Why call font-lock-add-keywords in a mode hook?

Why call font-lock-add-keywords in a mode hook?

[Marcin Borkowski]

Hi list,

this I found in the manual:

--8<---------------cut here---------------start------------->8---
 -- Function: font-lock-add-keywords mode keywords &optional how
     This function adds highlighting KEYWORDS, for the current buffer or
     for major mode MODE.  The argument KEYWORDS should be a list with
     the same format as the variable ‘font-lock-keywords’.

     If MODE is a symbol which is a major mode command name, such as
     ‘c-mode’, the effect is that enabling Font Lock mode in MODE will
     add KEYWORDS to ‘font-lock-keywords’.  Calling with a non-‘nil’
     value of MODE is correct only in your ‘~/.emacs’ file.

     If MODE is ‘nil’, this function adds KEYWORDS to
     ‘font-lock-keywords’ in the current buffer.  This way of calling
     ‘font-lock-add-keywords’ is usually used in mode hook functions.
--8<---------------cut here---------------end--------------->8---

My question: why would I want to call this function in a hook if I can
specify the mode in its invocation?  (Assuming that I use it in my
init.el, of course.)

TIA,

-- 
Marcin Borkowski
http://octd.wmi.amu.edu.pl/en/Marcin_Borkowski
Faculty of Mathematics and Computer Science
Adam Mickiewicz University

Re: Why call font-lock-add-keywords in a mode hook?

[Michael Heerdegen]

Marcin Borkowski <mbork@mbork.pl> writes:

> Hi list,
>
> this I found in the manual:
>
>  -- Function: font-lock-add-keywords mode keywords &optional how
>      This function adds highlighting KEYWORDS, for the current buffer or
>      for major mode MODE.  The argument KEYWORDS should be a list with
>      the same format as the variable ‘font-lock-keywords’.
>
>      If MODE is a symbol which is a major mode command name, such as
>      ‘c-mode’, the effect is that enabling Font Lock mode in MODE will
>      add KEYWORDS to ‘font-lock-keywords’.  Calling with a non-‘nil’
>      value of MODE is correct only in your ‘~/.emacs’ file.
>
>      If MODE is ‘nil’, this function adds KEYWORDS to
>      ‘font-lock-keywords’ in the current buffer.  This way of calling
>      ‘font-lock-add-keywords’ is usually used in mode hook functions.
>
> My question: why would I want to call this function in a hook if I can
> specify the mode in its invocation?  (Assuming that I use it in my
> init.el, of course.)

I can think of two reasons at least:

(1)  If MODE has not yet "been initialized", the HOW argument would not
have any effect.  In particular, if you want to append your keywords to
the generic ones, this will only work before the mode had been defined.

(2)  With MODE -> nil, using the function in a hook is a common use
case, I think.


Regards,

Michael.

Re: Why call font-lock-add-keywords in a mode hook?

[Stefan Monnier]

> My question: why would I want to call this function in a hook if I can
> specify the mode in its invocation?

Because the other use is fundamentally unclean, requiring
a re-implementation of major-mode's inheritance, whereas you get it for
free if you use nil for `mode' and call it from the mode's hook instead.


        Stefan

Re: Why call font-lock-add-keywords in a mode hook?

[Marcin Borkowski]

On 2016-01-26, at 05:14, Stefan Monnier <monnier@iro.umontreal.ca> wrote:

>> My question: why would I want to call this function in a hook if I can
>> specify the mode in its invocation?
>
> Because the other use is fundamentally unclean, requiring
> a re-implementation of major-mode's inheritance, whereas you get it for
> free if you use nil for `mode' and call it from the mode's hook instead.

Ah, so that means that if I use a non-nil value for the `mode' argument,
say x-mode, and launch a buffer in y-mode, which derives from x-mode,
then I won't have my keywords there?

>         Stefan

Best,

-- 
Marcin Borkowski
http://octd.wmi.amu.edu.pl/en/Marcin_Borkowski
Faculty of Mathematics and Computer Science
Adam Mickiewicz University

Re: Why call font-lock-add-keywords in a mode hook?

[Stefan Monnier]

>>> My question: why would I want to call this function in a hook if I can
>>> specify the mode in its invocation?
>> Because the other use is fundamentally unclean, requiring
>> a re-implementation of major-mode's inheritance, whereas you get it for
>> free if you use nil for `mode' and call it from the mode's hook instead.
> Ah, so that means that if I use a non-nil value for the `mode' argument,
> say x-mode, and launch a buffer in y-mode, which derives from x-mode,
> then I won't have my keywords there?

Yes (unless someone wrote the extra code to try and handle this case).


        Stefan