    {-# OPTIONS_GHC -XNoImplicitPrelude #-}
    -----------------------------------------------------------------------------
    -- |
    -- Module      :  Data.IORef
    -- Copyright   :  (c) The University of Glasgow 2001
    -- License     :  BSD-style (see the file libraries/base/LICENSE)
    --
    -- Maintainer  :  libraries@haskell.org
    -- Stability   :  experimental
    -- Portability :  portable
    --
    -- Mutable references in the IO monad.
    --
    -----------------------------------------------------------------------------

    module Data.IORef
      (
            -- * IORefs
            IORef,                -- abstract, instance of: Eq, Typeable
            newIORef,             -- :: a -> IO (IORef a)
            readIORef,            -- :: IORef a -> IO a
            writeIORef,           -- :: IORef a -> a -> IO ()
            modifyIORef,          -- :: IORef a -> (a -> a) -> IO ()
            atomicModifyIORef,    -- :: IORef a -> (a -> (a,b)) -> IO b

    #if !defined(__PARALLEL_HASKELL__) && defined(__GLASGOW_HASKELL__)
            mkWeakIORef,          -- :: IORef a -> IO () -> IO (Weak (IORef a))
    #endif
            ) where

    #ifdef __HUGS__
    import Hugs.IORef
    #endif

    #ifdef __GLASGOW_HASKELL__
    import GHC.Base
    import GHC.STRef
    import GHC.IOBase
    #if !defined(__PARALLEL_HASKELL__)
    import GHC.Weak
    #endif
    #endif /* __GLASGOW_HASKELL__ */

    #ifdef __NHC__
    import NHC.IOExtras
        ( IORef
        , newIORef
        , readIORef
        , writeIORef
        , excludeFinalisers
        )
    #endif

    #if defined(__GLASGOW_HASKELL__) && !defined(__PARALLEL_HASKELL__)
    -- |Make a 'Weak' pointer to an 'IORef'
    mkWeakIORef :: IORef a -> IO () -> IO (Weak (IORef a))
    mkWeakIORef r@(IORef (STRef r#)) f = IO $ \s ->
      case mkWeak# r# r f s of (# s1, w #) -> (# s1, Weak w #)
    #endif

    -- |Mutate the contents of an 'IORef'
    modifyIORef :: IORef a -> (a -> a) -> IO ()
    modifyIORef ref f = readIORef ref >>= writeIORef ref . f

    -- |Atomically modifies the contents of an 'IORef'.
    --
    -- This function is useful for using 'IORef' in a safe way in a multithreaded
    -- program.  If you only have one 'IORef', then using 'atomicModifyIORef' to
    -- access and modify it will prevent race conditions.
    --
    -- Extending the atomicity to multiple 'IORef's is problematic, so it
    -- is recommended that if you need to do anything more complicated
    -- then using 'Control.Concurrent.MVar.MVar' instead is a good idea.
    --
    atomicModifyIORef :: IORef a -> (a -> (a,b)) -> IO b
    #if defined(__GLASGOW_HASKELL__)
    atomicModifyIORef (IORef (STRef r#)) f = IO $ \s -> atomicModifyMutVar# r# f s

    #elif defined(__HUGS__)
    atomicModifyIORef = plainModifyIORef    -- Hugs has no preemption
      where plainModifyIORef r f = do
                    a <- readIORef r
                    case f a of (a',b) -> writeIORef r a' >> return b
    #elif defined(__NHC__)
    atomicModifyIORef r f =
      excludeFinalisers $ do
        a <- readIORef r
        let (a',b) = f a
        writeIORef r a'
        return b
    #endif