On 22 June 2004 15:59, Malcolm Wallace wrote:

"Simon Marlow" 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