On Mar 19, 2007, at 11:42 AM, AndrevanTonder wrote:
On Sun, 11 Mar 2007, R. Kent Dybvig wrote:
Okay, here's one example:
(library (L1)
(export y get-y set-y!)
(import (r6rs))
(define x (call/cc (lambda (k) (list 0 k values))))
(define y (car x))
(define z ((caddr x)))
(define get-y (lambda () y))
(define set-y!
(lambda (v)
(call/cc (lambda (k) ((cadr x) (list v (cadr x) k)))))))
By my reading of the current library description, the program:
(import (r6rs) (L1))
(write (list y (get-y))) (newline)
(set-y! 3)
(write (list y (get-y))) (newline)
Doesn't the example violate the LETREC* restriction, though?
One restriction on letrec* is very important: it must be possible
to evaluate
each <init> without assigning or referring to the value the
corresponding
<variable> or the <variable> of any of the bindings that follow
it in
<bindings>.
In particular, the second time the RHS of the (define x ....)
binding is
evaluated, you are actually referring (via the (cadr x) in set-y!)
to the
value of x itself. So the example may already be illegal.
The program does not violate the letrec* restriction since in the
second time,
the value of x has already been determined (the first time around).
The program
never refers or assigns to any uninitialized bindings.
Aziz,,,
_______________________________________________
r6rs-discuss mailing list
[email protected]
http://lists.r6rs.org/cgi-bin/mailman/listinfo/r6rs-discuss
