[bitc-dev] Capture problem with by-ref

[Jonathan S. Shapiro]

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.


shap