Portability | portable |
---|---|

Stability | provisional |

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

Safe Haskell | Safe |

- class Functor w => Extend w where
- duplicated :: w a -> w (w a)
- extended :: (w a -> b) -> w a -> w b

# Extendable Functors

There are two ways to define an `Extend`

instance:

I. Provide definitions for `extend`

satisfying this law:

extended f . extended g = extended (f . extended g)

II. Alternately, you may choose to provide definitions for `duplicate`

satisfying this law:

duplicated . duplicated = fmap duplicated . duplicated

These are both equivalent to the statement that (->-) is associative

(f ->- g) ->- h = f ->- (g ->- h)

You may of course, choose to define both `duplicate`

*and* `extend`

.
In that case you must also satisfy these laws:

extended f = fmap f . duplicated duplicated = extended id

These are the default definitions of `extended`

and `duplicated`

.