{-# 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