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Re: Can array-ref return (values ...) ?



Brad Lucier <lucier@xxxxxxxxxxxxxxx> writes:
> > Brad Lucier <lucier@xxxxxxxxxxxxxxx> writes:
> > > > I don't understand. Array-ref by me returns the contents of a single
> > > > element, so that would be just (values v) which is just v.
> > > 
> > > Yes, it returns the contents of a single element, but what are the contents
> > > of a single element?  ...  Can it be
> > > (values a b)?  If not, why not?  
> > 
> > No. Because (values ...) is not a data constructor. It is a special
> > syntax that explicitly applies its arguments to the current
> > continuation [see the recent messages on cls under the TCO thread].
> 
> In R5RS, (values ...) is a procedure.

Ok, it's a procedure that accesses the current continuation
then. It is a very odd procedure in since it can only be called in
tail position. It *still* does not construct a data object, so it
cannot be the "contents" of an array element. IOW,

        (vector-set! v n (values 4 3 2))

is completely meaningless. Therefore the contents of a single element
cannot be (values a b).

> > > And if not, perhaps it should be
> > > documented to be not possible.
> > 
> > OK. Except that it's really not possible because of R5RS.

> R5RS has nothing at all to say about arrays (or many other things).
> So we are free to use our imagination, if we wish.

But it has lots to say about tail calls and continuations. VALUES is a
continuation operator, not a data constructor.

david rush
-- 
Scheme: Because continuations are freaky.
	-- Anton van Straaten (the Scheme Marketing Dept from c.l.s)