Re: using command-error-function in emacspeak

[Jarek Czekalski]

W dniu 2013-11-05 20:01, Stefan Monnier pisze:
> > (add-function command-error-function something) ;; forgive me wrong syntax
> > here should do the same as
> > (setq command-error-function something)
> > in case when command-error-function is nil beforehand.
> But that can't be right.
> E.g.
>
>     (add-function :before command-error-function
>                   (lambda (&rest _ignore)
>                     (cl-incf command-error-function-counter)))
>
> Should not affect Emacs's behavior other than to keep a counter of calls
> to command-error-function.  But
>
>    (setq command-error-function
>          (lambda (&rest _ignore) (cl-incf command-error-function-counter))
>
> will not do that, since it will instead silence all the
> command-error messages.
>          
>
>          Stefan

So this is regarding command-error-function specific variable, right? 
Not add-function in general? This will be fixed soon, the patch is 
waiting for a review.

Are there examples of any other ...-function variables that work in this 
way: nil has a special meaning and nil value results in calling some 
system hidden handler? I think this is not a good logic to follow. The 
better is this:

nil value for a AAA-function variable means that no action is taken when 
AAA happens.

When this logic is used, add-function should operate on nil values as 
well as on function values.

Which logic is used in general in Emacs for ...-function variables - I 
don't know. If you can point me to some reading, that would help. 
Currently I try to understand it by intuition.

I also reviewed a couple of examples:

adaptive-fill-function - nil by default, means no action
browse-url-browser-function - not nil
buffer-stale-function - not nil
comment-indent-function - not nil
... stopped, this seems to confirm my logic.

add-function may be used by packages for their own goals, but nil value 
should always have a common meaning. No action seems to be intuitive.

Jarek