[bitc-dev] Capture problem with by-ref

[Swaroop Sridhar]

Jonathan S. Shapiro wrote:
> On Thu, 2008-03-13 at 14:19 -0400, Swaroop Sridhar wrote:
>>>>> A capture of a by-ref formal variable (x) by an
>>>>> inner lambda (L) is safe only if:
>>>>>
>>>>> (1) L does not escape,   or
>>>>> (2) The body of L does not use x as an lvalue that is a target of an
>>>>>      assignment.
> 
> Consider::
> 
>   (define i:(mutable int32) 1))
>   (define (g x: (by-ref (mutable int32)))
>     (lambda ()
>       x)) ;
> 
> Note that L escapes but the body of L does not use the captured by-ref
> formal parameter as an l-value.
> 
> Now consider:
>   
>   (define captured (g i))
> 
>   (pair (captured) (begin (set! i (+ i 1)) (captured))
> 
> Which must result in (1, 2).
> 
> The capture here is actually okay because x is capturing a global, but
> that cannot be determined at the point where G is admitted.
> 
> So I think that your conditions are not sufficient to ensure safety.

I see the problem, thanks for the clarification.

Swaroop.