define-minor-mode, auto-mode-alist & change-major-mode-with-file-name

[Sebastian Tennant]

Hi list,

Test sequence #1
================
(define-minor-mode test-me-mode "Test Me"
  :lighter " TM" (message "I have been Tested."))

(add-to-list 'auto-mode-alist '("\\.test-me\\'" . test-me-mode))

Commands                     *Messages*
--------                     ----------
C-x b foo.test-me RET y      No buffer matching `foo.test-me', create one? (y 
or n)

M-x test-me-mode             I have been Tested.

C-x C-w RET                  Toggling test-me-mode off; better pass an explicit 
argument.
                             I have been Tested.
                             Saving file ~/foo.test-me...
                             Wrote ~/foo.test-me

RESULT: buffer foo.test-me is no longer in test-me mode.


Test sequence #2
================
Kill buffer foo.test-me and delete file.

(define-minor-mode test-me-mode "Test Me"
  (set (make-local-variable 'change-major-mode-with-file-name) nil)
  :lighter " TM" (message "I have been Tested."))

;; unchanged - no need to re-evaluate
(add-to-list 'auto-mode-alist '("\\.test-me\\'" . test-me-mode))

Commands                     *Messages*
--------                     ----------
C-x b foo.test-me RET y      No buffer matching `foo.test-me', create one? (y 
or n)

M-x test-me-mode             I have been Tested.

C-x C-w RET                  Saving file ~/foo.test-me...
                             Wrote ~/foo.test-me         

RESULT: buffer foo.test-me remains in test-me mode.


Analysis
========
Macro define-minor-mode creates a toggle-able minor mode function (as it
should).  However, when a non-file visiting buffer is 'manually' put into a 
minor
mode defined using said macro and then written to disk using a file name which
triggers that minor mode (via auto-mode-alist), the effect of this
toggle-ability is to turn the minor mode off.  Test sequence #1.

One way of preventing this is to set change-major-mode-with-file-name to nil in
your minor mode function definition.  Test sequence #2.


Questions
=========

1. Is there a better way of preventing the minor mode from being turned off,
   (short of not using define-minor-mode) when this precise sequence of
   commands is performed?

2. Given that change-major-mode-with-file-name works with minor modes too (as
   demonstrated), shouldn't it be called change-mode-with-file-name instead?

3. Generally speaking, auto-mode-alist only kicks in when visiting a file.
   When a non-file visiting buffer is written to disk, it (the buffer) is
   immediately replaced by a file-visiting buffer, hence the apparent
   effectiveness of auto-mode-alist when writing a file (rather than visiting
   one).  Is this explanation more or less correct?


Sebastian

(emacs-version)
=> "GNU Emacs 23.2.1 (i486-pc-linux-gnu) of 2010-12-11 on raven, modified by 
Debian"

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