Re: [Chicken-hackers] [PATCH] R7RS delay/force/delay-force

[John Cowan]

Jörg F. Wittenberger scripsit:

> Rather uncontroversial: r7rs says "the result of a delayed
> expression returning multiple values is unspecified". Hence it's OK
> to actually return multiple values (as I'd prefer r7rs should have
> explicitely required). The downside is that a portable program must
> not depend on this to work.

The reason we didn't require this is that in order to cache the multiple
values, in practice the implementation would have to put them into a
data structure, so the user might just as well delay the data structure
explicitly. So instead of (delay (exact-integer-sqrt n)), you can instead
do (delay (call-with-values (lambda () (exact-integer-sqrt n)) list).
This may be a bit more messy, but it didn't seem worth complicating the
implementation for what is expected to be a rare case.

> It specifies "if it is forced a second time, the previously computed
> value is returned". This _could_ (and IMHO _should_) be understood
> as if the delayed expression must only ever be evaluated once.

That's the most plausible interpretation, yes.  But there are at
least two issues: re-entrancy via call/cc and via threads (which
in some implementations may be the same).  We left that open,
because it's not clear what the Right Thing is.

-- 
John Cowan  address@hidden  http://ccil.org/~cowan
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