| 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
A type f is a Functor if it provides a function fmap which, given any types a and b,
lets you apply any function of type (a -> b) to turn an f a into an f b, preserving the
structure of f.
Examples
Expand
>>>fmap show (Just 1) -- (a -> b) -> f a -> f bJust "1" -- (Int -> String) -> Maybe Int -> Maybe String
>>>fmap show Nothing -- (a -> b) -> f a -> f bNothing -- (Int -> String) -> Maybe Int -> Maybe String
>>>fmap show [1,2,3] -- (a -> b) -> f a -> f b["1", "2", "3"] -- (Int -> String) -> [Int] -> [String]
>>>fmap show [] -- (a -> b) -> f a -> f b[] -- (Int -> String) -> [Int] -> [String]
The fmap function is also available as the infix operator <$> :
>>>fmap show (Just 1) -- (Int -> String) -> Maybe Int -> Maybe StringJust "1">>>show <$> (Just 1) -- (Int -> String) -> Maybe Int -> Maybe StringJust "1"
Documentation
class Functor f where Source #
A type f is a Functor if it provides a function fmap which, given any types a and b
lets you apply any function from (a -> b) to turn an f a into an f b, preserving the
structure of f. Furthermore f needs to adhere to the following:
Note, that the second law follows from the free theorem of the type fmap and
the first law, so you need only check that the former condition holds.
Instances
Instances details
Instance details
Defined in Control.Arrow
Methods
fmap :: (a0 -> b) -> ArrowMonad a a0 -> ArrowMonad a b Source #
(<$) :: a0 -> ArrowMonad a b -> ArrowMonad a a0 Source #
Instance details
Defined in Control.Applicative
Methods
fmap :: (a -> b) -> WrappedMonad m a -> WrappedMonad m b Source #
(<$) :: a -> WrappedMonad m b -> WrappedMonad m a Source #
Instance details
Defined in Control.Applicative
Methods
fmap :: (a0 -> b0) -> WrappedArrow a b a0 -> WrappedArrow a b b0 Source #
(<$) :: a0 -> WrappedArrow a b b0 -> WrappedArrow a b a0 Source #
($>) :: Functor f => f a -> b -> f b infixl 4 Source #
Flipped version of <$ .
Examples
Expand
Replace the contents of a Maybe Int String :
>>>Nothing $> "foo"Nothing>>>Just 90210 $> "foo"Just "foo"
Replace the contents of an 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 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, <$> is function
application lifted over a Functor .
Examples
Expand
Convert from a Maybe Int Maybe
String show :
>>>show <$> NothingNothing>>>show <$> Just 3Just "3"
Convert from an Either Int Int Either Int String using show :
>>>show <$> Left 17Left 17>>>show <$> Right 17Right "17"
Double each element of a list:
>>>(*2) <$> [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 valueIO action.
Examples
Expand
Replace the contents of a Maybe Int
>>>void NothingNothing>>>void (Just 3)Just ()
Replace the contents of 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