    -----------------------------------------------------------------------------
    -- |
    -- Module      :  Control.Monad.ST
    -- 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 (requires universal quantification for runST)
    --
    -- This library provides support for /strict/ state threads, as
    -- described in the PLDI \'94 paper by John Launchbury and Simon Peyton
    -- Jones /Lazy Functional State Threads/.
    --
    -----------------------------------------------------------------------------

    module Control.Monad.ST
      (
            -- * The 'ST' Monad
            ST,             -- abstract, instance of Functor, Monad, Typeable.
            runST,          -- :: (forall s. ST s a) -> a
            fixST,          -- :: (a -> ST s a) -> ST s a

            -- * Converting 'ST' to 'IO'
            RealWorld,              -- abstract
            stToIO,                 -- :: ST RealWorld a -> IO a

            -- * Unsafe operations
            unsafeInterleaveST,     -- :: ST s a -> ST s a
            unsafeIOToST,           -- :: IO a -> ST s a
            unsafeSTToIO            -- :: ST s a -> IO a
          ) where

    #if defined(__GLASGOW_HASKELL__)
    import Control.Monad.Fix ()
    #else
    import Control.Monad.Fix
    #endif

    #include "Typeable.h"

    #if defined(__GLASGOW_HASKELL__)
    import GHC.ST           ( ST, runST, fixST, unsafeInterleaveST )
    import GHC.Base         ( RealWorld )
    import GHC.IO           ( stToIO, unsafeIOToST, unsafeSTToIO )
    #elif defined(__HUGS__)
    import Data.Typeable
    import Hugs.ST
    import qualified Hugs.LazyST as LazyST
    #endif

    #if defined(__HUGS__)
    INSTANCE_TYPEABLE2(ST,sTTc,"ST")
    INSTANCE_TYPEABLE0(RealWorld,realWorldTc,"RealWorld")

    fixST :: (a -> ST s a) -> ST s a
    fixST f = LazyST.lazyToStrictST (LazyST.fixST (LazyST.strictToLazyST . f))

    unsafeInterleaveST :: ST s a -> ST s a
    unsafeInterleaveST =
        LazyST.lazyToStrictST . LazyST.unsafeInterleaveST . LazyST.strictToLazyST
    #endif

    #if !defined(__GLASGOW_HASKELL__)
    instance MonadFix (ST s) where
            mfix = fixST
    #endif