+{-# LANGUAGE CPP #-}
+
+#ifdef __GLASGOW_HASKELL__
+{-# LANGUAGE MagicHash, DeriveDataTypeable #-}
+#endif
+
-----------------------------------------------------------------------------
---
+-- |
-- Module : Data.Unique
-- Copyright : (c) The University of Glasgow 2001
--- License : BSD-style (see the file libraries/core/LICENSE)
+-- License : BSD-style (see the file libraries/base/LICENSE)
--
-- Maintainer : libraries@haskell.org
-- Stability : experimental
-- Portability : non-portable
--
--- $Id: Unique.hs,v 1.1 2001/07/04 10:48:39 simonmar Exp $
---
--- An infinite supply of unique objects, supporting ordering and equality.
+-- An abstract interface to a unique symbol generator.
--
-----------------------------------------------------------------------------
module Data.Unique (
- Unique, -- instance (Eq, Ord)
- newUnique, -- :: IO Unique
- hashUnique -- :: Unique -> Int
+ -- * Unique objects
+ Unique, -- instance (Eq, Ord)
+ newUnique, -- :: IO Unique
+ hashUnique -- :: Unique -> Int
) where
import Prelude
-import Control.Concurrent
import System.IO.Unsafe (unsafePerformIO)
#ifdef __GLASGOW_HASKELL__
import GHC.Base
-import GHC.Num ( Integer(..) )
+import GHC.Num
+import GHC.Conc
+import Data.Typeable
#endif
-newtype Unique = Unique Integer deriving (Eq,Ord)
+-- | 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 :: 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
+-- 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)
+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
-#ifdef __GLASGOW_HASKELL__
-hashUnique (Unique (S# i)) = I# i
-hashUnique (Unique (J# s d)) | s ==# 0# = 0
- | otherwise = I# (indexIntArray# d 0#)
+#if defined(__GLASGOW_HASKELL__)
+hashUnique (Unique i) = I# (hashInteger i)
#else
-hashUnique (Unique u) = u `mod` (fromIntegral (maxBound :: Int) + 1)
+hashUnique (Unique u) = fromInteger (u `mod` (toInteger (maxBound :: Int) + 1))
#endif