Unique.hs

-----------------------------------------------------------------------------
-- |
-- 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