On Tue, Mar 11, 2008 at 05:54:01PM -0400, Swaroop Sridhar wrote:
> 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.
Then I'm confused, I thought that was the point of shap's original
email---the fact that by-refs can escape.
> 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).
If you're already doing some sort of region typing then this should be
being checked for and everything should be OK. I guess I'm missing
something then!
Hum, thinking about it a bit more. I'm not sure what you meant by
"there is no problem as long as the reference does not escape its
definition". The definition of what? Using shap's example:
(define (outer x:(by-ref 'a))
(lambda (y)
(pair y x)))
As an aside, shap spelt the inner expression (pair y,x) and I'm not sure
why; I thought it would be either (y,x) or (pair y x).
On a first reading, I thought you meant that x has escaped the
definition of "outer", but this, on its own, isn't unsafe. It's only
bad when x escapes the definition of whatever it's referencing. For
example, the following use of outer is safe:
(let ((z 1))
(snd (outer z)))
because the reference to z never escapes its definition. Again, I'm
unsure of the semantics, is the "let" even needed? from a functional
coding point of view it would be nice if it wasn't.
Is that sort of correct, or have I missed the point entirely?
Sam
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