# Functor-Applicative-Monad Proposal

### From HaskellWiki

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

A monad which requires custom handling for pattern match failures can implement <hask>MonadFail</hask>; otherwise, a failed pattern match will error in the same way as is does for pure code. |
A monad which requires custom handling for pattern match failures can implement <hask>MonadFail</hask>; otherwise, a failed pattern match will error in the same way as is does for pure code. |

## Revision as of 19:50, 2 January 2011

The standard class hierarchy is a consequence of Haskell's historical development, rather than logic. TheFunctor

Applicative

Monad

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 class Monad m => MonadFail m where fail :: String -> m a

fmap

liftM

map

liftA

(<*>)

ap

concat

join

MonadFail

Pointed

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. The class system extension proposal may also help.

Another variant might be to split aPointed

Applicative

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

Pointed

## See also

- A similar proposal exist on the wiki: The Other Prelude