Subject: bug#5572: 23.1; Undocumented key :value-to-internal in customize system

From: Teemu Likonen <[email-masked]>
Date: Sat, 13 Feb 2010 11:36:01 +0200

Some customize options use key :value-to-internal with customize type
"repeat". One example is variable message-ignored-news-headers in
lisp/gnus/message.el:

(defcustom message-ignored-news-headers
  "^NNTP-Posting-Host:\\|^Xref:\\|^[BGF]cc:\\|^Resent-Fcc: [...]
  "*Regexp of headers to be removed unconditionally before posting."
  :group 'message-news
  :group 'message-headers
  :link '(custom-manual "(message)Message Headers")
  :type '(repeat :value-to-internal (lambda (widget value)
				      (custom-split-regexp-maybe value))
		 :match (lambda (widget value)
			  (or (stringp value)
			      (widget-editable-list-match widget value)))
		 regexp))

(I cut one long line and added ellipsis "[...]".)

:value-to-internal key doesn't seem to be documented anywhere so I
consider this a documentation bug. Please add a description for the key
- perhaps to the info node "(elisp) Type Keywords" - because it seems
very useful for Emacs extension writers.