On 07/01/10 18:52, Michael Mossey wrote:
in the declaration of Functor:
class Functor f where fmap :: (a -> b) -> f a -> f b
does the compiler infer that f is a type constructor (not a type) because of the appearance of "f a" and "f b"?
What I'm thinking is that some classes are classes of types, not type constructors. Like
class CanMakeInt a where makeInt :: a -> Int
In this case a is a type, not a type constructor. But there is no difference in the form of the first line of the class declaration.
You are entirely correct! The compiler infers this from the entire class declaration (or maybe it's the entire module). The fact that f is a type-constructor, or a type, or whatever: that's its "kind", and this process the compiler does is called "kind inference". Only rarely can the compiler not figure it out, in which case (per standard) it defaults to * (the 'kind' of ordinary types). (If you use an extension you can also specify kinds explicitly, IIRC, like class Functor (f :: * -> *) where ... )