| 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.
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.
Minimal complete definition
Instances
| Functor [] | |
| Functor IO | |
| Functor Maybe | |
| Functor ReadP | |
| Functor ReadPrec | |
| Functor Last | |
| Functor First | |
| Functor STM | |
| Functor Handler | |
| Functor ZipList | |
| Functor Identity | |
| Functor ArgDescr | |
| Functor OptDescr | |
| Functor ArgOrder | |
| Functor ((->) r) | |
| Functor (Either a) | |
| Functor ((,) a) | |
| Functor (ST s) | |
| Functor (Proxy *) | |
| Arrow a => Functor (ArrowMonad a) | |
| Monad m => Functor (WrappedMonad m) | |
| Functor (Const m) | |
| Functor (ST s) | |
| Functor f => Functor (Alt * f) | |
| Arrow a => Functor (WrappedArrow a b) |
($>) :: Functor f => f a -> b -> f b infixl 4
Flipped version of <$.
Examples
Replace the contents of a with a constant Maybe IntString:
>>>Nothing $> "foo"Nothing>>>Just 90210 $> "foo"Just "foo"
Replace the contents of an with a constant
Either Int IntString, 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.
Examples
Convert from a to a Maybe Int using Maybe Stringshow:
>>>show <$> NothingNothing>>>show <$> Just 3Just "3"
Convert from an to an Either Int IntEither IntString 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 ()
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 NothingNothing>>>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