# Difference between revisions of "Functor-Applicative-Monad Proposal"

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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>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>(.)</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 19:48, 2 January 2011

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

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`

].

A monad which requires custom handling for pattern match failures can implement `MonadFail`

; otherwise, a failed pattern match will error in the same way as is does for pure code.

`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. The class system extension proposal may also help.

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.

## See also

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