In article <20040110040353.37C17AB8D@Adric.metnet.navy.mil>, oleg@pobox.com wrote:
(define circular (letrec ((c (cons 'x c))) c))
I'm afraid that is not a R5RS compliant code.
Indeed not, it merely demonstrates fixed-point behaviour. Nevertheless, allowing this as an extension does not make my implementation non-compliant. See section 1.3.2 on this point.
Do you have an example of use of Y for letrec where a program would violate R5RS?
http://google.com/groups?selm=7eb8ac3e.0304131423.4f103d4f%40posting.google.... m
The difference between the Y and set! approaches to letrec *is* observable.
I don't believe you. My implementation uses Haskell's "mfix", which looks like a Y to me. I certainly don't use anything like "set!". But my implementation passes Dorai Sitaram's test: (define *keep-track* '()) (letrec ((fact (lambda (n) (set! *keep-track* (cons fact *keep-track*)) (if (= n 0) 1 (* n (fact (- n 1))))))) (fact 8)) (eq? (car *keep-track*) (cadr *keep-track*)) My implementation returns 40320 #t ...which is apparently correct behaviour for R5RS. Indeed I get exactly the same result in mzscheme and guile. Again, I encourage you to try for yourself at http://hscheme.sourceforge.net/interpret.php (though it's a bit slow). -- Ashley Yakeley, Seattle WA