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bug#20613: Font locking is poorly documented

From: Dmitry Gutov
Subject: bug#20613: Font locking is poorly documented
Date: Wed, 20 May 2015 04:00:57 +0300
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:38.0) Gecko/20100101 Thunderbird/38.0

On 05/20/2015 01:42 AM, Paul Eggert wrote:

(defun  pretty-lambdas  ()
    nil `(("(\\(lambda\\>\\)"
           (0 (progn  (compose-region (match-beginning 1) (match-end 1)
                                     ,(make-char 'greek-iso8859-7 107))

but I can't find documentation explaining why a progn is needed, or why
it must return niil, or why one must use compose-region, or why one must
use it with a single λ rather than just having a λ character there.

The return value is documented in the docstring of font-lock-keywords.

This case is (MATCHER . HIGHLIGHT), where HIGHLIGHT is MATCH-HIGHLIGHT. The expression is supposed to return the face to use, but here it returns nil, because it doesn't apply a face.

Unfortunately, using compose-region in it is pretty much undocumented behavior. But since we can anticipate in which context the expression is evaluated, it's still relatively safe WRT future changes in font-lock code.

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