GNU bug report logs - #61098
29.0.60; Confusing behavior of show-paren-mode

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

From: Rah Guzar <rahguzar <at> zohomail.eu>
To: bug-gnu-emacs <at> gnu.org
Subject: 29.0.60; Confusing behavior of show-paren-mode
Date: Fri, 27 Jan 2023 17:03:14 +0100

Dear Emacs maintainers,
   After updating to Emacs 29, I saw in NEWS that show-paren-mode
   is no longer enabled in special-mode buffers. I think that is a
   welcome change. However in some cases (like reading the elisp
   manual) it is helpful to have it enabled. My impression after
   reading the NEWS entry, the manual entry for show-paren-mode
   and documentation for show-paren-local-mode was that I should
   be able to able to turn on show-paren-mode buffer locally by

            M-x show-paren-local-mode RET

    However this doesn't work and after much confusion I realized
    that calling show-paren-local-mode has effect only if I customize
    show-paren-predicate. My (incorrect) impression after reading
    the documentation was that show-paren-predicate determines only
    whether show-paren-mode is enabled in a buffer by default but
    doesn't prohibit the possibility of turning it off.

    I think a better default would be the equivalent of customizing
    show-buffer-predicate to t and then using

    (add-hook 'special-mode-hook
          (defun my-turn-off-show-paren () (show-paren-local-mode -1)))

    which is what I have added to my config. This was the result I was
    expecting from the NEWS and manual entry and I think those should
    be clarified.


In GNU Emacs 29.0.60 (build 1, x86_64-suse-linux-gnu, GTK+ Version
 3.24.35, cairo version 1.17.6) of 2023-01-22 built on sheep83
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Message #8 received at 61098 <at> debbugs.gnu.org (full text, mbox):

From: Eli Zaretskii <eliz <at> gnu.org>
To: Rah Guzar <rahguzar <at> zohomail.eu>, Stefan Monnier <monnier <at> iro.umontreal.ca>
Cc: 61098 <at> debbugs.gnu.org
Subject: Re: bug#61098: 29.0.60; Confusing behavior of show-paren-mode
Date: Sat, 28 Jan 2023 14:20:05 +0200

> Date: Fri, 27 Jan 2023 17:03:14 +0100
> From:  Rah Guzar via "Bug reports for GNU Emacs,
>  the Swiss army knife of text editors" <bug-gnu-emacs <at> gnu.org>
> 
> 
> Dear Emacs maintainers,
>    After updating to Emacs 29, I saw in NEWS that show-paren-mode
>    is no longer enabled in special-mode buffers. I think that is a
>    welcome change. However in some cases (like reading the elisp
>    manual) it is helpful to have it enabled. My impression after
>    reading the NEWS entry, the manual entry for show-paren-mode
>    and documentation for show-paren-local-mode was that I should
>    be able to able to turn on show-paren-mode buffer locally by
> 
>             M-x show-paren-local-mode RET
> 
>     However this doesn't work and after much confusion I realized
>     that calling show-paren-local-mode has effect only if I customize
>     show-paren-predicate. My (incorrect) impression after reading
>     the documentation was that show-paren-predicate determines only
>     whether show-paren-mode is enabled in a buffer by default but
>     doesn't prohibit the possibility of turning it off.

Your impression was correct: there should be no need to customize
show-paren-predicate to turn on show-paren-local-mode in a buffer that
is under a descendant of special-mode.  The patch below fixes the bug
and allows you to do what you want without messing with
show-paren-predicate.

However, there's one glitch: after applying the patch, when you invoke

   M-x show-paren-local-mode RET

in a special-mode buffer, it says the mode is _disabled_, and indeed
disables the mode.  You need to enable it by invoking the same command
again.

I'm not sure I understand the reason, but it is probably related to
the fact that we use the same show-paren-mode variable for the
localized mode, and the default value of that variable is t.

Stefan, can you help here? how do we make show-paren-local-mode be
enabled when the user explicitly invokes it?

Here's the patch I promised above:

diff --git a/lisp/paren.el b/lisp/paren.el
index b2a7962..de14de2 100644
--- a/lisp/paren.el
+++ b/lisp/paren.el
@@ -167,8 +167,11 @@ show-paren-local-mode
     (unless show-paren-mode
       (show-paren--delete-overlays))
     (kill-local-variable 'show-paren-mode))
-   ((not (default-value 'show-paren-mode))
-    ;; Locally enabled, but globally disabled.
+   ((or
+     ;; Locally enabled, but globally disabled.
+     (not (default-value 'show-paren-mode))
+     ;; Disabled by 'show-paren-predicate'.
+     (not (buffer-match-p show-paren-predicate (current-buffer))))
     (show-paren-mode 1)                ; Setup the timer.
     (setq-default show-paren-mode nil) ; But keep it globally disabled.
     )

Message #11 received at 61098 <at> debbugs.gnu.org (full text, mbox):

From: Rah Guzar <rahguzar <at> zohomail.eu>
To: Eli Zaretskii <eliz <at> gnu.org>
Cc: 61098 <at> debbugs.gnu.org, Stefan Monnier <monnier <at> iro.umontreal.ca>
Subject: Re: bug#61098: 29.0.60; Confusing behavior of show-paren-mode
Date: Mon, 30 Jan 2023 20:21:33 +0100

Hi,
  Thanks for the patch, it works as you described.

However I have questions about the show-paren-predicate variable.

When I see its default value  (not (derived-mode . special-mode))
it makes me think there is some dsl involved. The customize interface
mentions Lisp Expression as a possibility but the documentation doesn't
explain what form this expression is supposed to take. This makes me
think that the default is just a pseudo sentence intended to convey
what it does and there are only three possibilities.

If the latter is correct, I think a simpler way would be to disable
the mode in the body of special-mode if the value of `show-paren-predicate`
demands it. I think this will get rid of the problem that the mode is
disabled the first time show-paren-local-mode is called.

I am using the equivalent of this which adds a hook to special-mode-hook
to achieve the outcome and I am pretty happy with it.

Thanks again,
Rahguzar


Eli Zaretskii <eliz <at> gnu.org> writes:

>> Date: Fri, 27 Jan 2023 17:03:14 +0100
>> From:  Rah Guzar via "Bug reports for GNU Emacs,
>>  the Swiss army knife of text editors" <bug-gnu-emacs <at> gnu.org>
>>
>>
>> Dear Emacs maintainers,
>>    After updating to Emacs 29, I saw in NEWS that show-paren-mode
>>    is no longer enabled in special-mode buffers. I think that is a
>>    welcome change. However in some cases (like reading the elisp
>>    manual) it is helpful to have it enabled. My impression after
>>    reading the NEWS entry, the manual entry for show-paren-mode
>>    and documentation for show-paren-local-mode was that I should
>>    be able to able to turn on show-paren-mode buffer locally by
>>
>>             M-x show-paren-local-mode RET
>>
>>     However this doesn't work and after much confusion I realized
>>     that calling show-paren-local-mode has effect only if I customize
>>     show-paren-predicate. My (incorrect) impression after reading
>>     the documentation was that show-paren-predicate determines only
>>     whether show-paren-mode is enabled in a buffer by default but
>>     doesn't prohibit the possibility of turning it off.
>
> Your impression was correct: there should be no need to customize
> show-paren-predicate to turn on show-paren-local-mode in a buffer that
> is under a descendant of special-mode.  The patch below fixes the bug
> and allows you to do what you want without messing with
> show-paren-predicate.
>
> However, there's one glitch: after applying the patch, when you invoke
>
>    M-x show-paren-local-mode RET
>
> in a special-mode buffer, it says the mode is _disabled_, and indeed
> disables the mode.  You need to enable it by invoking the same command
> again.
>
> I'm not sure I understand the reason, but it is probably related to
> the fact that we use the same show-paren-mode variable for the
> localized mode, and the default value of that variable is t.
>
> Stefan, can you help here? how do we make show-paren-local-mode be
> enabled when the user explicitly invokes it?
>
> Here's the patch I promised above:
>
> diff --git a/lisp/paren.el b/lisp/paren.el
> index b2a7962..de14de2 100644
> --- a/lisp/paren.el
> +++ b/lisp/paren.el
> @@ -167,8 +167,11 @@ show-paren-local-mode
>      (unless show-paren-mode
>        (show-paren--delete-overlays))
>      (kill-local-variable 'show-paren-mode))
> -   ((not (default-value 'show-paren-mode))
> -    ;; Locally enabled, but globally disabled.
> +   ((or
> +     ;; Locally enabled, but globally disabled.
> +     (not (default-value 'show-paren-mode))
> +     ;; Disabled by 'show-paren-predicate'.
> +     (not (buffer-match-p show-paren-predicate (current-buffer))))
>      (show-paren-mode 1)                ; Setup the timer.
>      (setq-default show-paren-mode nil) ; But keep it globally disabled.
>      )

Message #14 received at 61098 <at> debbugs.gnu.org (full text, mbox):

From: Dmitry Gutov <dgutov <at> yandex.ru>
To: Eli Zaretskii <eliz <at> gnu.org>, Rah Guzar <rahguzar <at> zohomail.eu>,
 Stefan Monnier <monnier <at> iro.umontreal.ca>
Cc: 61098 <at> debbugs.gnu.org
Subject: Re: bug#61098: 29.0.60; Confusing behavior of show-paren-mode
Date: Tue, 31 Jan 2023 04:37:49 +0200

On 28/01/2023 14:20, Eli Zaretskii wrote:
> Here's the patch I promised above:

Your patch modifies the part which called the show-paren-mode to set up 
the timer, but the timer is started unconditionally, so predicate check 
is inside the timer function. So it seems incorrect to me.

I suggest trying the patch below.

It makes sure the buffer-local value of show-paren-mode is kept 
buffer-local even if it matches the global one (while it's = t), so that 
the later check

  (or (local-variable-p 'show-paren-mode)
      ;; If not, check that the predicate matches.
      (buffer-match-p show-paren-predicate (current-buffer)))

works correctly.

diff --git a/lisp/paren.el b/lisp/paren.el
index b2a79624c0f..437dc057c46 100644
--- a/lisp/paren.el
+++ b/lisp/paren.el
@@ -165,8 +165,8 @@ show-paren-local-mode
   (cond
    ((eq show-paren-mode (default-value 'show-paren-mode))
     (unless show-paren-mode
-      (show-paren--delete-overlays))
-    (kill-local-variable 'show-paren-mode))
+      (show-paren--delete-overlays)
+      (kill-local-variable 'show-paren-mode)))
    ((not (default-value 'show-paren-mode))
     ;; Locally enabled, but globally disabled.
     (show-paren-mode 1)                ; Setup the timer.

Message #17 received at 61098 <at> debbugs.gnu.org (full text, mbox):

From: Eli Zaretskii <eliz <at> gnu.org>
To: Rah Guzar <rahguzar <at> zohomail.eu>, Stefan Monnier <monnier <at> iro.umontreal.ca>
Cc: 61098 <at> debbugs.gnu.org
Subject: Re: bug#61098: 29.0.60; Confusing behavior of show-paren-mode
Date: Tue, 31 Jan 2023 14:03:28 +0200

> From: Rah Guzar <rahguzar <at> zohomail.eu>
> Cc: Stefan Monnier <monnier <at> iro.umontreal.ca>, 61098 <at> debbugs.gnu.org
> Date: Mon, 30 Jan 2023 20:21:33 +0100
> 
> Hi,
>   Thanks for the patch, it works as you described.

Thanks for testing.

I'm still waiting for Stefan to help me understand why after that
patch "M-x show-paren-local-mode" _disables_ the mode rather than
enabling it.  Stefan, any wisdom?

> However I have questions about the show-paren-predicate variable.
> 
> When I see its default value  (not (derived-mode . special-mode))
> it makes me think there is some dsl involved. The customize interface
> mentions Lisp Expression as a possibility but the documentation doesn't
> explain what form this expression is supposed to take.

It should be an expression that yields a boolean value (that's why the
variable is named "predicate").  I will improve the doc string when I
install the final version of the changes.

> This makes me think that the default is just a pseudo sentence
> intended to convey what it does and there are only three
> possibilities.

No, it's a real expression in Lisp, just quoted.

> If the latter is correct, I think a simpler way would be to disable
> the mode in the body of special-mode if the value of `show-paren-predicate`
> demands it. I think this will get rid of the problem that the mode is
> disabled the first time show-paren-local-mode is called.
> 
> I am using the equivalent of this which adds a hook to special-mode-hook
> to achieve the outcome and I am pretty happy with it.

I don't think I understand what you have in mind, but let's wait for
Stefan to chime in.

Message #20 received at 61098 <at> debbugs.gnu.org (full text, mbox):

From: Rah Guzar <rahguzar <at> zohomail.eu>
To: Eli Zaretskii <eliz <at> gnu.org>
Cc: 61098 <at> debbugs.gnu.org, Stefan Monnier <monnier <at> iro.umontreal.ca>
Subject: Re: bug#61098: 29.0.60; Confusing behavior of show-paren-mode
Date: Tue, 31 Jan 2023 14:03:20 +0100

Eli Zaretskii <eliz <at> gnu.org> writes:

> It should be an expression that yields a boolean value (that's why the
> variable is named "predicate").  I will improve the doc string when I
> install the final version of the changes.

What I had been missing was how is that expression supposed to yield
a boolean value. I think I gleaned the answer from Dmitry Gutov's message
which is that the expression corresponds to the "condition"
argument of buffer-match-p.

Rahguzar

Message #23 received at 61098 <at> debbugs.gnu.org (full text, mbox):

From: Eli Zaretskii <eliz <at> gnu.org>
To: Rah Guzar <rahguzar <at> zohomail.eu>
Cc: 61098 <at> debbugs.gnu.org, monnier <at> iro.umontreal.ca
Subject: Re: bug#61098: 29.0.60; Confusing behavior of show-paren-mode
Date: Tue, 31 Jan 2023 16:13:52 +0200

> From: Rah Guzar <rahguzar <at> zohomail.eu>
> Cc: Stefan Monnier <monnier <at> iro.umontreal.ca>, 61098 <at> debbugs.gnu.org
> Date: Tue, 31 Jan 2023 14:03:20 +0100
> 
> 
> Eli Zaretskii <eliz <at> gnu.org> writes:
> 
> > It should be an expression that yields a boolean value (that's why the
> > variable is named "predicate").  I will improve the doc string when I
> > install the final version of the changes.
> 
> What I had been missing was how is that expression supposed to yield
> a boolean value.

The function 'not' returns a boolean value.

Message #26 received at 61098 <at> debbugs.gnu.org (full text, mbox):

From: Dmitry Gutov <dgutov <at> yandex.ru>
To: Eli Zaretskii <eliz <at> gnu.org>, Rah Guzar <rahguzar <at> zohomail.eu>
Cc: 61098 <at> debbugs.gnu.org, monnier <at> iro.umontreal.ca
Subject: Re: bug#61098: 29.0.60; Confusing behavior of show-paren-mode
Date: Tue, 31 Jan 2023 16:41:42 +0200

On 31/01/2023 16:13, Eli Zaretskii wrote:
>> From: Rah Guzar<rahguzar <at> zohomail.eu>
>> Cc: Stefan Monnier<monnier <at> iro.umontreal.ca>,61098 <at> debbugs.gnu.org
>> Date: Tue, 31 Jan 2023 14:03:20 +0100
>>
>>
>> Eli Zaretskii<eliz <at> gnu.org>  writes:
>>
>>> It should be an expression that yields a boolean value (that's why the
>>> variable is named "predicate").  I will improve the doc string when I
>>> install the final version of the changes.
>> What I had been missing was how is that expression supposed to yield
>> a boolean value.
> The function 'not' returns a boolean value.

'not' is just a symbol in this case -- from a DSL.

(buffer-match-p show-paren-predicate (current-buffer)) is supposed to 
return non-nil.

See its docstring for the description of the DSL format.

Message #29 received at 61098 <at> debbugs.gnu.org (full text, mbox):

From: Stefan Monnier <monnier <at> iro.umontreal.ca>
To: Eli Zaretskii <eliz <at> gnu.org>
Cc: Rah Guzar <rahguzar <at> zohomail.eu>, 61098 <at> debbugs.gnu.org
Subject: Re: bug#61098: 29.0.60; Confusing behavior of show-paren-mode
Date: Tue, 31 Jan 2023 17:42:09 -0500

> I'm still waiting for Stefan to help me understand why after that
> patch "M-x show-paren-local-mode" _disables_ the mode rather than
> enabling it.  Stefan, any wisdom?

Not sure.  I don't quite understand how your patch intends to work.
From where I sit I get the impression that it will result in an
inconsistent state in various "corner" cases.

Dmitry's patch seems closer to my intuition (I might even suggest
removing the `kill-local-variable` altogether).


        Stefan

Message #32 received at 61098 <at> debbugs.gnu.org (full text, mbox):

From: Eli Zaretskii <eliz <at> gnu.org>
To: Stefan Monnier <monnier <at> iro.umontreal.ca>
Cc: rahguzar <at> zohomail.eu, dgutov <at> yandex.ru, 61098 <at> debbugs.gnu.org
Subject: Re: bug#61098: 29.0.60; Confusing behavior of show-paren-mode
Date: Wed, 01 Feb 2023 19:48:11 +0200

> From: Stefan Monnier <monnier <at> iro.umontreal.ca>
> Cc: Rah Guzar <rahguzar <at> zohomail.eu>,  61098 <at> debbugs.gnu.org
> Date: Tue, 31 Jan 2023 17:42:09 -0500
> 
> > I'm still waiting for Stefan to help me understand why after that
> > patch "M-x show-paren-local-mode" _disables_ the mode rather than
> > enabling it.  Stefan, any wisdom?
> 
> Not sure.  I don't quite understand how your patch intends to work.
> >From where I sit I get the impression that it will result in an
> inconsistent state in various "corner" cases.
> 
> Dmitry's patch seems closer to my intuition (I might even suggest
> removing the `kill-local-variable` altogether).

I guess I don't understand what show-paren-local-mode does, then.  Let
me explain my line of reasoning:

  (define-minor-mode show-paren-local-mode
    "Toggle `show-paren-mode' only in this buffer."
    :variable ( show-paren-mode .
		(lambda (val) (setq-local show-paren-mode val)))
    (cond
     ((eq show-paren-mode (default-value 'show-paren-mode))
      (unless show-paren-mode
	(show-paren--delete-overlays))
      (kill-local-variable 'show-paren-mode))
     ((not (default-value 'show-paren-mode))
      ;; Locally enabled, but globally disabled.
      (show-paren-mode 1)                ; Setup the timer.
      (setq-default show-paren-mode nil) ; But keep it globally disabled.
      )
     (t ;; Locally disabled only.
      (show-paren--delete-overlays))))

When I looked at the above function and stepped through it in the OP's
scenario, what I saw was that the 2nd condition of 'cond' doesn't
fire, because the default-value of show-paren-mode is t nowadays.  So
I concluded that this code relies on the mode being globally off by
default, and attempted to fix the condition that turns the mode on
locally even though it is on globally.

The predicate used by the timer requires either a match of the buffer
(which doesn't happen in this case) or that the local value of
show-paren-mode is non-nil.  Setting the local value non-nil is the
job of the mode function, isn't it?  So I tried to get the mode
function do its job in this scenario.

What Dmitry's patch does, and why it is supposed to fix this, I simply
don't understand, perhaps for the same reason that I misinterpret what
the mode function does.

Message #35 received at 61098 <at> debbugs.gnu.org (full text, mbox):

From: Stefan Monnier <monnier <at> iro.umontreal.ca>
To: Eli Zaretskii <eliz <at> gnu.org>
Cc: rahguzar <at> zohomail.eu, dgutov <at> yandex.ru, 61098 <at> debbugs.gnu.org
Subject: Re: bug#61098: 29.0.60; Confusing behavior of show-paren-mode
Date: Sun, 05 Feb 2023 10:19:24 -0500

> I guess I don't understand what show-paren-local-mode does, then.  Let
> me explain my line of reasoning:
>
>   (define-minor-mode show-paren-local-mode
>     "Toggle `show-paren-mode' only in this buffer."
>     :variable ( show-paren-mode .
> 		(lambda (val) (setq-local show-paren-mode val)))
>     (cond
>      ((eq show-paren-mode (default-value 'show-paren-mode))
>       (unless show-paren-mode
> 	(show-paren--delete-overlays))
>       (kill-local-variable 'show-paren-mode))
>      ((not (default-value 'show-paren-mode))
>       ;; Locally enabled, but globally disabled.
>       (show-paren-mode 1)                ; Setup the timer.
>       (setq-default show-paren-mode nil) ; But keep it globally disabled.
>       )
>      (t ;; Locally disabled only.
>       (show-paren--delete-overlays))))
>
> When I looked at the above function and stepped through it in the OP's
> scenario, what I saw was that the 2nd condition of 'cond' doesn't
> fire, because the default-value of show-paren-mode is t nowadays.  So
> I concluded that this code relies on the mode being globally off by
> default, and attempted to fix the condition that turns the mode on
> locally even though it is on globally.

But if the mode is already globally enabled, the (setq-default show-paren-mode nil)
will definitely not be right: you'd end up disabling it globally (while
maybe keeping it enabled locally).

> The predicate used by the timer requires either a match of the buffer
> (which doesn't happen in this case) or that the local value of
> show-paren-mode is non-nil.  Setting the local value non-nil is the
> job of the mode function, isn't it?

Yes, but the first branch of the `cond` incorrectly does
`kill-local-variable` under the assumption that the local value doesn't
matter if it's the same as the global one, but that's not true here
because (as a wise man said):

   The predicate used by the timer requires either a match of the buffer
   (which doesn't happen in this case) or that the local value of
   show-paren-mode is non-nil.


-- Stefan

Message #38 received at 61098 <at> debbugs.gnu.org (full text, mbox):

From: Eli Zaretskii <eliz <at> gnu.org>
To: Stefan Monnier <monnier <at> iro.umontreal.ca>
Cc: rahguzar <at> zohomail.eu, dgutov <at> yandex.ru, 61098 <at> debbugs.gnu.org
Subject: Re: bug#61098: 29.0.60; Confusing behavior of show-paren-mode
Date: Sun, 05 Feb 2023 17:24:57 +0200

> From: Stefan Monnier <monnier <at> iro.umontreal.ca>
> Cc: rahguzar <at> zohomail.eu,  61098 <at> debbugs.gnu.org,  dgutov <at> yandex.ru
> Date: Sun, 05 Feb 2023 10:19:24 -0500
> 
> > I guess I don't understand what show-paren-local-mode does, then.  Let
> > me explain my line of reasoning:
> >
> >   (define-minor-mode show-paren-local-mode
> >     "Toggle `show-paren-mode' only in this buffer."
> >     :variable ( show-paren-mode .
> > 		(lambda (val) (setq-local show-paren-mode val)))
> >     (cond
> >      ((eq show-paren-mode (default-value 'show-paren-mode))
> >       (unless show-paren-mode
> > 	(show-paren--delete-overlays))
> >       (kill-local-variable 'show-paren-mode))
> >      ((not (default-value 'show-paren-mode))
> >       ;; Locally enabled, but globally disabled.
> >       (show-paren-mode 1)                ; Setup the timer.
> >       (setq-default show-paren-mode nil) ; But keep it globally disabled.
> >       )
> >      (t ;; Locally disabled only.
> >       (show-paren--delete-overlays))))
> >
> > When I looked at the above function and stepped through it in the OP's
> > scenario, what I saw was that the 2nd condition of 'cond' doesn't
> > fire, because the default-value of show-paren-mode is t nowadays.  So
> > I concluded that this code relies on the mode being globally off by
> > default, and attempted to fix the condition that turns the mode on
> > locally even though it is on globally.
> 
> But if the mode is already globally enabled, the (setq-default show-paren-mode nil)
> will definitely not be right: you'd end up disabling it globally (while
> maybe keeping it enabled locally).
> 
> > The predicate used by the timer requires either a match of the buffer
> > (which doesn't happen in this case) or that the local value of
> > show-paren-mode is non-nil.  Setting the local value non-nil is the
> > job of the mode function, isn't it?
> 
> Yes, but the first branch of the `cond` incorrectly does
> `kill-local-variable` under the assumption that the local value doesn't
> matter if it's the same as the global one, but that's not true here
> because (as a wise man said):
> 
>    The predicate used by the timer requires either a match of the buffer
>    (which doesn't happen in this case) or that the local value of
>    show-paren-mode is non-nil.

This answers only half of my questions, but if you and Dmitry think
Dmitry's patch will solve this problem, then Dmitry, please install it
and close this bug.

Thanks.

Message #41 received at 61098 <at> debbugs.gnu.org (full text, mbox):

From: Dmitry Gutov <dgutov <at> yandex.ru>
To: Eli Zaretskii <eliz <at> gnu.org>, Stefan Monnier <monnier <at> iro.umontreal.ca>
Cc: rahguzar <at> zohomail.eu, 61098 <at> debbugs.gnu.org
Subject: Re: bug#61098: 29.0.60; Confusing behavior of show-paren-mode
Date: Sun, 5 Feb 2023 19:21:29 +0200

On 05/02/2023 17:24, Eli Zaretskii wrote:
>> From: Stefan Monnier<monnier <at> iro.umontreal.ca>
>> Cc:rahguzar <at> zohomail.eu,61098 <at> debbugs.gnu.org,dgutov <at> yandex.ru
>> Date: Sun, 05 Feb 2023 10:19:24 -0500
>>
>>> I guess I don't understand what show-paren-local-mode does, then.  Let
>>> me explain my line of reasoning:
>>>
>>>    (define-minor-mode show-paren-local-mode
>>>      "Toggle `show-paren-mode' only in this buffer."
>>>      :variable ( show-paren-mode .
>>> 		(lambda (val) (setq-local show-paren-mode val)))
>>>      (cond
>>>       ((eq show-paren-mode (default-value 'show-paren-mode))
>>>        (unless show-paren-mode
>>> 	(show-paren--delete-overlays))
>>>        (kill-local-variable 'show-paren-mode))
>>>       ((not (default-value 'show-paren-mode))
>>>        ;; Locally enabled, but globally disabled.
>>>        (show-paren-mode 1)                ; Setup the timer.
>>>        (setq-default show-paren-mode nil) ; But keep it globally disabled.
>>>        )
>>>       (t ;; Locally disabled only.
>>>        (show-paren--delete-overlays))))
>>>
>>> When I looked at the above function and stepped through it in the OP's
>>> scenario, what I saw was that the 2nd condition of 'cond' doesn't
>>> fire, because the default-value of show-paren-mode is t nowadays.  So
>>> I concluded that this code relies on the mode being globally off by
>>> default, and attempted to fix the condition that turns the mode on
>>> locally even though it is on globally.
>> But if the mode is already globally enabled, the (setq-default show-paren-mode nil)
>> will definitely not be right: you'd end up disabling it globally (while
>> maybe keeping it enabled locally).
>>
>>> The predicate used by the timer requires either a match of the buffer
>>> (which doesn't happen in this case) or that the local value of
>>> show-paren-mode is non-nil.  Setting the local value non-nil is the
>>> job of the mode function, isn't it?
>> Yes, but the first branch of the `cond` incorrectly does
>> `kill-local-variable` under the assumption that the local value doesn't
>> matter if it's the same as the global one, but that's not true here
>> because (as a wise man said):
>>
>>     The predicate used by the timer requires either a match of the buffer
>>     (which doesn't happen in this case) or that the local value of
>>     show-paren-mode is non-nil.
> This answers only half of my questions, but if you and Dmitry think
> Dmitry's patch will solve this problem, then Dmitry, please install it
> and close this bug.

Installed.

Some testing revealed a certain issue, which until now was not really a 
problem:

The first time I 'M-x show-paren-local-mode' with show-paren-mode 
enabled globally, it _turns off_ the mode locally. Which kind of makes 
sense for a lot of cases, but not when the user's intention is to 
forcibly enable it.

Evaluating (show-paren-local-mode 1) still does the intended thing, though.

Just as well as (add-hook 'some-special-mode #'show-paren-local-mode).

Toggling 'M-x show-paren-local-mdoe' twice also does that.

If we do consider this a problem, a further fix could look like this:

diff --git a/lisp/paren.el b/lisp/paren.el
index 7ee4e9ae682..f2756db6cd9 100644
--- a/lisp/paren.el
+++ b/lisp/paren.el
@@ -161,7 +161,9 @@ show-paren--delete-overlays
 ;;;###autoload
 (define-minor-mode show-paren-local-mode
   "Toggle `show-paren-mode' only in this buffer."
-  :variable ( show-paren-mode .
+  :variable ( (and show-paren-mode
+                   (buffer-match-p show-paren-predicate (current-buffer)))
+              .
               (lambda (val) (setq-local show-paren-mode val)))
   (cond
    ((eq show-paren-mode (default-value 'show-paren-mode))

Message #44 received at 61098 <at> debbugs.gnu.org (full text, mbox):

From: Eli Zaretskii <eliz <at> gnu.org>
To: Dmitry Gutov <dgutov <at> yandex.ru>
Cc: rahguzar <at> zohomail.eu, monnier <at> iro.umontreal.ca, 61098 <at> debbugs.gnu.org
Subject: Re: bug#61098: 29.0.60; Confusing behavior of show-paren-mode
Date: Sun, 05 Feb 2023 20:29:26 +0200

> Date: Sun, 5 Feb 2023 19:21:29 +0200
> Cc: rahguzar <at> zohomail.eu, 61098 <at> debbugs.gnu.org
> From: Dmitry Gutov <dgutov <at> yandex.ru>
> 
> Some testing revealed a certain issue, which until now was not really a 
> problem:
> 
> The first time I 'M-x show-paren-local-mode' with show-paren-mode 
> enabled globally, it _turns off_ the mode locally.

That's exactly what I asked about in

  https://debbugs.gnu.org/cgi/bugreport.cgi?bug=61098#8

> Which kind of makes sense for a lot of cases, but not when the
> user's intention is to forcibly enable it.

In a buffer where the mode is turned off, either effectively by virtue
of the predicate, or explicitly via the local value of the variable,
"M-x show-paren-local-mode" should turn the mode ON, not OFF.

Message #47 received at 61098 <at> debbugs.gnu.org (full text, mbox):

From: Stefan Monnier <monnier <at> iro.umontreal.ca>
To: Dmitry Gutov <dgutov <at> yandex.ru>
Cc: rahguzar <at> zohomail.eu, Eli Zaretskii <eliz <at> gnu.org>, 61098 <at> debbugs.gnu.org
Subject: Re: bug#61098: 29.0.60; Confusing behavior of show-paren-mode
Date: Sun, 05 Feb 2023 13:57:06 -0500

> +  :variable ( (and show-paren-mode
> +                   (buffer-match-p show-paren-predicate (current-buffer)))
> +              .
>                (lambda (val) (setq-local show-paren-mode val)))

This doesn't look quote right because it doesn't pay attention to
`local-variable-p`.  We should extract the test performed in the timer
and reuse it here.


        Stefan

Message #50 received at 61098 <at> debbugs.gnu.org (full text, mbox):

From: Dmitry Gutov <dgutov <at> yandex.ru>
To: Eli Zaretskii <eliz <at> gnu.org>
Cc: rahguzar <at> zohomail.eu, monnier <at> iro.umontreal.ca, 61098 <at> debbugs.gnu.org
Subject: Re: bug#61098: 29.0.60; Confusing behavior of show-paren-mode
Date: Sun, 5 Feb 2023 21:06:33 +0200

On 05/02/2023 20:29, Eli Zaretskii wrote:
>> Date: Sun, 5 Feb 2023 19:21:29 +0200
>> Cc:rahguzar <at> zohomail.eu,61098 <at> debbugs.gnu.org
>> From: Dmitry Gutov<dgutov <at> yandex.ru>
>>
>> Some testing revealed a certain issue, which until now was not really a
>> problem:
>>
>> The first time I 'M-x show-paren-local-mode' with show-paren-mode
>> enabled globally, it_turns off_  the mode locally.
> That's exactly what I asked about in
> 
>    https://debbugs.gnu.org/cgi/bugreport.cgi?bug=61098#8
> 
>> Which kind of makes sense for a lot of cases, but not when the
>> user's intention is to forcibly enable it.
> In a buffer where the mode is turned off, either effectively by virtue
> of the predicate, or explicitly via the local value of the variable,
> "M-x show-paren-local-mode" should turn the mode ON, not OFF.

All right.

Message #55 received at 61098-done <at> debbugs.gnu.org (full text, mbox):

From: Dmitry Gutov <dgutov <at> yandex.ru>
To: Stefan Monnier <monnier <at> iro.umontreal.ca>
Cc: rahguzar <at> zohomail.eu, Eli Zaretskii <eliz <at> gnu.org>,
 61098-done <at> debbugs.gnu.org
Subject: Re: bug#61098: 29.0.60; Confusing behavior of show-paren-mode
Date: Sun, 5 Feb 2023 21:09:36 +0200

On 05/02/2023 20:57, Stefan Monnier via Bug reports for GNU Emacs, the 
Swiss army knife of text editors wrote:
>> +  :variable ( (and show-paren-mode
>> +                   (buffer-match-p show-paren-predicate (current-buffer)))
>> +              .
>>                 (lambda (val) (setq-local show-paren-mode val)))
> This doesn't look quote right because it doesn't pay attention to
> `local-variable-p`.  We should extract the test performed in the timer
> and reuse it here.

Thank you, makes sense.

Here's the patch I installed:

diff --git a/lisp/paren.el b/lisp/paren.el
index 7ee4e9ae682..4c91fd29490 100644
--- a/lisp/paren.el
+++ b/lisp/paren.el
@@ -161,8 +161,9 @@ show-paren--delete-overlays
 ;;;###autoload
 (define-minor-mode show-paren-local-mode
   "Toggle `show-paren-mode' only in this buffer."
-  :variable ( show-paren-mode .
-              (lambda (val) (setq-local show-paren-mode val)))
+  :variable ((show-paren--enabled-p)
+             .
+             (lambda (val) (setq-local show-paren-mode val)))
   (cond
    ((eq show-paren-mode (default-value 'show-paren-mode))
     (unless show-paren-mode
@@ -428,14 +429,17 @@ show-paren--show-context-in-overlay
 ;; `show-paren-delay'.
 (defvar-local show-paren--last-pos nil)

+(defun show-paren--enabled-p ()
+  (and show-paren-mode
+       ;; If we're using `show-paren-local-mode', then
+       ;; always heed the value.
+       (or (local-variable-p 'show-paren-mode)
+           ;; If not, check that the predicate matches.
+           (buffer-match-p show-paren-predicate (current-buffer)))))
+
 (defun show-paren-function ()
   "Highlight the parentheses until the next input arrives."
-  (let ((data (and show-paren-mode
-                   ;; If we're using `show-paren-local-mode', then
-                   ;; always heed the value.
-                   (or (local-variable-p 'show-paren-mode)
-                       ;; If not, check that the predicate matches.
-                       (buffer-match-p show-paren-predicate 
(current-buffer)))
+  (let ((data (and (show-paren--enabled-p)
                    (funcall show-paren-data-function))))
     (if (not data)
         (progn

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