(add-hook 'font-lock-mode-hook ...) keeps adding to font-lock-keywords

(add-hook 'font-lock-mode-hook ...) keeps adding to font-lock-keywords

[Klaus Zeitler]

For a long time I've been using the simple show-tabs function below. With CVS
emacs I've noticed now that the same element gets appended to global
font-lock-keywords over and over again.


1. emacs -Q&

2. M-x global-font-lock-mode

3. in scratch buffer evaluate the following 2 expressions:

(defun show-tabs () "Show tabs with a slightly changed background"
   (setq font-lock-keywords
	(append font-lock-keywords '(("[\t]+"  (0 'tab-face t))))))
(add-hook 'font-lock-mode-hook 'show-tabs)

4. in scratch buffer: C-h v font-lock-keywords

Local in buffer *scratch*; global value is 
(("[	]+"
  (0 'tab-face t)))

5. every C-h v font-lock-keywords adds one more entry to global
   font-lock-keywords

Value:
(t
 (("[	]+"
   (0 'tab-face t)))
 ("[	]+"
  (0 'tab-face t)))



Moreover if I load e.g. a text file, then C-h v in that buffer will give me
something like:

Value:
(t
 (("[	]+"
   (0 'tab-face t)))
 ("[	]+"
  (0 'tab-face t)))

Local in buffer x.txt; global value is 
(("[	]+"
  (0 'tab-face t))
 ("[	]+"
  (0 'tab-face t))
 ("[	]+"
  (0 'tab-face t))
 ("[	]+"
  (0 'tab-face t))
 ("[	]+"
  (0 'tab-face t)))

-- 
 ------------------------------------------
|  Klaus Zeitler      Lucent Technologies  |
|  Email:             kzeitler@lucent.com  |
 ------------------------------------------
---
Why bother building any more nuclear warheads until we use the ones we have?

Re: (add-hook 'font-lock-mode-hook ...) keeps adding to font-lock-keywords

[Stefan Monnier]

> (defun show-tabs () "Show tabs with a slightly changed background"
>    (setq font-lock-keywords
> 	(append font-lock-keywords '(("[\t]+"  (0 'tab-face t))))))
> (add-hook 'font-lock-mode-hook 'show-tabs)

font-lock-mode-hook is called both when turning the mode ON and when turning
the mode OFF.

font-lock-keywords should not be modified as you do.  Instead you should use
font-lock-add-keywords, which knows about font-lock-compile-keywords.


        Stefan

Re: (add-hook 'font-lock-mode-hook ...) keeps adding to font-lock-keywords

[Klaus Zeitler]

>>>>> "Stefan" == Stefan Monnier <monnier@iro.umontreal.ca> writes:
    Stefan> 
    Stefan> font-lock-mode-hook is called both when turning the mode ON and
    Stefan> when turning the mode OFF.

But I didn't turn it off.

    Stefan> font-lock-keywords should not be modified as you do.  Instead you

Is that documented somewhere. I've seen similar constructs used quite a few
times (e.g. emacs wiki page).

    Stefan> should use font-lock-add-keywords, which knows about
    Stefan> font-lock-compile-keywords.

ok, changed it to:

(defun show-tabs () "Show tabs with a slightly changed background"
   (font-lock-add-keywords nil '(("[\t]+"  (0 'tab-face t)))))
(add-hook 'font-lock-mode-hook 'show-tabs)

and now it looks better, but it still looks a bit odd to me:

(t
 (("[	]+"
   (0 'tab-face t)))
 ("[	]+"
  (0 'tab-face t)))

But at least it doesn't get added over and over again.

-- 
 ------------------------------------------
|  Klaus Zeitler      Lucent Technologies  |
|  Email:             kzeitler@lucent.com  |
 ------------------------------------------
---
If a listener nods his head when you're
explaining your program, wake him up.

Re: (add-hook 'font-lock-mode-hook ...) keeps adding to font-lock-keywords

[Stefan Monnier]

Stefan> font-lock-mode-hook is called both when turning the mode ON and
Stefan> when turning the mode OFF.
> But I didn't turn it off.

No, but it still means your hook code is wrong: you should check that
font-lock-mode is non-nil before calling your `show-tabs'.

Stefan> font-lock-keywords should not be modified as you do.  Instead you
> Is that documented somewhere. I've seen similar constructs used quite a few
> times (e.g. emacs wiki page).

Not really, but it's always been that way.  Take a look at what
font-lock-compile-keywords does.  It has partly changed over time, but some
form of it has existed for "ever".
Also the docstring suggests using font-lock-add-keywords:

   [...]
   contained expressions.  You can also alter it by calling
   `font-lock-add-keywords' or `font-lock-remove-keywords' with MODE = nil.
   [...]

> and now it looks better, but it still looks a bit odd to me:

Only to you.  Not to Emacs.


        Stefan