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|
{-# LANGUAGE Trustworthy #-}
-----------------------------------------------------------------------------
-- |
-- 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,
newUnique,
hashUnique
) where
import System.IO.Unsafe (unsafePerformIO)
import GHC.Num
import Data.IORef
-- $setup
-- >>> import Prelude
-- | An abstract unique object. Objects of type 'Unique' may be
-- compared for equality and ordering and hashed into 'Int'.
--
-- >>> :{
-- do x <- newUnique
-- print (x == x)
-- y <- newUnique
-- print (x == y)
-- :}
-- True
-- False
newtype Unique = Unique Integer deriving (Eq,Ord)
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)
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.
-- IGL (08/06/2013): changed to using atomicModifyIORef' instead.
-- This feels a little safer, from the point of view of not leaking
-- memory, but the resulting core is identical.
-- | 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
hashUnique (Unique i) = integerToInt i
|
