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[ghc-base.git] / Data / Unique.hs

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+-----------------------------------------------------------------------------
+-- |
+-- 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 Control.Concurrent.MVar
+import System.IO.Unsafe (unsafePerformIO)
+
+#ifdef __GLASGOW_HASKELL__
+import GHC.Base
+import GHC.Num
+#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)
+
+uniqSource :: MVar Integer
+uniqSource = unsafePerformIO (newMVar 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)
+
+-- | 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