GNU bug report logs - #40865
26.3; Electric pair mode and syntax class $

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

From: Augusto Stoffel <arstoffel <at> gmail.com>
To: bug-gnu-emacs <at> gnu.org
Subject: 26.3; Electric pair mode and syntax class $
Date: Sun, 26 Apr 2020 12:18:32 +0200

Electric pair mode does not treat string quote characters and paired
delimiters (syntax class "$") in the same way, but it should probably
do so.

For instance, in TeX mode with electric pair mode active, typing a
dollar sign in the situation

    $a<point here>$

shold result in

    $a$<point here>

but instead results in

    $a$<point here>$$

The issue seems to be related with the function
`electric-pair-skip-if-helps-balance'.

In GNU Emacs 26.3 (build 1, x86_64-redhat-linux-gnu, GTK+ Version
3.24.13)
 of 2019-12-10 built on buildhw-07.phx2.fedoraproject.org
Windowing system distributor 'Fedora Project', version 11.0.12006000
Recent messages:

Quit [2 times]
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next-line: End of buffer
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(New file)
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Auto-saving...done
delete-backward-char: Beginning of buffer
Configured using:
 'configure --build=x86_64-redhat-linux-gnu
 --host=x86_64-redhat-linux-gnu --program-prefix=
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 LDFLAGS=-Wl,-z,relro
 PKG_CONFIG_PATH=:/usr/lib64/pkgconfig:/usr/share/pkgconfig'

Configured features:
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ACL LIBSELINUX GNUTLS LIBXML2 FREETYPE M17N_FLT LIBOTF XFT ZLIB
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Important settings:
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  value of $XMODIFIERS: @im=ibus
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Major mode: Emacs-Lisp

Minor modes in effect:
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  electric-pair-mode: t
  tooltip-mode: t
  global-eldoc-mode: t
  eldoc-mode: t
  electric-indent-mode: t
  mouse-wheel-mode: t
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  font-lock-mode: t
  blink-cursor-mode: t
  auto-composition-mode: t
  auto-encryption-mode: t
  auto-compression-mode: t
  line-number-mode: t
  transient-mark-mode: t

Load-path shadows:
None found.

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Message #8 received at 40865 <at> debbugs.gnu.org (full text, mbox):

From: João Távora <joaotavora <at> gmail.com>
To: Augusto Stoffel <arstoffel <at> gmail.com>
Cc: 40865 <at> debbugs.gnu.org
Subject: Re: bug#40865: 26.3; Electric pair mode and syntax class $
Date: Tue, 12 May 2020 22:58:36 +0100

Augusto Stoffel <arstoffel <at> gmail.com> writes:

Hi Augusto, thank you for the report.

> Electric pair mode does not treat string quote characters and paired
> delimiters (syntax class "$") in the same way, but it should probably
> do so.

Paired delimiters don't formally define any kind of syntactic region
between them, contrary to string delimiters.

Therefore, electric-pair-mode can't easily know if a decision to skip or
auto-pair would be benefitting the balance of the syntax buffer.
Therefore, it decides to be dumb and always autopair, never autoskip.

If I had chosen the alternative, having your point just before a '$foo$'
would make autoskip put it inside the group, instead of creating a new
'$$$foo$', which is probably what you wanted in that particular
situation.

Maybe, even though flawed, that is more defensible as a default
behaviour than the current one.  At least it doesn't seem to break any
tests.

The "correct" way to fix this is to make latex-mode treat paired
delimiters like it does string delimiteres.  But that probably has other
negative implications that I'm not aware of.

I have the vague recollection that this came up when I was developing
electric-pair-mode, particularly in latex-mode, some 6 years ago, but I
can't remember anymore.

Anyway, if you do prefer the alternative I gave earlier you can do this:

(add-hook 'latex-mode-hook
          (lambda ()
            (add-function :before-until
                          (local 'electric-pair-skip-self)
                          (lambda (char) (eq char ?$))
                          '((name . latex-skip-paired-delimiters)))))

João

PS: On a related note, note that AUCTex mode (a different mode to edit
TeX files) has a special binding for the dollar sign, TeX-insert-dollar,
which doesn't follow _any_ of electric-pair-mode.

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