On 16/10/2008, at 21:34, Simon Peyton-Jones wrote:
For strict *constructors*, on the other hand, we *do* guarantee to evaluate the argument before building the constructor. We generate a wrapper thus wC = \ab. case a of { a' -> C a' b } (Remember 'case' always evaluates in Core.) So for strict constructors we could take advantage of the known evaluated-ness of the result to avoid the test.
BUT people who care probably UNPACK their strict fields too, which is even better. The time you can't do that is for sum types data T = MkT ![Int]
You also can't do it for polymorphic components. I've used code like: data T a = MkT !a foo :: T (a,b) -> a foo (MkT (x,y)) = x Here, unpacking doesn't work but foo could still access the components of the pair directly. Roman