Functor, Pointed, Applicative, Monad

As pointed out on StackOverflow, Functor, Pointed, Applicative and Monad all belong in a hierarchy thus:

class Functor f where
    map :: (a -> b) -> f a -> f b

class Functor f => Pointed f where
    pure :: a -> f a

class Pointed f => Applicative f where
    (<*>) :: f (a -> b) -> f a -> f b
    (*>) :: f a -> f b -> f b
    (<*) :: f a -> f b -> f a

class Applicative m => Monad m where
    (>>=) :: (m a) -> (a -> m b) -> m b
    (>>) :: m a -> m b -> m b
    return :: a -> m a

As well as eliminating much duplication (where every Applicative must also be given a Monad instance), this will make it easier to eliminate such duplication as [fmap,liftM,map,liftA], [(<*>),ap], and [concat,join].

fail does not belong in Monad; it only makes sense monadically as mzero.

Trac metadata

Trac field	Value
Version	6.12.3
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	libraries/base
Test case
Differential revisions
BlockedBy
Related
Blocking
CC	gideon@accursoft.com
Operating system
Architecture