Data/Functor.hs

    {-# LANGUAGE Trustworthy #-}
    {-# LANGUAGE NoImplicitPrelude #-}

    -----------------------------------------------------------------------------
    -- |
    -- Module      :  Data.Functor
    -- Copyright   :  (c) The University of Glasgow 2001
    -- License     :  BSD-style (see the file libraries/base/LICENSE)
    --
    -- Maintainer  :  libraries@haskell.org
    -- Stability   :  provisional
    -- Portability :  portable
    --
    -- Functors: uniform action over a parameterized type, generalizing the
    -- 'Data.List.map' function on lists.

    module Data.Functor
        (
          Functor(fmap),
          (<$),
          ($>),
          (<$>),
          void,
        ) where

    import GHC.Base ( Functor(..), flip )

    -- $setup
    -- Allow the use of Prelude in doctests.
    -- >>> import Prelude hiding ((<$>))

    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 <$> (2,2)
    -- (2,True)
    --
    (<$>) :: Functor f => (a -> b) -> f a -> f b
    (<$>) = fmap

    infixl 4 $>

    -- | Flipped version of '<$'.
    --
    -- @since 4.7.0.0
    --
    -- ==== __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) $> "foo"
    -- (1,"foo")
    --
    ($>) :: Functor f => f a -> b -> f b
    ($>) = flip (<$)

    -- | @'void' value@ discards or ignores the result of evaluation, such
    -- as the return value of an 'System.IO.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 'System.IO.IO' action:
    --
    -- >>> mapM print [1,2]
    -- 1
    -- 2
    -- [(),()]
    -- >>> void $ mapM print [1,2]
    -- 1
    -- 2
    --
    void :: Functor f => f a -> f ()
    void x = () <$ x