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Data.Functor

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

Description

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

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 (<$) :: a -> IO b -> IO a Source
Functor V1
Methods fmap :: (a -> b) -> V1 a -> V1 b Source (<$) :: a -> V1 b -> V1 a Source
Functor U1
Methods fmap :: (a -> b) -> U1 a -> U1 b Source (<$) :: a -> U1 b -> U1 a Source
Functor Par1
Methods fmap :: (a -> b) -> Par1 a -> Par1 b Source (<$) :: a -> Par1 b -> Par1 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 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 (<$) :: a -> ArgOrder b -> ArgOrder a Source
Functor Complex
Methods fmap :: (a -> b) -> Complex a -> Complex b Source (<$) :: 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 (<$) :: a -> Identity b -> Identity a Source
Functor ((->) r)
Methods fmap :: (a -> b) -> (r -> a) -> r -> b Source (<$) :: a -> (r -> b) -> r -> a Source
Functor (Either a)
Methods fmap :: (a -> b) -> Either a a -> Either a b Source (<$) :: a -> Either a b -> Either a a Source
Functor f => Functor (Rec1 f)
Methods fmap :: (a -> b) -> Rec1 f a -> Rec1 f b Source (<$) :: a -> Rec1 f b -> Rec1 f a Source
Functor (URec Char)
Methods fmap :: (a -> b) -> URec Char a -> URec Char b Source (<$) :: a -> URec Char b -> URec Char a Source
Functor (URec Double)
Methods fmap :: (a -> b) -> URec Double a -> URec Double b Source (<$) :: a -> URec Double b -> URec Double a Source
Functor (URec Float)
Methods fmap :: (a -> b) -> URec Float a -> URec Float b Source (<$) :: a -> URec Float b -> URec Float a Source
Functor (URec Int)
Methods fmap :: (a -> b) -> URec Int a -> URec Int b Source (<$) :: a -> URec Int b -> URec Int a Source
Functor (URec Word)
Methods fmap :: (a -> b) -> URec Word a -> URec Word b Source (<$) :: a -> URec Word b -> URec Word a Source
Functor (URec (Ptr ()))
Methods fmap :: (a -> b) -> URec (Ptr ()) a -> URec (Ptr ()) b Source (<$) :: a -> URec (Ptr ()) b -> URec (Ptr ()) 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 *)
Methods fmap :: (a -> b) -> Proxy * a -> Proxy * b Source (<$) :: a -> Proxy * b -> Proxy * a Source
Arrow a => Functor (ArrowMonad a)
Methods fmap :: (a -> b) -> ArrowMonad a a -> ArrowMonad a b Source (<$) :: 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 (<$) :: a -> Arg a b -> Arg a a Source
Functor (K1 i c)
Methods fmap :: (a -> b) -> K1 i c a -> K1 i c b Source (<$) :: a -> K1 i c b -> K1 i c a Source
(Functor f, Functor g) => Functor ((:+:) f g)
Methods fmap :: (a -> b) -> (f :+: g) a -> (f :+: g) b Source (<$) :: a -> (f :+: g) b -> (f :+: g) a Source
(Functor f, Functor g) => Functor ((:*:) f g)
Methods fmap :: (a -> b) -> (f :: g) a -> (f :: g) b Source (<$) :: a -> (f :: g) b -> (f :: g) a Source
(Functor f, Functor g) => Functor ((:.:) f g)
Methods fmap :: (a -> b) -> (f :.: g) a -> (f :.: g) b Source (<$) :: a -> (f :.: g) b -> (f :.: g) a Source
Functor f => Functor (Alt * f)
Methods fmap :: (a -> b) -> Alt * f a -> Alt * f b Source (<$) :: a -> Alt * f b -> Alt * f a Source
Functor (Const * m)
Methods fmap :: (a -> b) -> Const * m a -> Const * m b Source (<$) :: a -> Const * m b -> Const * m a Source
Arrow a => Functor (WrappedArrow a b)
Methods fmap :: (a -> b) -> WrappedArrow a b a -> WrappedArrow a b b Source (<$) :: a -> WrappedArrow a b b -> WrappedArrow a b a Source
Functor f => Functor (M1 i c f)
Methods fmap :: (a -> b) -> M1 i c f a -> M1 i c f b Source (<$) :: a -> M1 i c f b -> M1 i c f a Source
(Functor f, Functor g) => Functor (Product * f g)
Methods fmap :: (a -> b) -> Product * f g a -> Product * f g b Source (<$) :: a -> Product * f g b -> Product * f g a Source
(Functor f, Functor g) => Functor (Sum * f g)
Methods fmap :: (a -> b) -> Sum * f g a -> Sum * f g b Source (<$) :: a -> Sum * f g b -> Sum * f g a Source
(Functor f, Functor g) => Functor (Compose * * f g)
Methods fmap :: (a -> b) -> Compose * * f g a -> Compose * * f g b Source (<$) :: a -> Compose * * f g b -> Compose * * 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.

The name of this operator is an allusion to $. Note the similarities between their types:

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

Whereas $ is function application, <$> is function application lifted over a Functor.

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

© The University of Glasgow and others
Licensed under a BSD-style license (see top of the page).
https://downloads.haskell.org/~ghc/8.0.1/docs/html/libraries/base-4.9.0.0/Data-Functor.html