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

[Paul Pogonyshev <pogonyshev@gmail.com>]

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Hi,

Suppose I have a library with

    (defun my-foo (callback)
      ...
      (funcall callback computed-data))

Now, _some_ users of my library found out that it would be nice to have
`this-value-gets-computed-as-side-effect' too, though it is not really
necessary for callbacks in general. I wouldn't want to change interface of
the library, instead I would just improve it like this:

    (defun my-foo (callback)
      ...
      (if (accepts-2-arguments callback)
          (funcall callback computed-data
this-value-gets-computed-as-side-effect)
        (funcall callback computed-data)))

This way `my-foo' automagically accepts callbacks that expect one argument
(as in initial form), as well as those that want two arguments.

The problem, of course, is to write `accepts-2-arguments'.  I found
`subr-arity', which provides exactly what I need, but only for builtins...
I was also told about `help-function-arglist', but it returns the raw
argument list in which you still have to handle &optional or &rest (maybe
something else too?). This looks like a bit too much work for simple thing
and likely not good for performance-critical places either.

Feature request: add a builtin like `subr-arity' that works for _any_
function. Evaluator sure knows how to call the functions, so the
information is already there.

Paul

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