In article <20040102051625.053B1AB8D@Adric.metnet.navy.mil>, oleg@pobox.com wrote:
Similarly, R5RS obligates any Scheme implementation to resort to assignments when processing a letrec form.
Not mine! I do use a polyvariadic fixed-point function. (define circular (letrec ((c (cons 'x c))) c)) (list-head circular 10) => (x x x x x x x x x x) Try it yourself at http://hscheme.sourceforge.net/interpret.php. I also make the fixed-point function available as "call-with-result", it's more or less equivalent to this: (lambda (f) (letrec ((x (f x))) x))
An implementation may not use a (polyvariadic) Y to implement letrec, unless the implementation can prove that the difference is unobservable for the form in question.
Do you have an example of use of Y for letrec where a program would violate R5RS? -- Ashley Yakeley, Seattle WA