Functor-Applicative-Monad Proposal

From HaskellWiki
Revision as of 08:28, 15 December 2010 by Gidyn (talk | contribs) (fail in MonadPlus)

Jump to: navigation, search

The standard class hierarchy is a consequence of Haskell's historical development, rather than logic. The Functor, Applicative, and Monad type classes could be defined as:

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

class Functor f => Applicative f where
    return :: a -> f a
    (<*>) :: 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
    f >>= x = join $ map f x

    join :: m (m a) -> m a
    join x = x >>= id

This would eliminate the necessity of declaring a Monad instance for every Applicative, and eliminate the need for sets of duplicate functions such as [fmap, liftM, map, liftA], [(<*>), ap], and [concat, join].

fail should be removed from Monad; a failed pattern match should error in the same way as is does for pure code, while in MonadPlus, the current behaviour could be maintained with mzero. This would, however, complicate the Monad machinery.

Pointed has not been included due to controversy as to whether it should be a subclass of Functor, a superclass of Functor, independent of Functor, or perhaps it is not sufficiently useful to include at all.

Backward compatibility could be eased with a legacy module, such as:

module Legacy where

fmap :: Functor f => (a -> b) -> f a -> f b
fmap = map

liftA :: Applicative f => (a -> b) -> f a -> f b
liftA = map

liftM :: Monad m => (a -> b) -> m a -> m b
liftM = map

ap :: Monad m => m (a -> b) -> m a -> m b
ap = (<*>)

(>>) :: Monad m => m a -> m b -> m b
(>>) = (*>)

concat :: [[a]] -> [a]
concat = join

etc.

And for those who really want a list map,

listMap :: (a -> b) -> [a] -> [b]
listMap = map

Context alias would also be a great help with backwards compatibility.

Another variant might be to split a Pointed class from the Applicative class.

class Pointed f where
    return :: a -> f a

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

Such Pointed functionality by itself could be useful, for example, in a DSL in which it is only possible to embed values and not to lift functions to functions over those embedded values.