Data/Tuple.hs

    {-# LANGUAGE Trustworthy #-}
    {-# LANGUAGE NoImplicitPrelude #-}
    {-# OPTIONS_GHC -fno-warn-unused-imports #-}
    -- XXX -fno-warn-unused-imports needed for the GHC.Tuple import below. Sigh.

    -----------------------------------------------------------------------------
    -- |
    -- Module      :  Data.Tuple
    -- Copyright   :  (c) The University of Glasgow 2001
    -- License     :  BSD-style (see the file libraries/base/LICENSE)
    --
    -- Maintainer  :  libraries@haskell.org
    -- Stability   :  experimental
    -- Portability :  portable
    --
    -- The tuple data types, and associated functions.
    --
    -----------------------------------------------------------------------------

    module Data.Tuple
      ( fst
      , snd
      , curry
      , uncurry
      , swap
      )
        where

    import GHC.Base
    -- We need to depend on GHC.Base so that
    -- a) so that we get GHC.Classes, GHC.Types

    -- b) so that GHC.Base.inline is available, which is used
    --    when expanding instance declarations

    import GHC.Tuple
    -- We must import GHC.Tuple, to ensure sure that the
    -- data constructors of `(,)' are in scope when we do
    -- the standalone deriving instance for Eq (a,b) etc

    default ()              -- Double isn't available yet

    -- ---------------------------------------------------------------------------
    -- Standard functions over tuples

    -- | Extract the first component of a pair.
    fst                     :: (a,b) -> a
    fst (x,_)               =  x

    -- | Extract the second component of a pair.
    snd                     :: (a,b) -> b
    snd (_,y)               =  y

    -- | 'curry' converts an uncurried function to a curried function.
    curry                   :: ((a, b) -> c) -> a -> b -> c
    curry f x y             =  f (x, y)

    -- | 'uncurry' converts a curried function to a function on pairs.
    uncurry                 :: (a -> b -> c) -> ((a, b) -> c)
    uncurry f p             =  f (fst p) (snd p)

    -- | Swap the components of a pair.
    swap                    :: (a,b) -> (b,a)
    swap (a,b)              = (b,a)