+{-# LANGUAGE CPP #-}
+
+#ifdef __GLASGOW_HASKELL__
+{-# LANGUAGE MagicHash, DeriveDataTypeable #-}
+#endif
+
-----------------------------------------------------------------------------
-- |
-- Module : Data.Unique
import Prelude
-import Control.Concurrent.MVar
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)
+newtype Unique = Unique Integer deriving (Eq,Ord
+#ifdef __GLASGOW_HASKELL__
+ ,Typeable
+#endif
+ )
-uniqSource :: MVar Integer
-uniqSource = unsafePerformIO (newMVar 0)
+uniqSource :: TVar Integer
+uniqSource = unsafePerformIO (newTVarIO 0)
{-# NOINLINE uniqSource #-}
-- | Creates a new object of type 'Unique'. The value returned will
-- 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)
+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'