Dominic Orchard wrote:
I was just playing around and noticed that the kind of the function arrow in GHC is (?? -> ? -> *) when I (naively) expected it to be (* -> * -> *). After looking at (http://hackage.haskell.org/packages/archive/ghc/6.10.2/doc/html/Type.html#5) I see that the kind of (->) means that the parameter type cannot be an unboxed tuple, whilst the result type can be anything. Why is this? After reading this documentation I would expect the kind (? -> ? -> *).
Unboxed tuples don't exist. That is, they are in evidence, but they don't actually have any physical representation. The elements of an unboxed tuple are stored in registers when returning from a function, so that the caller can access them immediately (rather than needing to indirect through a pointer to a tuple). ISTR that because of this strategy, there are restrictions on what types of tuples can be unboxed. Theoretically GHC could also allow passing certain arguments in registers when invoking a function, but this isn't supported as yet. -- Live well, ~wren