    -----------------------------------------------------------------------------
    -- |
    -- Module      :  Data.STRef
    -- 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 (uses Control.Monad.ST)
    --
    -- Mutable references in the (strict) ST monad.
    --
    -----------------------------------------------------------------------------

    module Data.STRef (
            -- * STRefs
            STRef,          -- abstract, instance Eq
            newSTRef,       -- :: a -> ST s (STRef s a)
            readSTRef,      -- :: STRef s a -> ST s a
            writeSTRef,     -- :: STRef s a -> a -> ST s ()
            modifySTRef     -- :: STRef s a -> (a -> a) -> ST s ()
     ) where

    import Prelude

    #ifdef __GLASGOW_HASKELL__
    import GHC.ST
    import GHC.STRef
    #endif

    #ifdef __HUGS__
    import Hugs.ST
    import Data.Typeable

    #include "Typeable.h"
    INSTANCE_TYPEABLE2(STRef,stRefTc,"STRef")
    #endif

    -- |Mutate the contents of an 'STRef'
    modifySTRef :: STRef s a -> (a -> a) -> ST s ()
    modifySTRef ref f = writeSTRef ref . f =<< readSTRef ref