On Tue, Jun 12, 2012 at 6:09 AM, John Cowan <cowan@mercury.ccil.org> wrote:
> Emmanuel Medernach scripsit:
>
>>   I agree that making a promise with an immediate value could evaluate
>> to that immediate instead of creating a promise.
>
> That's where make-promise (formerly known as eager) comes in: it allows
> you to return an object to your caller that is guaranteed to be a
> promise.  This allows promise-based APIs, where the caller knows he can
> safely force the return value.

make-promise no more guarantees a promise than delay
does, since there's no way to test for this and the standard
gives explicit license to cheat here.

>> IMHO for any other usage auto-forcing in primitives strongly sounds
>> as being in the "it seems a good idea at that time" department:
>> auto-forcing means that primitives have to check if something is a
>> promise and forcing it in that case, adding this check add a cost and
>> it has deep impact on the language semantics.
>
> It does add a cost, but the cost is paid only by users of an
> implementation (such as Kawa) that does auto-forcing.  The impact
> is actually quite shallow: it simply makes some functions mildly
> polymorphic.

Implicit forcing can be done without any overhead except
when actually making use of it.

--
Alex

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