Haskell Code by HsColour

-----------------------------------------------------------------------------
-- |
-- Module : Data.Unique
-- Copyright : (c) The University of Glasgow 2001
-- License : BSD-style (see the file libraries/base/LICENSE)
-- 
-- Maintainer : libraries@haskell.org
-- Stability : experimental
-- Portability : non-portable
--
-- An abstract interface to a unique symbol generator.
--
-----------------------------------------------------------------------------

module Data.Unique (
 -- * Unique objects
 Unique, -- instance (Eq, Ord)
 newUnique, -- :: IO Unique
 hashUnique -- :: Unique -> Int
 ) where

import Prelude

import Control.Concurrent.MVar
import System.IO.Unsafe (unsafePerformIO)

#ifdef __GLASGOW_HASKELL__
import GHC.Base
import GHC.Num
#endif

-- | An abstract unique object. Objects of type 'Unique' may be
-- compared for equality and ordering and hashed into 'Int'.
newtype Unique = Unique Integer deriving (Eq,Ord)

uniqSource :: MVar Integer
uniqSource = unsafePerformIO (newMVar 0)
{-# NOINLINE uniqSource #-}

-- | Creates a new object of type 'Unique'. The value returned will
-- not compare equal to any other value of type 'Unique' returned by
-- previous calls to 'newUnique'. There is no limit on the number of
-- times 'newUnique' may be called.
newUnique :: IO Unique
newUnique = do
 val <- takeMVar uniqSource
 let next = val+1
 putMVar uniqSource next
 return (Unique next)

-- | Hashes a 'Unique' into an 'Int'. Two 'Unique's may hash to the
-- same value, although in practice this is unlikely. The 'Int'
-- returned makes a good hash key.
hashUnique :: Unique -> Int
#if defined(__GLASGOW_HASKELL__)
hashUnique (Unique i) = I# (hashInteger i)
#else
hashUnique (Unique u) = fromInteger (u `mod` (toInteger (maxBound :: Int) + 1))
#endif