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 [] | Since: 2.1 |
Functor Maybe | Since: 2.1 |
Functor IO | Since: 2.1 |
Functor Par1 | |
Functor ReadP | Since: 2.1 |
Functor ReadPrec | Since: 2.1 |
Functor Last | |
Functor First | |
Functor Product | Since: 4.8.0.0 |
Functor Sum | Since: 4.8.0.0 |
Functor Dual | Since: 4.8.0.0 |
Functor STM | Since: 4.3.0.0 |
Functor Handler | Since: 4.6.0.0 |
Functor Identity | Since: 4.8.0.0 |
Functor ZipList | |
Functor ArgDescr | Since: 4.6.0.0 |
Functor OptDescr | Since: 4.6.0.0 |
Functor ArgOrder | Since: 4.6.0.0 |
Functor NonEmpty | Since: 4.9.0.0 |
Functor Option | Since: 4.9.0.0 |
Functor Last | Since: 4.9.0.0 |
Functor First | Since: 4.9.0.0 |
Functor Max | Since: 4.9.0.0 |
Functor Min | Since: 4.9.0.0 |
Functor Complex | |
Functor (Either a) | Since: 3.0 |
Functor (V1 *) | |
Functor (U1 *) | Since: 4.9.0.0 |
Functor ((,) a) | Since: 2.1 |
Functor (ST s) | Since: 2.1 |
Functor (Proxy *) | Since: 4.7.0.0 |
Arrow a => Functor (ArrowMonad a) | Since: 4.6.0.0 |
Monad m => Functor (WrappedMonad m) | Since: 2.1 |
Functor (ST s) | Since: 2.1 |
Functor (Arg a) | Since: 4.9.0.0 |
Functor f => Functor (Rec1 * f) | |
Functor (URec * Char) | |
Functor (URec * Double) | |
Functor (URec * Float) | |
Functor (URec * Int) | |
Functor (URec * Word) | |
Functor (URec * (Ptr ())) | |
Functor f => Functor (Alt * f) | |
Functor (Const * m) | Since: 2.1 |
Arrow a => Functor (WrappedArrow a b) | Since: 2.1 |
Functor ((->) LiftedRep LiftedRep r) | Since: 2.1 |
Functor (K1 * i c) | |
(Functor g, Functor f) => Functor ((:+:) * f g) | |
(Functor g, Functor f) => Functor ((:*:) * f g) | |
(Functor f, Functor g) => Functor (Sum * f g) | Since: 4.9.0.0 |
(Functor f, Functor g) => Functor (Product * f g) | Since: 4.9.0.0 |
Functor f => Functor (M1 * i c f) | |
(Functor g, Functor f) => Functor ((:.:) * * f g) | |
(Functor f, Functor g) => Functor (Compose * * f g) | Since: 4.9.0.0 |
(<$) :: Functor f => 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.
($>) :: Functor f => f a -> b -> f b infixl 4 Source
Flipped version of <$
.
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, <$>
is function application lifted over a Functor
.
Convert from a Maybe Int
to a Maybe String
using show
:
>>>
show <$> 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.
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/8.2.1/docs/html/libraries/base-4.10.0.0/Data-Functor.html