Re: tail recursion hack in Emacs Lisp?

[Oliver Scholz <alkibiades@gmx.de>]

[Sorry for talking to myself today.]

Oliver Scholz <alkibiades@gmx.de> writes:

> Oliver Scholz <alkibiades@gmx.de> writes:
>
> [`iterate' macro with code-walker]
>>
>> So my question is: Can anybody think of a case where this approach
>> would break?
>
> Silly me. The usual case for breaking such things would apply:
>
> (iterate fact ((n 10)
> 	       (r 1))
>   (if (= n 1)
>       r
>     (funcall (intern "fact") (1- n) (* n r))))
>
> Not to mention `eval'.

And of course that could be circumwented by `fset'ing NAME:

(defmacro iterate (name arglist &rest body)
  (let ((catch-symbol (make-symbol "--repeat"))
	(continue (make-symbol "--continue"))
	(result (make-symbol "--result"))
	(fdef (make-symbol "--fdef"))
	(lambda-list (mapcar 'car arglist))
	(initial-args (mapcar 'cadr arglist)))
    `(let ((,fdef (and (fboundp ',name)
			   (symbol-function ',name))))
       (unwind-protect
	   (let ((,continue t)
		 (,result (list ,@initial-args)))
	     (fset ',name (lambda (&rest args)
			    (throw ',catch-symbol args)))
	     (while ,continue
	       (setq ,result
		     (catch ',catch-symbol
		       (setq ,result
			     (apply
			      (lambda ,lambda-list
				,@body)
			      ,result))
		       (setq ,continue nil)
		       ,result)))
	     ,result)
	 (if (null ,fdef)
	     (fmakunbound ',name)
	   (fset ',name ,fdef))))))


Yet, I dislike it more and more.


    Oliver
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