Re: ELisp function prototypes and local function name

[Ted Zlatanov <tzz@lifelogs.com>]

On Thu, 07 Apr 2011 12:27:52 +0900 "Stephen J. Turnbull" <stephen@xemacs.org> wrote: 

SJT> Ted Zlatanov writes:
>> But it doesn't correspond to the advertised prototype
>> "(mapc FUNCTION SEQUENCE)" and loses the argument names.

SJT> Oh well.

SJT> (function-arglist 'mapc) => (mapc FUNCTION SEQUENCE &rest SEQUENCES)

SJT> where I live.  I think Aidan Kehoe knows most about that code
SJT> currently, but I don't know what the assignment status is.

Interesting.  So, disregarding the string-oriented help.el functions, in
Emacs we have `semanticdb-elisp-sym-function-arglist' which extracts the
actual function from the symbol:

#+begin_src lisp
(defun semanticdb-elisp-sym-function-arglist (sym)
  "Get the argument list for SYM.
Deal with all different forms of function.
This was snarfed out of eldoc."
  (let* ((prelim-def
	  (let ((sd (and (fboundp sym)
			 (symbol-function sym))))
	    (and (symbolp sd)
		 (condition-case err
		     (setq sd (indirect-function sym))
		   (error (setq sd nil))))
	    sd))
         (def (if (eq (car-safe prelim-def) 'macro)
                  (cdr prelim-def)
                prelim-def))
         (arglist (cond ((null def) nil)
			((byte-code-function-p def)
			 ;; This is an eieio compatibility function.
			 ;; We depend on EIEIO, so use this.
			 (eieio-compiled-function-arglist def))
                        ((eq (car-safe def) 'lambda)
                         (nth 1 def))
                        (t nil))))
    arglist))

#+end_src

but that returns nil for everything when I tried it.  Also it calls
`eieio-compiled-function-arglist' which is:

#+begin_src lisp
(eval-and-compile
;; About the above.  EIEIO must process its own code when it compiles
;; itself, thus, by eval-and-compiling outselves, we solve the problem.

;; Compatibility
(if (fboundp 'compiled-function-arglist)

    ;; XEmacs can only access a compiled functions arglist like this:
    (defalias 'eieio-compiled-function-arglist 'compiled-function-arglist)

  ;; Emacs doesn't have this function, but since FUNC is a vector, we can just
  ;; grab the appropriate element.
  (defun eieio-compiled-function-arglist (func)
    "Return the argument list for the compiled function FUNC."
    (aref func 0))

  )
#+end_src

So `eieio-compiled-function-arglist' is what I need, but `aref' doesn't
work on any functions I've tried:

(aref (indirect-function 'mapcar) 0)
=> (wrong-type-argument arrayp #<subr mapcar>)

(aref (symbol-function 'mapcar) 0)
=> (wrong-type-argument arrayp #<subr mapcar>)

;; aref still doesn't work, but nth does on lambdas
(nth 1 (lambda (a b)))
=> (a b)

It looks like the XEmacs `function-arglist' is not useful, in any case.
It depends too much on the underlying implementation.

Ted