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

From: Abdulaziz Ghuloum <aghuloum>
Date: Tue Mar 20 04:09:34 2007

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,,,
Received on Tue Mar 20 2007 - 04:09:10 UTC