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minor mode problem

From: Stephen Berman
Subject: minor mode problem
Date: Fri, 27 Jun 2008 01:06:33 +0200
User-agent: Gnus/5.13 (Gnus v5.13) Emacs/23.0.60 (gnu/linux)

In GNU Emacs (i686-pc-linux-gnu, GTK+ Version 2.12.0) of
2008-06-25 on escher I start with -Q and evaluate the following minor
mode definition:

(define-minor-mode srb-mode
  "Toggle srb mode."
  :lighter " srb"
  (if srb-mode
        (message "font-lock-mode: %s" font-lock-mode)
        (sit-for 1)
        (if font-lock-mode (font-lock-mode -1))
        (message "font-lock-mode: %s" font-lock-mode))
    (unless font-lock-mode (font-lock-mode 1))
    (message "font-lock-mode: %s" font-lock-mode)))

I write the following files to disk:

,----[ /tmp/srb-test1.el ]
| ;; blah

,----[ /tmp/srb-test1 ]
| ;; blah
| ;; Local Variables:
| ;; mode: emacs-lisp
| ;; End:

,----[ /tmp/srb-test2 ]
| ;; blah

Now I evaluate the following function definitions:

(defun srb-mode-test (file)
  "Enable srb-mode."
  (interactive "fFile: ")
  (find-file file)
  (srb-mode 1))

(defun srb-mode-test2 ()
  "Put a file in Emacs Lisp mode and enable srb-mode."
  (find-file "/tmp/srb-test2")
  (srb-mode 1))

(1) I then call srb-mode-test twice, passing it first the file
/tmp/srb-test1.el and then the file /tmp/srb-test1.  In both cases I
first see the buffer text fontified with font-lock-comment-face, then
after a second the fontification turns off.  This is as I expect.

(2) Then I call srb-mode-test2: I see the text in font-lock-comment-face
and the message "font-lock-mode: t", then a second later the message
"font-lock-mode: nil", but the text remains fontified, and `C-h v
font-lock-mode' in that buffer says its value is t, although it was nil
according to the last message.  This I did not expect and do not

(3) Next I try edebugging srb-mode-test2.  
(a) When I execute each sexp (with `f'), after (emacs-lisp-mode) I see
the buffer text become fontified with font-lock-comment-face, and after
(srb-mode 1) it loses fontification.  This is what I expect but, again,
not what happens when I execute the whole function outside of edebug.
(b) However, when I step into rather than over (emacs-lisp-mode) (`i'
instead of `f') and then execute each sexp of that function, the buffer
text never displays font-lock-comment-face.
(c) And finally, when I instrument srb-mode but not srb-mode-test2, then
when edebug stops execution at srb-mode, the buffer text displays
font-lock-comment-face, and when I step through srb-mode, I see the
buffer text lose fontification at the end of the progn, but when I then
continue stepping through to the end of srb-mode, the fontification
returns, so the result is as in (2) above.

(4) As a last case, I write the following file to disk:

,----[ /tmp/srb-test3 ]
| ;; blah
| ;; Local Variables:
| ;; mode: emacs-lisp
| ;; mode: srb
| ;; End:

(a) When I do `C-x C-f /tmp/srb-test3', the result is as in (2).
(b) When I instrument emacs-lisp-mode, and after typing `C-x C-f
/tmp/srb-test3' step over it in edebug, the result is as in (3b).
(c) When I instrument srb-mode, and after typing `C-x C-f
/tmp/srb-test3' step over it in edebug, the result is as in (3a) (not as
in (3c)).

Can anyone explain what's happening in (2)-(4) (especially cases (2) and
(4a)), or at least offer some advice about how to debug them better?

Steve Berman

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