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{-# 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/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
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