Sam Mason wrote: > On Tue, Mar 11, 2008 at 09:09:39AM -0400, Jonathan S. Shapiro wrote: >> Problem: BY-REF parameters can escape: >> >> (define (outer x:'a) >> (define (capture xc:(by-ref 'a)) >> (lambda (y) >> (pair y, x))) >> >> Note that "x" is in the call frame of OUTER, it is aliased by "xc", "xc" >> is in turn captured by the lambda, and the lambda escapes. This is bad, >> because we now have a heap-based object (the closure) that points back >> into a stack-based object. > > If I understand, once by-ref's have been included you've pretty much > subverted your value/ref dichotomy. I.e. it's the strong distinction > between value and ref's that ensure type safety and by-ref allows value > types to be propagated to refs. > > As another, much more complicated solution, how about some limited form > of region typing. Each pointer would be annotated with the (highest) > stack frame it came from and if a pointer ever escapes from a frame > with the same annotation then a compile error is thrown. Some form of > parametrisation is also needed, which is where the (implementation) > complexity arises when I think about it.
The by-ref operator is a limited form of the address (&) operator. There is actually no problem as long as the reference does not escape its definition. The by-ref model is actually a degenerate form of region typing, since we do not allow escape at all (rather than allowing it as long as the target region is guaranteed to be in a later activation). Swaroop. _______________________________________________ bitc-dev mailing list bitc-dev@coyotos.org http://www.coyotos.org/mailman/listinfo/bitc-dev
