GNU bug report logs - #5129
23.1.50; adding font-lock-keywords for face highlight

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Message #5 received at submit <at> emacsbugs.donarmstrong.com (full text, mbox):

From: "Roland Winkler" <Roland.Winkler <at> physik.uni-erlangen.de>
To: emacs-pretest-bug <at> gnu.org
Subject: 23.1.50; adding font-lock-keywords for face highlight
Date: Sat, 05 Dec 2009 13:15:31 -0600

The highlight face is somehow treated special in an odd way:

If I eval the following in a buffer it will highlight all occurences
of "foo" as expected:

  (font-lock-add-keywords nil '(("foo" . font-lock-constant-face)))

Yet when I do instead

  (font-lock-add-keywords nil '(("foo" . highlight)))

it gives me the message (in the *Messages* buffer)

  Invalid face reference: 0

When I put point at the position of the string "foo" and evaluate

  (get-text-property (point) 'face)

this gives me

  (0 highlight)

which probably explains the "invalid face reference" error.
Why is the face `0' added?


In GNU Emacs 23.1.50.1 (i686-pc-linux-gnu, GTK+ Version 2.14.4)
 of 2009-12-04 on regnitz
Windowing system distributor `The X.Org Foundation', version 11.0.10502000

Important settings:
  value of $LC_ALL: nil
  value of $LC_COLLATE: C
  value of $LC_CTYPE: nil
  value of $LC_MESSAGES: nil
  value of $LC_MONETARY: nil
  value of $LC_NUMERIC: nil
  value of $LC_TIME: en_GB.utf8
  value of $LANG: en_US.ISO-8859-15
  value of $XMODIFIERS: nil
  locale-coding-system: iso-latin-9-unix
  default enable-multibyte-characters: t

Message #8 received at 5129 <at> emacsbugs.donarmstrong.com (full text, mbox):

From: Glenn Morris <rgm <at> gnu.org>
To: Roland Winkler <Roland.Winkler <at> physik.uni-erlangen.de>
Cc: 5129 <at> debbugs.gnu.org
Subject: Re: bug#5129: 23.1.50; adding font-lock-keywords for face highlight
Date: Sat, 05 Dec 2009 15:32:07 -0500

"Roland Winkler" wrote:

>   (font-lock-add-keywords nil '(("foo" . font-lock-constant-face)))

works

>   (font-lock-add-keywords nil '(("foo" . highlight)))

doesn't work.

I think you are running into the old issue that faces should be quoted
in font-lock keywords, else they will be treated as variables. For
historical reasons, font-lock-constant-face (etc) as well as being a
face, is a variable whose value is that face.

Instead, try

(font-lock-add-keywords nil '(("foo" . 'highlight)))

Message #13 received at 5129-done <at> emacsbugs.donarmstrong.com (full text, mbox):

From: Stefan Monnier <monnier <at> iro.umontreal.ca>
To: Roland Winkler <Roland.Winkler <at> physik.uni-erlangen.de>
Subject: Re: bug#5129: 23.1.50; adding font-lock-keywords for face highlight
Date: Sat, 05 Dec 2009 16:04:29 -0500

>   (font-lock-add-keywords nil '(("foo" . font-lock-constant-face)))
                                           ^^^^^^^^^^^^^^^^^^^^^^^
> Yet when I do instead

>   (font-lock-add-keywords nil '(("foo" . highlight)))
                                           ^^^^^^^^^

Thos two things are treated by font-lock as a Lisp expression
to evaluate.  Since font-lock-constant-face is a variable (whose
content is usually the symbol font-lock-constant-face), the first will
work, whereas the second will either signal en error (because the
variable `highlight' doesn't exist) or lead to odd behavior (because
`highlight' just happens to be let-bound by one of the caller).

I.e. you want

   (font-lock-add-keywords nil '(("foo" . 'highlight)))
                                         ^^^

-- Stefan

Message #18 received at 5129 <at> emacsbugs.donarmstrong.com (full text, mbox):

From: "Roland Winkler" <Roland.Winkler <at> physik.uni-erlangen.de>
To: Glenn Morris <rgm <at> gnu.org>
Cc: 5129 <at> debbugs.gnu.org, Stefan Monnier <monnier <at> iro.umontreal.ca>
Subject: Re: bug#5129: 23.1.50; adding font-lock-keywords for face highlight
Date: Sat, 5 Dec 2009 17:32:44 -0600

On Sat Dec 5 2009 Glenn Morris wrote:
> Instead, try
> 
> (font-lock-add-keywords nil '(("foo" . 'highlight)))

Oh well, I thought I ran into this often enough. But still it's too
easy to forget it again.

Thanks for helping me out and sorry for the noise!

Roland

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