Re: Arbitrary function: find the number(s) of expected arguments

[Eli Zaretskii <eliz@gnu.org>]

> From: Michael Heerdegen <michael_heerdegen@web.de>
> Cc: emacs-devel@gnu.org
> Date: Sat, 19 Mar 2016 18:59:14 +0100
> 
> Eli Zaretskii <eliz@gnu.org> writes:
> 
> > > > I disagree with this conclusion.  The fact is that the Emacs Lisp
> > > > interpreter does know how to produce a clear answer.
> > > 
> > > Imagine `f' accepts, say, three arguments.  Now, let's
> > > 
> > > (advice-add 'f :around
> > >             (defun my-f-around-advice (orig-f &rest args)
> > >               (let ((some-value (g args)))
> > >                 (if (function-p (car-safe some-value))
> > >                     (apply (car some-value) (cdr some-value))
> > >                   (if (h args)
> > >                       (k args)
> > >                     (apply orig-f some-value))))))
> > > 
> > > What should (function-arity #'f) return?
> >
> > How is that relevant to the cited text?
> 
> I had concluded that it is not possible to implement `function-arity' so
> that it will always give a meaningful return value.  You disagreed
> (that's the cited text), so I tried to give an example proving my
> conclusion.  What did I miss?

The second sentence in the cited text.