On 29 June 2011 16:36, Haisheng Wu
Well, how did you know Monad is with kind :: * -> * ?
If I was looking at the docs I'd see these two pieces
class Monad m where (>>=) :: forall a b. m a -> (a -> m b) -> m b
As (m a) and (m b) are used in the method (>>=) and but only m is specified in the class declaration, it would tell me m is :: * -> *. There is an extension in GHC (probably {-# LANGUAGE KindSignatures #-} ) where you can specify kinds in the class declaration:
class Monad (m :: * -> *) where
Personally I wouldn't be upset if this was the only valid syntax - it's redundant but I find it clearer. I'm not sure what references are available. I think constructor classes were originally implemented in Gofer (a Haskell variant) and when they were in Gofer but not Haskell, people seemed to make a clear distinction between the two. Nowadays people just call them "type classes" and have got used to the kinds of the common classes.