Safe Haskell | Safe |
---|---|
Language | Haskell2010 |
Documentation
class Functor (f :: * -> *) where #
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.
Functor [] | Since: 2.1 |
Functor Maybe | Since: 2.1 |
Functor IO | Since: 2.1 |
Functor Min | Since: 4.9.0.0 |
Functor Max | Since: 4.9.0.0 |
Functor First | Since: 4.9.0.0 |
Functor Last | Since: 4.9.0.0 |
Functor Option | Since: 4.9.0.0 |
Functor NonEmpty | Since: 4.9.0.0 |
Functor ZipList | |
Functor Dual | Since: 4.8.0.0 |
Functor Sum | Since: 4.8.0.0 |
Functor Product | Since: 4.8.0.0 |
Functor First | |
Functor Last | |
Functor (Either a) | Since: 3.0 |
Functor ((,) a) | Since: 2.1 |
Functor (Arg a) | Since: 4.9.0.0 |
Monad m => Functor (WrappedMonad m) | Since: 2.1 |
Arrow a => Functor (WrappedArrow a b) | Since: 2.1 |
Functor (Const * m) | Since: 2.1 |
Functor f => Functor (Alt * f) | |
Functor ((->) LiftedRep LiftedRep r) | Since: 2.1 |
($>) :: Functor f => f a -> b -> f b infixl 4 #
Flipped version of <$
.
Examples
Replace the contents of a
with a constant Maybe
Int
String
:
>>>
Nothing $> "foo"
Nothing>>>
Just 90210 $> "foo"
Just "foo"
Replace the contents of an
with a constant
Either
Int
Int
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 #
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, <$>
is function
application lifted over a Functor
.
Examples
Convert from a
to a Maybe
Int
using Maybe
String
show
:
>>>
show <$> Nothing
Nothing>>>
show <$> Just 3
Just "3"
Convert from an
to an Either
Int
Int
Either
Int
String
using show
:
>>>
show <$> 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 () #
discards or ignores the result of evaluation, such
as the return value of an void
valueIO
action.
Examples
Replace the contents of a
with unit:Maybe
Int
>>>
void Nothing
Nothing>>>
void (Just 3)
Just ()
Replace the contents of an
with unit,
resulting in an Either
Int
Int
: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