Data/Unique.hs

{-# LANGUAGE Trustworthy #-}
{-# 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 Data.Typeable
import Data.IORef
#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 :: 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)
 r `seq` 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.

-- | 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