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|
{-# LANGUAGE Trustworthy #-}
{-# LANGUAGE NoImplicitPrelude #-}
-----------------------------------------------------------------------------
-- |
-- Module : Data.Ord
-- Copyright : (c) The University of Glasgow 2005
-- License : BSD-style (see the file libraries/base/LICENSE)
--
-- Maintainer : libraries@haskell.org
-- Stability : stable
-- Portability : portable
--
-- Orderings
--
-----------------------------------------------------------------------------
module Data.Ord (
Ord(..),
Ordering(..),
Down(..),
comparing,
) where
import GHC.Base
import GHC.Show
import GHC.Read
-- |
-- > comparing p x y = compare (p x) (p y)
--
-- Useful combinator for use in conjunction with the @xxxBy@ family
-- of functions from "Data.List", for example:
--
-- > ... sortBy (comparing fst) ...
comparing :: (Ord a) => (b -> a) -> b -> b -> Ordering
comparing p x y = compare (p x) (p y)
-- | The 'Down' type allows you to reverse sort order conveniently. A value of type
-- @'Down' a@ contains a value of type @a@ (represented as @'Down' a@).
-- If @a@ has an @'Ord'@ instance associated with it then comparing two
-- values thus wrapped will give you the opposite of their normal sort order.
-- This is particularly useful when sorting in generalised list comprehensions,
-- as in: @then sortWith by 'Down' x@
--
-- Provides 'Show' and 'Read' instances (/since: 4.7.0.0/).
--
-- @since 4.6.0.0
newtype Down a = Down a deriving (Eq, Show, Read)
-- | @since 4.6.0.0
instance Ord a => Ord (Down a) where
compare (Down x) (Down y) = y `compare` x
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