# Functor-Applicative-Monad Proposal

### From HaskellWiki

(Difference between revisions)

(GHC Proposal) |

## Revision as of 10:28, 3 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

## 1 GHC Proposal

A subset of this proposal has been formally proposed for GHC. The patches attached to the ticket make Applicative into a superclass of Monad, but does not deprecate any names.

## 2 See also

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