GNU bug report logs - #24176
Confusing interaction between define-derived-mode and font-lock-add-keywords

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

From: Clément Pit--Claudel <clement.pitclaudel <at> live.com>
To: bug-gnu-emacs <at> gnu.org
Subject: Confusing interaction between define-derived-mode and
 font-lock-add-keywords
Date: Sun, 7 Aug 2016 02:41:30 -0400

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

Hi bug-gnu-emacs,

Given the following mode definitions, I was expecting ~/a to have one keyword, ~/b to have two, and ~/c to have three:

(define-derived-mode ~/a fundamental-mode
  (font-lock-add-keywords nil `(("a" 0 'font-lock-keyword-face))))
(define-derived-mode ~/b ~/a
  (font-lock-add-keywords nil `(("b" 0 'font-lock-builtin-face))))
(define-derived-mode ~/c ~/b
  (font-lock-add-keywords nil `(("c" 0 'font-lock-constant-face))))

This assumption is wrong, as can be easily verified by opening a new buffer, inserting "abc", and switching between ~/a, ~/b, ~/c.

What's going on? Is this a font-lock bug, a doc bug, or a user-got-confused issue?

Clément.

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Message #8 received at 24176 <at> debbugs.gnu.org (full text, mbox):

From: Clément Pit--Claudel <clement.pitclaudel <at> live.com>
To: 24176 <at> debbugs.gnu.org
Subject: Re: bug#24176: Acknowledgement (Confusing interaction between
 define-derived-mode and font-lock-add-keywords)
Date: Sun, 7 Aug 2016 02:57:19 -0400

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

Sorry, the original report had a confusing example; here is a fixed copy.

(define-derived-mode ~/a fundamental-mode "~/a"
  (font-lock-add-keywords nil `(("a" 0 'font-lock-keyword-face))))
(define-derived-mode ~/b ~/a "~/b"
  (font-lock-add-keywords nil `(("b" 0 'font-lock-builtin-face))))
(define-derived-mode ~/c ~/b "~/c"
  (font-lock-add-keywords nil `(("c" 0 'font-lock-constant-face))))

Explicitly calling (setq font-lock-major-mode major-mode) before calling font-lock-add-keywords yields the expected behaviour.

Clément.

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Message #11 received at 24176 <at> debbugs.gnu.org (full text, mbox):

From: npostavs <at> users.sourceforge.net
To: Clément Pit--Claudel <clement.pitclaudel <at> live.com>
Cc: 24176 <at> debbugs.gnu.org
Subject: Re: bug#24176: Confusing interaction between define-derived-mode and
 font-lock-add-keywords
Date: Wed, 24 Aug 2016 20:53:15 -0400

severity 24176 minor
tags 24176 confirmed
found 24176 25.1
quit

Clément Pit--Claudel <clement.pitclaudel <at> live.com> writes:

> Sorry, the original report had a confusing example; here is a fixed copy.
>
> (define-derived-mode ~/a fundamental-mode "~/a"
>   (font-lock-add-keywords nil `(("a" 0 'font-lock-keyword-face))))
> (define-derived-mode ~/b ~/a "~/b"
>   (font-lock-add-keywords nil `(("b" 0 'font-lock-builtin-face))))
> (define-derived-mode ~/c ~/b "~/c"
>   (font-lock-add-keywords nil `(("c" 0 'font-lock-constant-face))))
>
> Explicitly calling (setq font-lock-major-mode major-mode) before calling font-lock-add-keywords yields the expected behaviour.

Yeah, it seems `font-lock-set-defaults' (called from
`font-lock-add-keywords') deletes the parent mode's added keywords since
they were not from the "correct" mode.


    (defun font-lock-set-defaults ()
      "Set fontification defaults appropriately for this mode.
    Sets various variables using `font-lock-defaults' and
    `font-lock-maximum-decoration'."
      ;; Set fontification defaults if not previously set for correct major mode.
      (unless (and font-lock-set-defaults
               (eq font-lock-major-mode major-mode))
        (setq font-lock-major-mode major-mode)
        (setq font-lock-set-defaults t)
        ...

Message #20 received at 24176 <at> debbugs.gnu.org (full text, mbox):

From: Lars Ingebrigtsen <larsi <at> gnus.org>
To: npostavs <at> users.sourceforge.net
Cc: Clément Pit--Claudel <clement.pitclaudel <at> live.com>,
 Stefan Monnier <monnier <at> iro.umontreal.ca>, 24176 <at> debbugs.gnu.org
Subject: Re: bug#24176: Confusing interaction between define-derived-mode
 and font-lock-add-keywords
Date: Sun, 17 Nov 2019 07:59:53 +0100

npostavs <at> users.sourceforge.net writes:

> Clément Pit--Claudel <clement.pitclaudel <at> live.com> writes:
>
>> Sorry, the original report had a confusing example; here is a fixed copy.
>>
>> (define-derived-mode ~/a fundamental-mode "~/a"
>>   (font-lock-add-keywords nil `(("a" 0 'font-lock-keyword-face))))
>> (define-derived-mode ~/b ~/a "~/b"
>>   (font-lock-add-keywords nil `(("b" 0 'font-lock-builtin-face))))
>> (define-derived-mode ~/c ~/b "~/c"
>>   (font-lock-add-keywords nil `(("c" 0 'font-lock-constant-face))))
>>
>> Explicitly calling (setq font-lock-major-mode major-mode) before
>> calling font-lock-add-keywords yields the expected behaviour.
>
> Yeah, it seems `font-lock-set-defaults' (called from
> `font-lock-add-keywords') deletes the parent mode's added keywords since
> they were not from the "correct" mode.
>
>     (defun font-lock-set-defaults ()
>       "Set fontification defaults appropriately for this mode.
>     Sets various variables using `font-lock-defaults' and
>     `font-lock-maximum-decoration'."
>       ;; Set fontification defaults if not previously set for correct major mode.
>       (unless (and font-lock-set-defaults
>                (eq font-lock-major-mode major-mode))
>         (setq font-lock-major-mode major-mode)
>         (setq font-lock-set-defaults t)
>         ...

Stefan, is this working as designed?  If so, the behaviour should
perhaps be mentioned in the doc for font-lock-add-keywords and/or
define-derived-mode?

-- 
(domestic pets only, the antidote for overdose, milk.)
   bloggy blog: http://lars.ingebrigtsen.no

Message #23 received at 24176 <at> debbugs.gnu.org (full text, mbox):

From: Stefan Monnier <monnier <at> iro.umontreal.ca>
To: Lars Ingebrigtsen <larsi <at> gnus.org>
Cc: Clément Pit--Claudel <clement.pitclaudel <at> live.com>,
 24176 <at> debbugs.gnu.org, npostavs <at> users.sourceforge.net
Subject: Re: bug#24176: Confusing interaction between define-derived-mode
 and font-lock-add-keywords
Date: Sun, 17 Nov 2019 13:55:49 -0500

> Stefan, is this working as designed?

Not really, no.  The design didn't take this into account at least.

> If so, the behaviour should perhaps be mentioned in the doc for
> font-lock-add-keywords and/or define-derived-mode?

I think it'd be better to fix it ;-)


        Stefan

Message #26 received at 24176 <at> debbugs.gnu.org (full text, mbox):

From: Lars Ingebrigtsen <larsi <at> gnus.org>
To: Stefan Monnier <monnier <at> iro.umontreal.ca>
Cc: Clément Pit--Claudel <clement.pitclaudel <at> live.com>,
 npostavs <at> users.sourceforge.net, 24176 <at> debbugs.gnu.org
Subject: Re: bug#24176: Confusing interaction between define-derived-mode
 and font-lock-add-keywords
Date: Mon, 24 Jan 2022 11:56:18 +0100

Stefan Monnier <monnier <at> iro.umontreal.ca> writes:

>> Stefan, is this working as designed?
>
> Not really, no.  The design didn't take this into account at least.
>
>> If so, the behaviour should perhaps be mentioned in the doc for
>> font-lock-add-keywords and/or define-derived-mode?
>
> I think it'd be better to fix it ;-)

I think I've fixed this in Emacs 29.

-- 
(domestic pets only, the antidote for overdose, milk.)
   bloggy blog: http://lars.ingebrigtsen.no

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