Data.Functor

base-4.9.0.0: Basic libraries

Copyright | (c) The University of Glasgow 2001
License | BSD-style (see the file libraries/base/LICENSE)
Maintainer | libraries@haskell.org
Stability | provisional
Portability | portable
Safe Haskell | Trustworthy
Language | Haskell2010

Data.Functor

Description

Functors: uniform action over a parameterized type, generalizing the `map` function on lists.

Synopsis

- class Functor f where
- (<$) :: Functor f => a -> f b -> f a
- ($>) :: Functor f => f a -> b -> f b
- (<$>) :: Functor f => (a -> b) -> f a -> f b
- void :: Functor f => f a -> f ()

# Documentation

class Functor f where Source

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.

Minimal complete definition

fmap

Methods

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.

Instances

Functor [] |
Methods fmap :: (a -> b) -> [a] -> [b] Source (<$) :: a -> [b] -> [a] Source
Functor Maybe |
Methods fmap :: (a -> b) -> Maybe a -> Maybe b Source (<$) :: a -> Maybe b -> Maybe a Source
Functor IO |
Methods fmap :: (a -> b) -> IO a -> IO b Source (<$) :: a -> IO b -> IO a Source
Functor ReadP |
Methods fmap :: (a -> b) -> ReadP a -> ReadP b Source (<$) :: a -> ReadP b -> ReadP a Source
Functor ReadPrec |
Methods fmap :: (a -> b) -> ReadPrec a -> ReadPrec b Source (<$) :: a -> ReadPrec b -> ReadPrec a Source
Functor Last |
Methods fmap :: (a -> b) -> Last a -> Last b Source (<$) :: a -> Last b -> Last a Source
Functor First |
Methods fmap :: (a -> b) -> First a -> First b Source (<$) :: a -> First b -> First a Source
Functor Product |
Methods fmap :: (a -> b) -> Product a -> Product b Source (<$) :: a -> Product b -> Product a Source
Functor Sum |
Methods fmap :: (a -> b) -> Sum a -> Sum b Source (<$) :: a -> Sum b -> Sum a Source
Functor Dual |
Methods fmap :: (a -> b) -> Dual a -> Dual b Source (<$) :: a -> Dual b -> Dual a Source
Functor STM |
Methods fmap :: (a -> b) -> STM a -> STM b Source (<$) :: a -> STM b -> STM a Source
Functor Handler |
Methods fmap :: (a -> b) -> Handler a -> Handler b Source (<$) :: a -> Handler b -> Handler a Source
Functor ZipList |
Methods fmap :: (a -> b) -> ZipList a -> ZipList b Source (<$) :: a -> ZipList b -> ZipList a Source
Functor Complex |
Methods fmap :: (a -> b) -> Complex a -> Complex b Source (<$) :: a -> Complex b -> Complex a Source
Functor NonEmpty |
Methods fmap :: (a -> b) -> NonEmpty a -> NonEmpty b Source (<$) :: a -> NonEmpty b -> NonEmpty a Source
Functor Option |
Methods fmap :: (a -> b) -> Option a -> Option b Source (<$) :: a -> Option b -> Option a Source
Functor Last |
Methods fmap :: (a -> b) -> Last a -> Last b Source (<$) :: a -> Last b -> Last a Source
Functor First |
Methods fmap :: (a -> b) -> First a -> First b Source (<$) :: a -> First b -> First a Source
Functor Max |
Methods fmap :: (a -> b) -> Max a -> Max b Source (<$) :: a -> Max b -> Max a Source
Functor Min |
Methods fmap :: (a -> b) -> Min a -> Min b Source (<$) :: a -> Min b -> Min a Source
Functor Identity |
Methods fmap :: (a -> b) -> Identity a -> Identity b Source (<$) :: a -> Identity b -> Identity a Source
Functor ArgDescr |
Methods fmap :: (a -> b) -> ArgDescr a -> ArgDescr b Source (<$) :: a -> ArgDescr b -> ArgDescr a Source
Functor OptDescr |
Methods fmap :: (a -> b) -> OptDescr a -> OptDescr b Source (<$) :: a -> OptDescr b -> OptDescr a Source
Functor ArgOrder |
Methods fmap :: (a -> b) -> ArgOrder a -> ArgOrder b Source (<$) :: a -> ArgOrder b -> ArgOrder a Source
Functor ((->) r) |
Methods fmap :: (a -> b) -> (r -> a) -> r -> b Source (<$) :: a -> (r -> b) -> r -> a Source
Functor (Either a) |
Methods fmap :: (a -> b) -> Either a a -> Either a b Source (<$) :: a -> Either a b -> Either a a Source
Functor ((,) a) |
Methods fmap :: (a -> b) -> (a, a) -> (a, b) Source (<$) :: a -> (a, b) -> (a, a) Source
Functor (ST s) |
Methods fmap :: (a -> b) -> ST s a -> ST s b Source (<$) :: a -> ST s b -> ST s a Source
Functor (Proxy (TYPE Lifted)) |
Methods fmap :: (a -> b) -> Proxy (TYPE Lifted) a -> Proxy (TYPE Lifted) b Source (<$) :: a -> Proxy (TYPE Lifted) b -> Proxy (TYPE Lifted) a Source
Arrow a => Functor (ArrowMonad a) |
Methods fmap :: (a -> b) -> ArrowMonad a a -> ArrowMonad a b Source (<$) :: a -> ArrowMonad a b -> ArrowMonad a a Source
Monad m => Functor (WrappedMonad m) |
Methods fmap :: (a -> b) -> WrappedMonad m a -> WrappedMonad m b Source (<$) :: a -> WrappedMonad m b -> WrappedMonad m a Source
Functor (ST s) |
Methods fmap :: (a -> b) -> ST s a -> ST s b Source (<$) :: a -> ST s b -> ST s a Source
Functor (Arg a) |
Methods fmap :: (a -> b) -> Arg a a -> Arg a b Source (<$) :: a -> Arg a b -> Arg a a Source
Functor f => Functor (Alt (TYPE Lifted) f) |
Methods fmap :: (a -> b) -> Alt (TYPE Lifted) f a -> Alt (TYPE Lifted) f b Source (<$) :: a -> Alt (TYPE Lifted) f b -> Alt (TYPE Lifted) f a Source
Functor (Const (TYPE Lifted) m) |
Methods fmap :: (a -> b) -> Const (TYPE Lifted) m a -> Const (TYPE Lifted) m b Source (<$) :: a -> Const (TYPE Lifted) m b -> Const (TYPE Lifted) m a Source
Arrow a => Functor (WrappedArrow a b) |
Methods fmap :: (a -> b) -> WrappedArrow a b a -> WrappedArrow a b b Source (<$) :: a -> WrappedArrow a b b -> WrappedArrow a b a Source
(Functor f, Functor g) => Functor (Product (TYPE Lifted) f g) |
Methods fmap :: (a -> b) -> Product (TYPE Lifted) f g a -> Product (TYPE Lifted) f g b Source (<$) :: a -> Product (TYPE Lifted) f g b -> Product (TYPE Lifted) f g a Source
(Functor f, Functor g) => Functor (Sum (TYPE Lifted) f g) |
Methods fmap :: (a -> b) -> Sum (TYPE Lifted) f g a -> Sum (TYPE Lifted) f g b Source (<$) :: a -> Sum (TYPE Lifted) f g b -> Sum (TYPE Lifted) f g a Source
(Functor f, Functor g) => Functor (Compose (TYPE Lifted) (TYPE Lifted) f g) |
Methods fmap :: (a -> b) -> Compose (TYPE Lifted) (TYPE Lifted) f g a -> Compose (TYPE Lifted) (TYPE Lifted) f g b Source (<$) :: a -> Compose (TYPE Lifted) (TYPE Lifted) f g b -> Compose (TYPE Lifted) (TYPE Lifted) f g a Source

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

Flipped version of `<$`.

#### Examples

Replace the contents of a ``Maybe` `Int`` with a constant `String`:

    >>> Nothing $> "foo"
    Nothing
    >>> Just 90210 $> "foo"
    Just "foo"

Replace the contents of an ``Either` `Int` `Int`` with a constant `String`, resulting in an ``Either` `Int` `String``:

    >>> Left 8675309 $> "foo"
    Left 8675309
    >>> Right 8675309 $> "foo"
    Right "foo"

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

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

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

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

Since: 4.7.0.0

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

An infix synonym for `fmap`.

#### Examples

Convert from a ``Maybe` `Int`` to a ``Maybe` `String`` using `show`:

    >>> show <$> Nothing
    Nothing
    >>> show <$> Just 3
    Just "3"

Convert from an ``Either` `Int` `Int`` to an ``Either` `Int`` `String` using `show`:

    >>> show <$> Left 17
    Left 17
    >>> show <$> Right 17
    Right "17"

Double each element of a list:

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

Apply `even` to the second element of a pair:

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

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.

#### Examples

Replace the contents of a ``Maybe` `Int`` with unit:

    >>> void Nothing
    Nothing
    >>> void (Just 3)
    Just ()

Replace the contents of an ``Either` `Int` `Int`` with unit, resulting in an ``Either` `Int` '()'`:

    >>> void (Left 8675309)
    Left 8675309
    >>> void (Right 8675309)
    Right ()

Replace every element of a list with unit:

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

Replace the second element of a pair with unit:

    >>> void (1,2)
    (1,())

Discard the result of an `IO` action:

    >>> mapM print [1,2]
    1
    2
    [(),()]
    >>> void $ mapM print [1,2]
    1
    2