Functor hierarchy proposal
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Currently the standard libraries include a Functor class and a Monad class, and Functor is not a superclass of Monad, even though logically every monad is a functor. New classes have been added to HEAD to create a richer hierarchy of Functor classes:
class Functor f where
fmap :: (a -> b) -> f a -> f b
class Functor f => Applicative f where
pure :: a -> f a
(<*>) :: f (a -> b) -> f a -> f b
(*>) :: Applicative f => f a -> f b -> f b
fa *> fb = (fmap (const id) fa) <*> fb
I propose that "pure" be renamed "return", "*>" be renamed ">>", and that Monad be made a subclass of Applicative:
class (Applicative f) => Monad f where
(>>=) :: f a -> (a -> f b) -> f b
The downside of this is that every instance declaration of Monad would have to be accompanied by instance declarations for the superclasses, but see Class system extension proposal.