On Mar 12, 2007, at 4:06 PM, Nathan Thern wrote:
Here is a modification to library L1 that gets around
the above "loophole fix":
(library (L1)
(export y get-y set-y!)
(import (r6rs))
(define x #f)
(define y (car x))
(define z ((caddr x)))
(define get-y (lambda () y))
(define set-y! #f)
(set! x (call/cc (lambda (k) (list 0 k values))))
(set! set-y!
(lambda (v)
(call/cc (lambda (k) ((cadr x) (list v (cadr x) k)))))))
Note that this is an invalid library (cannot export mutable bindings).
The point was that Kent's program was exploiting the "invisible" set!
that comes from the letrec* implementation in order to change the value
of the "immutable" y. The once-only restriction makes it an error to
modify the value of any library identifier by returning more than once
to the code that initialized the variable.
Aziz,,,
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