Copyright | (c) The University of Glasgow 2001 |
---|---|

License | BSD-style (see the file libraries/base/LICENSE) |

Maintainer | [email protected] |

Stability | provisional |

Portability | portable |

Safe Haskell | Trustworthy |

Language | Haskell2010 |

Functors: uniform action over a parameterized type, generalizing the `map`

function on lists.

The `Functor`

class is used for types that can be mapped over. Instances of `Functor`

should satisfy the following laws:

fmap id == id fmap (f . g) == fmap f . fmap g

The instances of `Functor`

for lists, `Maybe`

and `IO`

satisfy these laws.

fmap :: (a -> b) -> f a -> f b Source

(<$) :: a -> f b -> f a infixl 4 Source

Replace all locations in the input with the same value. The default definition is `fmap . const`

, but this may be overridden with a more efficient version.

Functor [] | |

Functor IO | |

Functor Maybe | |

Functor ReadP | |

Functor ReadPrec | |

Functor Last | |

Functor First | |

Functor STM | |

Functor Handler | |

Functor ZipList | |

Functor Identity | |

Functor ArgDescr | |

Functor OptDescr | |

Functor ArgOrder | |

Functor ((->) r) | |

Functor (Either a) | |

Functor ((,) a) | |

Functor (ST s) | |

Functor (Proxy *) | |

Arrow a => Functor (ArrowMonad a) | |

Monad m => Functor (WrappedMonad m) | |

Functor (Const m) | |

Functor (ST s) | |

Functor f => Functor (Alt * f) | |

Arrow a => Functor (WrappedArrow a b) |

($>) :: Functor f => f a -> b -> f b infixl 4 Source

Flipped version of `<$`

.

Replace the contents of a `Maybe Int`

with a constant `String`

:

`>>>`

Nothing`Nothing $> "foo"`

`>>>`

Just "foo"`Just 90210 $> "foo"`

Replace the contents of an `Either Int Int`

with a constant `String`

, resulting in an `Either Int String`

:

`>>>`

Left 8675309`Left 8675309 $> "foo"`

`>>>`

Right "foo"`Right 8675309 $> "foo"`

Replace each element of a list with a constant `String`

:

`>>>`

["foo","foo","foo"]`[1,2,3] $> "foo"`

Replace the second element of a pair with a constant `String`

:

`>>>`

(1,"foo")`(1,2) $> "foo"`

Since: 4.7.0.0

(<$>) :: Functor f => (a -> b) -> f a -> f b infixl 4 Source

An infix synonym for `fmap`

.

Convert from a `Maybe Int`

to a `Maybe String`

using `show`

:

`>>>`

Nothing`show <$> Nothing`

`>>>`

Just "3"`show <$> Just 3`

Convert from an `Either Int Int`

to an `Either Int`

`String`

using `show`

:

`>>>`

Left 17`show <$> Left 17`

`>>>`

Right "17"`show <$> Right 17`

Double each element of a list:

`>>>`

[2,4,6]`(*2) <$> [1,2,3]`

Apply `even`

to the second element of a pair:

`>>>`

(2,True)`even <$> (2,2)`

void :: Functor f => f a -> f () Source

`void value`

discards or ignores the result of evaluation, such as the return value of an `IO`

action.

Replace the contents of a `Maybe Int`

with unit:

`>>>`

Nothing`void Nothing`

`>>>`

Just ()`void (Just 3)`

Replace the contents of an `Either Int Int`

with unit, resulting in an `Either Int '()'`

:

`>>>`

Left 8675309`void (Left 8675309)`

`>>>`

Right ()`void (Right 8675309)`

Replace every element of a list with unit:

`>>>`

[(),(),()]`void [1,2,3]`

Replace the second element of a pair with unit:

`>>>`

(1,())`void (1,2)`

Discard the result of an `IO`

action:

`>>>`

1 2 [(),()]`mapM print [1,2]`

`>>>`

1 2`void $ mapM print [1,2]`

© The University of Glasgow and others

Licensed under a BSD-style license (see top of the page).

https://downloads.haskell.org/~ghc/7.10.3/docs/html/libraries/base-4.8.2.0/Data-Functor.html