On 22 June 2004 15:59, Malcolm Wallace wrote:
> "Simon Marlow" <[EMAIL PROTECTED]> writes:
>
>> Nope. You can't return something without evaluating it to head
>> normal form in Haskell. Every value that is "returned" is a value,
>> never a thunk. If you want to return something unevaluated, you
>> have to wrap it in a constructor.
>
> Actually, there are two possible strategies for who evaluates a thunk
> to head normal form. The caller or the callee. GHC says that the
> callee always evaluates to HNF before returning the value. But the
> language does not force this implementation choice. It is equally
> possible for an implementation of the function to return a thunk and
> for the caller to evaluate it if necessary. Both strategies give the
> same result semantically, since if the value in question is demanded,
> it gets evaluated.
If a function is called, then the result has been demanded. There are
no situations in which a function has been called but the caller will
accept a thunk as the result without further evaluating it.
So what is the advantage of returning a possibly-unevaluated result?
The caller will definitely have to evaluate it, so every eval will need
to loop until the value is in HNF.
And what about updates?
Cheers,
Simon
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