Re: How many parameters does an elisp function take?

[Kevin Rodgers <ihs_4664@yahoo.com>]

David Kastrup wrote:
> Kevin Rodgers <ihs_4664@yahoo.com> writes:
>>Alan Mackenzie wrote:
>>>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.
>>
>>I would start with eldoc-function-arglist.
> 
> For built-in functions, subr-arity might help.

And now for lisp functions, lambda-arity:

(require 'eldoc)

(defun lambda-arity (function)
   "Return minimum and maximum number of args allowed for FUNCTION.
FUNCTION must be a symbol whose function binding is a lambda expression
or a macro.
The returned value is a pair (MIN . MAX).  MIN is the minimum number
of args.  MAX is the maximum number or the symbol `many', for a lambda
or macro with `&rest' args."
   (let* ((arglist (eldoc-function-arglist function))
          (optional-arglist (memq '&optional arglist))
          (rest-arglist (memq '&rest arglist)))
     (cons (- (length arglist)
              (cond (optional-arglist (length optional-arglist))
                    (rest-arglist (length rest-arglist))
                    (t 0)))
           (cond (rest-arglist 'many)
                 (optional-arglist (+ (length arglist)
                                      (length optional-arglist)
                                      -1))
                 (t (length arglist))))))

-- 
Kevin Rodgers