    {-# 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 (Unique -> Unique -> Bool
    forall a. (a -> a -> Bool) -> (a -> a -> Bool) -> Eq a
    /= :: Unique -> Unique -> Bool
    $c/= :: Unique -> Unique -> Bool
    == :: Unique -> Unique -> Bool
    $c== :: Unique -> Unique -> Bool
    Eq,Eq Unique
    Unique -> Unique -> Bool
    Unique -> Unique -> Ordering
    Unique -> Unique -> Unique
    forall a.
    Eq a
    -> (a -> a -> Ordering)
    -> (a -> a -> Bool)
    -> (a -> a -> Bool)
    -> (a -> a -> Bool)
    -> (a -> a -> Bool)
    -> (a -> a -> a)
    -> (a -> a -> a)
    -> Ord a
    min :: Unique -> Unique -> Unique
    $cmin :: Unique -> Unique -> Unique
    max :: Unique -> Unique -> Unique
    $cmax :: Unique -> Unique -> Unique
    >= :: Unique -> Unique -> Bool
    $c>= :: Unique -> Unique -> Bool
    > :: Unique -> Unique -> Bool
    $c> :: Unique -> Unique -> Bool
    <= :: Unique -> Unique -> Bool
    $c<= :: Unique -> Unique -> Bool
    < :: Unique -> Unique -> Bool
    $c< :: Unique -> Unique -> Bool
    compare :: Unique -> Unique -> Ordering
    $ccompare :: Unique -> Unique -> Ordering
    Ord)

    uniqSource :: IORef Integer
    uniqSource :: IORef Integer
    uniqSource = forall a. IO a -> a
    unsafePerformIO (forall a. a -> IO (IORef a)
    newIORef Integer
    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 :: IO Unique
    newUnique = do
      Integer
    r <- forall a b. IORef a -> (a -> (a, b)) -> IO b
    atomicModifyIORef' IORef Integer
    uniqSource forall a b. (a -> b) -> a -> b
    $ \Integer
    x -> let z :: Integer
    z = Integer
    xforall a. Num a => a -> a -> a
    +Integer
    1 in (Integer
    z,Integer
    z)
      forall (m :: * -> *) a. Monad m => a -> m a
    return (Integer -> Unique
    Unique Integer
    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 -> Int
    hashUnique (Unique Integer
    i) = Integer -> Int
    integerToInt Integer
    i