-dirtrack-mode: t
  which-function-mode: t
  cua-mode: t
  display-time-mode: t
  electric-pair-mode: t
  show-paren-mode: t
  tooltip-mode: t
  mouse-wheel-mode: t
  file-name-shadow-mode: t
  global-font-lock-mode: t
  font-lock-mode: t
  auto-composition-mode: t
  auto-encryption-mode: t
  auto-compression-mode: t
  line-number-mode: t
  transient-mark-mode: t

[Message part 2 (text/html, inline)]

Message #8 received at 13122 <at> debbugs.gnu.org (full text, mbox):

From: martin rudalics <rudalics <at> gmx.at>
To: Jyothis V <jyothisv <at> gmail.com>
Cc: 13122 <at> debbugs.gnu.org
Subject: Re: bug#13122: save-excursion not saving point
Date: Sat, 08 Dec 2012 12:21:08 +0100

> I wrote the following version of transpose-chars which is supposed to
> always transform the last two chars without moving the point.
> (defun gosmacs-tranpose-chars (n)
>   (interactive "p")
>   (save-excursion
>     (forward-char (- n))
>     (transpose-chars 1)))
>
> The trouble is, even though I have used save-excursion, the point does
> move 2 characters backwards when n=1! For other values of n, this seems to
> be working fine.

This is a limitation (or bug) in the code of `transpose-subr' and
`transpose-subr-1' due to the use of `delete-and-extract-region' (IIRC
transposing mangles `undo' as well.)

martin

Message #17 received at 13122-done <at> debbugs.gnu.org (full text, mbox):

From: Chong Yidong <cyd <at> gnu.org>
To: martin rudalics <rudalics <at> gmx.at>
Cc: Jyothis V <jyothisv <at> gmail.com>, 13122-done <at> debbugs.gnu.org
Subject: Re: bug#13122: save-excursion not saving point
Date: Fri, 21 Dec 2012 13:43:55 +0800

martin rudalics <rudalics <at> gmx.at> writes:

> This is a limitation (or bug) in the code of `transpose-subr' and
> `transpose-subr-1' due to the use of `delete-and-extract-region' (IIRC
> transposing mangles `undo' as well.)

I've committed a fix to trunk.

Message #20 received at 13122 <at> debbugs.gnu.org (full text, mbox):

From: Alexander Kjeldaas <alexander.kjeldaas <at> gmail.com>
To: 13122 <at> debbugs.gnu.org
Subject: sort-regexp-fields also seems to not save point
Date: Mon, 7 Jan 2013 13:32:18 +0100

[Message part 1 (text/plain, inline)]

I think this might be related.  In the following function, point is also
not restored:

(defun my-java-organize-imports2 ()
  "Sorts import statements"
  (interactive)
  (message "Point %d" (point))
  (sit-for 3)
  (save-excursion
    (let (deactivate-mark)
      ;; Remove empty lines between imports
      (replace-regexp "^import \\(.*\\)\n\\(\n\\)*import \\(.*\\)$"
                      "import \\1\nimport \\3"
                      nil (point-min) (point-max))
      ;; Sort imports in file
      (sort-regexp-fields nil "^.*$" "^import .*$" (point-min)
(point-max))))
  (message "Point %d" (point))
  (sit-for 3))

Thanks,
Alexander

[Message part 2 (text/html, inline)]

Message #27 received at 13122 <at> debbugs.gnu.org (full text, mbox):

From: Tino Calancha <tino.calancha <at> gmail.com>
To: 13122 <at> debbugs.gnu.org
Subject: Re: bug#13122: save-excursion not saving point
Date: Sat, 27 Apr 2019 04:53:56 +0900

;; Following expression should eval as non-nil
(let ((pos 3))
  (with-temp-buffer
    (insert "abcdef")
    (goto-char pos)
    (save-excursion (transpose-chars 2))
    (= pos (point))))
=> nil

In GNU Emacs 27.0.50 (build 3, x86_64-pc-linux-gnu, GTK+ Version 3.22.11)
 of 2019-04-23 built on calancha-pc.dy.bbexcite.jp
Repository revision: 981470e3590534a4d2947dfe5626cae832c6502d
Repository branch: master
Windowing system distributor 'The X.Org Foundation', version 11.0.11902000
System Description: Debian GNU/Linux 9 (stretch)


Jyothis V <jyothisv <at> gmail.com> writes:


> I wrote the following version of transpose-chars which is supposed to
> always transform the last two chars without moving the point.
> (defun gosmacs-tranpose-chars (n)
>   (interactive "p")
>   (save-excursion
>     (forward-char (- n))
>     (transpose-chars 1)))
>
> The trouble is, even though I have used save-excursion, the point does
> move 2 characters backwards when n=1! For other values of n, this seems to be working fine.
>
> Regards,
> Jyothis V
>
> In GNU Emacs 24.2.1 (x86_64-unknown-linux-gnu, GTK+ Version 3.6.2)
>  of 2012-11-18 on eric
> Windowing system distributor `The X.Org Foundation', version 11.0.11300000

Message #30 received at 13122 <at> debbugs.gnu.org (full text, mbox):

From: Eli Zaretskii <eliz <at> gnu.org>
To: Tino Calancha <tino.calancha <at> gmail.com>
Cc: 13122 <at> debbugs.gnu.org
Subject: Re: bug#13122: save-excursion not saving point
Date: Sat, 27 Apr 2019 09:57:24 +0300

> From: Tino Calancha <tino.calancha <at> gmail.com>
> Date: Sat, 27 Apr 2019 04:53:56 +0900
> 
> ;; Following expression should eval as non-nil
> (let ((pos 3))
>   (with-temp-buffer
>     (insert "abcdef")
>     (goto-char pos)
>     (save-excursion (transpose-chars 2))
>     (= pos (point))))
> => nil

Why did you think it should evaluate to non-nil?  The ELisp manual has
a warning near the end of description of save-excursion; I think it's
relevant to this use case.

Message #33 received at 13122 <at> debbugs.gnu.org (full text, mbox):

From: Tino Calancha <tino.calancha <at> gmail.com>
To: Eli Zaretskii <eliz <at> gnu.org>
Cc: 13122 <at> debbugs.gnu.org, Tino Calancha <tino.calancha <at> gmail.com>
Subject: Re: bug#13122: save-excursion not saving point
Date: Sat, 27 Apr 2019 16:44:52 +0900 (JST)

On Sat, 27 Apr 2019, Eli Zaretskii wrote:

>> From: Tino Calancha <tino.calancha <at> gmail.com>
>> Date: Sat, 27 Apr 2019 04:53:56 +0900
>>
>> ;; Following expression should eval as non-nil
>> (let ((pos 3))
>>   (with-temp-buffer
>>     (insert "abcdef")
>>     (goto-char pos)
>>     (save-excursion (transpose-chars 2))
>>     (= pos (point))))
>> => nil

I read that warning; maybe getting `nil' is above is expected; if that's 
the case then we can close the report again.

The reason why I shared the above snippet is because Chong applied a patch
(commit: aa26f345096166bd8c135876dbab9b671ae232e3)
to fix the original recipe:
a call to `traspose-chars' inside a `save-excursion' moving the 
point if the argument is n != 1.

> Why did you think it should evaluate to non-nil?  The ELisp manual has
> a warning near the end of description of save-excursion; I think it's
> relevant to this use case.

Message #36 received at 13122 <at> debbugs.gnu.org (full text, mbox):

From: Andreas Schwab <schwab <at> linux-m68k.org>
To: Tino Calancha <tino.calancha <at> gmail.com>
Cc: Eli Zaretskii <eliz <at> gnu.org>, 13122 <at> debbugs.gnu.org
Subject: Re: bug#13122: save-excursion not saving point
Date: Sat, 27 Apr 2019 11:59:48 +0200

On Apr 27 2019, Tino Calancha <tino.calancha <at> gmail.com> wrote:

> On Sat, 27 Apr 2019, Eli Zaretskii wrote:
>
>>> From: Tino Calancha <tino.calancha <at> gmail.com>
>>> Date: Sat, 27 Apr 2019 04:53:56 +0900
>>>
>>> ;; Following expression should eval as non-nil
>>> (let ((pos 3))
>>>   (with-temp-buffer
>>>     (insert "abcdef")
>>>     (goto-char pos)
>>>     (save-excursion (transpose-chars 2))
>>>     (= pos (point))))
>>> => nil
>
> I read that warning; maybe getting `nil' is above is expected; if that's
> the case then we can close the report again.

Since the form inside save-excursion can modify the text before point,
there is never a guarantee that the numeric value of point stays the
same.  And if point points inside the modified text, it is generally
unspecified how point moves during the modification.

Andreas.

-- 
Andreas Schwab, schwab <at> linux-m68k.org
GPG Key fingerprint = 7578 EB47 D4E5 4D69 2510  2552 DF73 E780 A9DA AEC1
"And now for something completely different."

Message #39 received at 13122-done <at> debbugs.gnu.org (full text, mbox):

From: Tino Calancha <tino.calancha <at> gmail.com>
To: 13122-done <at> debbugs.gnu.org
Subject: Re: bug#13122: save-excursion not saving point
Date: Sat, 4 May 2019 17:39:11 +0900 (JST)

Saving the point, in the general case, would rely on unspecified 
behaviour; so I close this bug.
>>>> ;; Following expression should eval as non-nil
>>>> (let ((pos 3))
>>>>   (with-temp-buffer
>>>>     (insert "abcdef")
>>>>     (goto-char pos)
>>>>     (save-excursion (transpose-chars 2))
>>>>     (= pos (point))))
>>>> => nil
> Since the form inside save-excursion can modify the text before point,
> there is never a guarantee that the numeric value of point stays the
> same.  And if point points inside the modified text, it is generally
> unspecified how point moves during the modification.

Previous Next

GNU bug tracking system
Copyright (C) 1999 Darren O. Benham, 1997,2003 nCipher Corporation Ltd, 1994-97 Ian Jackson.