Re: How many parameters does an elisp function take?

[Alan Mackenzie]

Stefan Monnier <monnier@iro.umontreal.ca> wrote on Fri, 18 Feb 2005
08:59:24 -0500:
>> Is it possible to determine at run time how many parameters an elisp
>> function takes?  For example, I'd like to write something like:

>> (how-many-params 'null)

>> and have it evaluate to 1.  Or something like that.  Together with
>> some reasonable convention for indicating &optional and &rest
>> arguments.

> Why do you want to know?

I'd like to fix `beginning-of-defun-raw', where it does (funcall
beginning-of-defun-function).  The parameter `arg' from
beginning-of-defun should be passed through to b-o-d-f.  We've talked
about this before.  Unfortunately, there is no guarantee that existing
user supplied functions can accept an `arg'.  I would thus replace the
call with

(if (>= (how-many-params beginnin-of-defun-function) 1)
    (funcall beginning-of-defun-function arg)
  (funcall beginning-of-defun-function))

> Every time this has shown up for me, what I truly wanted to know was
> more like "can I call this with 4 args?", and the reason why I wanted
> to know was to know whether to call it with 4 args or otherwise do
> something else (e.g. call it with fewer args).

Exactly.

> In practice, it's simpler to just do

>     (condition-case nil
>         (fooo)
>       (wrong-number-of-arguments
>         (bar)))

> It's not perfect, but I've found it to suffer from fewer problems than
> other solutions.  It's also faster.

Good idea!

Why is there not such a function in the Emacs core?  It seems such an
incredibly useful function, say for debuggers or code-analysers, or for
the uses mentioned above.  Did somebody just overlook it in the early
days, perhaps?

>         Stefan

-- 
Alan Mackenzie (Munich, Germany)
Email: aacm@muuc.dee; to decode, wherever there is a repeated letter
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