Functor

protolude-0.1.7: A sensible set of defaults for writing custom Preludes.

Safe Haskell | Safe
Language | Haskell2010

Functor

Synopsis

- class Functor f where
  - fmap :: (a -> b) -> f a -> f b
  - (<$) :: a -> f b -> f a
- ($>) :: Functor f => f a -> b -> f b
- (<$>) :: Functor f => (a -> b) -> f a -> f b
- void :: Functor f => f a -> f ()

# 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

fmap

Instances

Functor [] |
Methods fmap :: (a -> b) -> [a] -> [b] # (<$) :: a -> [b] -> [a] #
Functor Maybe |
Methods fmap :: (a -> b) -> Maybe a -> Maybe b # (<$) :: a -> Maybe b -> Maybe a #
Functor IO |
Methods fmap :: (a -> b) -> IO a -> IO b # (<$) :: a -> IO b -> IO a #
Functor V1 |
Methods fmap :: (a -> b) -> V1 a -> V1 b # (<$) :: a -> V1 b -> V1 a #
Functor U1 |
Methods fmap :: (a -> b) -> U1 a -> U1 b # (<$) :: a -> U1 b -> U1 a #
Functor Par1 |
Methods fmap :: (a -> b) -> Par1 a -> Par1 b # (<$) :: a -> Par1 b -> Par1 a #
Functor Async |
Methods fmap :: (a -> b) -> Async a -> Async b # (<$) :: a -> Async b -> Async a #
Functor Concurrently |
Methods fmap :: (a -> b) -> Concurrently a -> Concurrently b # (<$) :: a -> Concurrently b -> Concurrently a #
Functor Id |
Methods fmap :: (a -> b) -> Id a -> Id b # (<$) :: a -> Id b -> Id a #
Functor Identity |
Methods fmap :: (a -> b) -> Identity a -> Identity b # (<$) :: a -> Identity b -> Identity a #
Functor Min |
Methods fmap :: (a -> b) -> Min a -> Min b # (<$) :: a -> Min b -> Min a #
Functor Max |
Methods fmap :: (a -> b) -> Max a -> Max b # (<$) :: a -> Max b -> Max a #
Functor First |
Methods fmap :: (a -> b) -> First a -> First b # (<$) :: a -> First b -> First a #
Functor Last |
Methods fmap :: (a -> b) -> Last a -> Last b # (<$) :: a -> Last b -> Last a #
Functor Option |
Methods fmap :: (a -> b) -> Option a -> Option b # (<$) :: a -> Option b -> Option a #
Functor NonEmpty |
Methods fmap :: (a -> b) -> NonEmpty a -> NonEmpty b # (<$) :: a -> NonEmpty b -> NonEmpty a #
Functor Complex |
Methods fmap :: (a -> b) -> Complex a -> Complex b # (<$) :: a -> Complex b -> Complex a #
Functor ZipList |
Methods fmap :: (a -> b) -> ZipList a -> ZipList b # (<$) :: a -> ZipList b -> ZipList a #
Functor Handler |
Methods fmap :: (a -> b) -> Handler a -> Handler b # (<$) :: a -> Handler b -> Handler a #
Functor STM |
Methods fmap :: (a -> b) -> STM a -> STM b # (<$) :: a -> STM b -> STM a #
Functor Dual |
Methods fmap :: (a -> b) -> Dual a -> Dual b # (<$) :: a -> Dual b -> Dual a #
Functor Sum |
Methods fmap :: (a -> b) -> Sum a -> Sum b # (<$) :: a -> Sum b -> Sum a #
Functor Product |
Methods fmap :: (a -> b) -> Product a -> Product b # (<$) :: a -> Product b -> Product a #
Functor First |
Methods fmap :: (a -> b) -> First a -> First b # (<$) :: a -> First b -> First a #
Functor Last |
Methods fmap :: (a -> b) -> Last a -> Last b # (<$) :: a -> Last b -> Last a #
Functor Put |
Methods fmap :: (a -> b) -> Put a -> Put b # (<$) :: a -> Put b -> Put a #
Functor Digit |
Methods fmap :: (a -> b) -> Digit a -> Digit b # (<$) :: a -> Digit b -> Digit a #
Functor Node |
Methods fmap :: (a -> b) -> Node a -> Node b # (<$) :: a -> Node b -> Node a #
Functor Elem |
Methods fmap :: (a -> b) -> Elem a -> Elem b # (<$) :: a -> Elem b -> Elem a #
Functor FingerTree |
Methods fmap :: (a -> b) -> FingerTree a -> FingerTree b # (<$) :: a -> FingerTree b -> FingerTree a #
Functor Seq |
Methods fmap :: (a -> b) -> Seq a -> Seq b # (<$) :: a -> Seq b -> Seq a #
Functor ViewL |
Methods fmap :: (a -> b) -> ViewL a -> ViewL b # (<$) :: a -> ViewL b -> ViewL a #
Functor ViewR |
Methods fmap :: (a -> b) -> ViewR a -> ViewR b # (<$) :: a -> ViewR b -> ViewR a #
Functor IntMap |
Methods fmap :: (a -> b) -> IntMap a -> IntMap b # (<$) :: a -> IntMap b -> IntMap a #
Functor ((->) r) |
Methods fmap :: (a -> b) -> (r -> a) -> r -> b # (<$) :: a -> (r -> b) -> r -> a #
Functor (Either a) |
Methods fmap :: (a -> b) -> Either a a -> Either a b # (<$) :: a -> Either a b -> Either a a #
Functor f => Functor (Rec1 f) |
Methods fmap :: (a -> b) -> Rec1 f a -> Rec1 f b # (<$) :: a -> Rec1 f b -> Rec1 f a #
Functor (URec Char) |
Methods fmap :: (a -> b) -> URec Char a -> URec Char b # (<$) :: a -> URec Char b -> URec Char a #
Functor (URec Double) |
Methods fmap :: (a -> b) -> URec Double a -> URec Double b # (<$) :: a -> URec Double b -> URec Double a #
Functor (URec Float) |
Methods fmap :: (a -> b) -> URec Float a -> URec Float b # (<$) :: a -> URec Float b -> URec Float a #
Functor (URec Int) |
Methods fmap :: (a -> b) -> URec Int a -> URec Int b # (<$) :: a -> URec Int b -> URec Int a #
Functor (URec Word) |
Methods fmap :: (a -> b) -> URec Word a -> URec Word b # (<$) :: a -> URec Word b -> URec Word a #
Functor (URec (Ptr ())) |
Methods fmap :: (a -> b) -> URec (Ptr ()) a -> URec (Ptr ()) b # (<$) :: a -> URec (Ptr ()) b -> URec (Ptr ()) a #
Functor ((,) a) |
Methods fmap :: (a -> b) -> (a, a) -> (a, b) # (<$) :: a -> (a, b) -> (a, a) #
Functor (Array i) |
Methods fmap :: (a -> b) -> Array i a -> Array i b # (<$) :: a -> Array i b -> Array i a #
Functor (StateL s) |
Methods fmap :: (a -> b) -> StateL s a -> StateL s b # (<$) :: a -> StateL s b -> StateL s a #
Functor (StateR s) |
Methods fmap :: (a -> b) -> StateR s a -> StateR s b # (<$) :: a -> StateR s b -> StateR s a #
Functor (Arg a) |
Methods fmap :: (a -> b) -> Arg a a -> Arg a b # (<$) :: a -> Arg a b -> Arg a a #
Monad m => Functor (WrappedMonad m) |
Methods fmap :: (a -> b) -> WrappedMonad m a -> WrappedMonad m b # (<$) :: a -> WrappedMonad m b -> WrappedMonad m a #
Arrow a => Functor (ArrowMonad a) |
Methods fmap :: (a -> b) -> ArrowMonad a a -> ArrowMonad a b # (<$) :: a -> ArrowMonad a b -> ArrowMonad a a #
Functor (Proxy *) |
Methods fmap :: (a -> b) -> Proxy * a -> Proxy * b # (<$) :: a -> Proxy * b -> Proxy * a #
Functor (State s) |
Methods fmap :: (a -> b) -> State s a -> State s b # (<$) :: a -> State s b -> State s a #
Functor (Map k) |
Methods fmap :: (a -> b) -> Map k a -> Map k b # (<$) :: a -> Map k b -> Map k a #
Functor (K1 i c) |
Methods fmap :: (a -> b) -> K1 i c a -> K1 i c b # (<$) :: a -> K1 i c b -> K1 i c a #
(Functor f, Functor g) => Functor ((:+:) f g) |
Methods fmap :: (a -> b) -> (f :+: g) a -> (f :+: g) b # (<$) :: a -> (f :+: g) b -> (f :+: g) a #
(Functor f, Functor g) => Functor ((:*:) f g) |
Methods fmap :: (a -> b) -> (f :*: g) a -> (f :*: g) b # (<$) :: a -> (f :*: g) b -> (f :*: g) a #
(Functor f, Functor g) => Functor ((:.:) f g) |
Methods fmap :: (a -> b) -> (f :.: g) a -> (f :.: g) b # (<$) :: a -> (f :.: g) b -> (f :.: g) a #
Arrow a => Functor (WrappedArrow a b) |
Methods fmap :: (a -> b) -> WrappedArrow a b a -> WrappedArrow a b b # (<$) :: a -> WrappedArrow a b b -> WrappedArrow a b a #
Functor (Const * m) |
Methods fmap :: (a -> b) -> Const * m a -> Const * m b # (<$) :: a -> Const * m b -> Const * m a #
Functor f => Functor (Alt * f) |
Methods fmap :: (a -> b) -> Alt * f a -> Alt * f b # (<$) :: a -> Alt * f b -> Alt * f a #
Functor m => Functor (ErrorT e m) |
Methods fmap :: (a -> b) -> ErrorT e m a -> ErrorT e m b # (<$) :: a -> ErrorT e m b -> ErrorT e m a #
Functor f => Functor (M1 i c f) |
Methods fmap :: (a -> b) -> M1 i c f a -> M1 i c f b # (<$) :: a -> M1 i c f b -> M1 i c f a #

($>) :: Functor f => f a -> b -> f b infixl 4 #

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 #

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 () #

``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