From: joswig@... (Rainer Joswig)
Date: 2004-07-15T15:22:18+09:00
Subject: Re: Functional Ruby (Re: Why I don't use Ruby.)

Lennon Day-Reynolds <rcoder@gmail.com> wrote in message news:<5d4c612404071411082f012bb8@mail.gmail.com>...
> On Wed, 14 Jul 2004 21:57:20 +0900, Rainer Joswig
> <joswig@corporate-world.lisp.de> wrote:
> > how should the PROD function mentioned in that authoritative ;-) explanation
> > work? Looks not very convincing to me. M needs to be incremented
> > in the recursive call - otherwise it would be running for a very
> > LOOOOONG time. Obviously that example code has never been
> > checked.
> Tail-call elimination -- the recursive function will be optimized into
> a loop by the compiler.
> 
> Lennon

Hmm

a) it can't be optimized because the function call is not a tail call
  (unless the compiler does some other high-level transformations).
b) still some value needs to be changed. the thing just does not compute
   a product in any useful way.

Say, if you want to compute a product from 1 to n, you need to
count somewhere. The function in the paper does not.

Here is the recursive version in Lisp:

(defun prod (m n f)
  (if (= m n)
      m
    (* (funcall f m)
       (prod (+ 1 m) n f))))

And here is a tail-recursive version:

(defun prod (m n f &optional (acc n))
  (if (= m n)
      acc
    (prod (+ 1 m) n f
          (* (funcall f m) acc))))

; product of numbers from 1 to 4
(prod 1 4 #'identity)  -> 24