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| 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 ()
(font-lock-add-keywords
nil `(("(\\(lambda\\>\\)"
(0 (progn (compose-region (match-beginning 1) (match-end 1)
,(make-char 'greek-iso8859-7 107))
nil))))))
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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