    -----------------------------------------------------------------------------
    -- |
    -- 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 System.IO.Unsafe (unsafePerformIO)

    #ifdef __GLASGOW_HASKELL__
    import GHC.Base
    import GHC.Num
    import GHC.Conc
    import Data.Typeable
    #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
    #ifdef __GLASGOW_HASKELL__
       ,Typeable
    #endif
       )

    uniqSource :: TVar Integer
    uniqSource = unsafePerformIO (newTVarIO 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 = atomically $ do
      val <- readTVar uniqSource
      let next = val+1
      writeTVar uniqSource $! next
      return (Unique next)

    -- SDM (18/3/2010): changed from MVar to STM.  This fixes
    --  1. there was no async exception protection
    --  2. there was a space leak (now new value is strict)
    --  3. using atomicModifyIORef would be slightly quicker, but can
    --     suffer from adverse scheduling issues (see #3838)
    --  4. also, the STM version is faster.

    -- | 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