[r6rs-discuss] [Formal] eliminate library export immutability loophole

[Abdulaziz Ghuloum]

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,,,