    {-# LANGUAGE Trustworthy #-}
    {-# LANGUAGE MagicHash, AutoDeriveTypeable #-}

    -----------------------------------------------------------------------------
    -- |
    -- 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,
       newUnique,
       hashUnique
     ) where

    import System.IO.Unsafe (unsafePerformIO)

    import GHC.Base
    import GHC.Num
    import Data.Typeable
    import Data.IORef

    -- | 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,Typeable)

    uniqSource :: IORef Integer
    uniqSource = unsafePerformIO (newIORef 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
      r <- atomicModifyIORef' uniqSource $ \x -> let z = x+1 in (z,z)
      return (Unique r)

    -- 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.

    -- SDM (30/4/2012): changed to IORef using atomicModifyIORef.  Reasons:
    --  1. STM version could not be used inside unsafePerformIO, if it
    --     happened to be poked inside an STM transaction.
    --  2. IORef version can be used with unsafeIOToSTM inside STM,
    --     because if the transaction retries then we just get a new
    --     Unique.
    --  3. IORef version is very slightly faster.

    -- IGL (08/06/2013): changed to using atomicModifyIORef' instead.
    --  This feels a little safer, from the point of view of not leaking
    --  memory, but the resulting core is identical.

    -- | 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
    hashUnique (Unique i) = I# (hashInteger i)