#4481: Functor, Pointed, Applicative, Monad
---------------------------------+------------------------------------------
Reporter: gidyn | Owner:
Type: proposal | Status: new
Priority: normal | Component: libraries/base
Version: 6.12.3 | Keywords:
Testcase: | Blockedby:
Os: Unknown/Multiple | Blocking:
Architecture: Unknown/Multiple | Failure: None/Unknown
---------------------------------+------------------------------------------
As pointed out on [http://stackoverflow.com/questions/1634911/can-liftm-
differ-from-lifta 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.
--
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/4481>
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