[r6rs-discuss] A suggestion about LETREC

From: Abdulaziz Ghuloum <aghuloum>
Date: Wed Mar 28 16:45:27 2007

On Mar 28, 2007, at 12:51 PM, Joe Marshall wrote:

> Since we're discussing the semantics of LETREC, I've always thought
> it was
> a wart in the language to specify LETREC in terms of SET! It would
> be nice
> if implementations were permitted (but not required) to implement
> LETREC
> in terms of a fixed-point operator as an alternative to SET!

Do you mean that an implementation should be allowed to treat E1 as E2
or E3 instead of E4?

E1=
(letrec ((fact (lambda (n) (if (zero? n) 1 (fact (- n 1))))))
   (fact 5))

E2=
(fix ((fact (lambda (n) (if (zero? n) 1 (fact (- n 1))))))
   (fact 5))

E3=
(let ([fact (Y (lambda (fact)
                  (lambda (n) (if (zero? n) 1 (fact (- n 1))))))])
   (fact 5))

E4=
(let ((fact (unspecified)))
   (let ((t (lambda (n) (if (zero? n) 1 (fact (- n 1))))))
     (set! fact t))
   (fact 5))


> One advantage of this is that it would be much easier to create a
> purely
> functional subset of Scheme.

How do you do the following functionally?

(letrec ([f (g (lambda () f))])
   f)

Aziz,,,
Received on Wed Mar 28 2007 - 16:44:33 UTC