Copyright | (C) 2011-2015 Edward Kmett |
---|---|

License | BSD-style (see the file LICENSE) |

Maintainer | Edward Kmett <ekmett@gmail.com> |

Stability | provisional |

Portability | portable |

Safe Haskell | Trustworthy |

Language | Haskell98 |

## Synopsis

- class Functor f => Alt f where
- (<!>) :: f a -> f a -> f a
- some :: Applicative f => f a -> f [a]
- many :: Applicative f => f a -> f [a]

- optional :: (Alt f, Applicative f) => f a -> f (Maybe a)
- module Data.Functor.Apply

# Documentation

class Functor f => Alt f where #

Laws:

<!> is associative: (a <!> b) <!> c = a <!> (b <!> c) <$> left-distributes over <!>: f <$> (a <!> b) = (f <$> a) <!> (f <$> b)

If extended to an `Alternative`

then `<!>`

should equal `<|>`

.

Ideally, an instance of `Alt`

also satisfies the "left distributon" law of
MonadPlus with respect to `<.>`

:

<.> right-distributes over <!>: (a <!> b) <.> c = (a <.> c) <!> (b <.> c)

But `Maybe`

, `IO`

,

, `Either`

a

, and `ErrorT`

e m`STM`

satisfy the alternative
"left catch" law instead:

pure a <!> b = pure a

However, this variation cannot be stated purely in terms of the dependencies of `Alt`

.

When and if MonadPlus is successfully refactored, this class should also be refactored to remove these instances.

The right distributive law should extend in the cases where the a `Bind`

or `Monad`

is
provided to yield variations of the right distributive law:

(m <!> n) >>- f = (m >>- f) <!> (m >>- f) (m <!> n) >>= f = (m >>= f) <!> (m >>= f)

(<!>) :: f a -> f a -> f a infixl 3 #

`<|>`

without a required `empty`

some :: Applicative f => f a -> f [a] #

many :: Applicative f => f a -> f [a] #

## Instances

optional :: (Alt f, Applicative f) => f a -> f (Maybe a) #

One or none.

module Data.Functor.Apply