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On 06/09/2012 07:22 PM, Alex Shinn wrote:
> On Sun, Jun 10, 2012 at 9:57 AM, Ray Dillinger <bear@sonic.net>
> wrote:
>>
>> Scheme's "delay" and "force" have never really made strong
>> enough guarantees that delayed promises will not be computed
>> unless forced.  The language allows implicit forcing whenever a
>> promise is referred to.
>
> If this were true it would definitely make delay completely
> unusable, however it is not true.
>
> What RnRS allow is "implicit forcing" when passing a promise to a
> primitive "like `cdr' and`+'", with the example
>
> (+ (delay (* 3 7)) 13) => 34
>
> given.  This is obviously vague, but the intent is that primitives
> which immediately need the actual value are allowed to do an
> implicit force.  Other primitives like "cons" for instance, don't
> care what the value is and should not perform an implicit force -
> otherwise you couldn't build a stream.

Due to the "vagueness," I have (since R3RS at least)
regarded streams relying on delayed functions as a bit
of magic that works, not because the implementors are
following the spec, but rather because implementors are
flatly refusing to follow a spec which permits a
completely useless result in such a way as to actually
*produce* a completely useless result.

If you claim that the behavior people have been
implementing is something that the spec has required
all along, I think you are wrong.  If you claim that
it is something the spec has permitted and ought to
have required all along, I think you are right.

			Bear

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