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bug#20613: Font locking is poorly documented

From: Paul Eggert
Subject: bug#20613: Font locking is poorly documented
Date: Tue, 19 May 2015 15:42:05 -0700
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:31.0) Gecko/20100101 Thunderbird/31.7.0

I was recently urged to use font locking to support a feature in Emacs, and I must say that I am still confused about how it works even after reading the Elisp manual several times. The manual has a lot of low-level discussion of mechanism, but it doesn't explain policy and since crucial features seem to be undocumented, I'm at a loss as to how to use it, or how to debug it when it's not working. I finally gave up and used Google and found cookbook stuff, but I have no idea how or why it works as well as it does, or why it fails when it does. For example:


gives this example:

(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.

One small suggestion: add a section explaining how the above example works.

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