    {-# OPTIONS_GHC -fno-warn-orphans #-}
    {-# LANGUAGE CPP, MagicHash, UnboxedTuples #-}

    #if __GLASGOW_HASKELL__ >= 701
    {-# LANGUAGE Trustworthy #-}
    #endif

    -----------------------------------------------------------------------------
    -- |
    -- Module      :  Control.Monad.STM
    -- Copyright   :  (c) The University of Glasgow 2004
    -- License     :  BSD-style (see the file libraries/base/LICENSE)
    --
    -- Maintainer  :  libraries@haskell.org
    -- Stability   :  experimental
    -- Portability :  non-portable (requires STM)
    --
    -- Software Transactional Memory: a modular composable concurrency
    -- abstraction.  See
    --
    --  * /Composable memory transactions/, by Tim Harris, Simon Marlow, Simon
    --    Peyton Jones, and Maurice Herlihy, in /ACM Conference on Principles
    --    and Practice of Parallel Programming/ 2005.
    --    <http://research.microsoft.com/Users/simonpj/papers/stm/index.htm>
    --
    -- This module only defines the 'STM' monad; you probably want to
    -- import "Control.Concurrent.STM" (which exports "Control.Monad.STM").
    -----------------------------------------------------------------------------

    module Control.Monad.STM (
      	STM,
    	atomically,
    #ifdef __GLASGOW_HASKELL__
            always,
            alwaysSucceeds,
    	retry,
    	orElse,
    	check,
    #endif
            throwSTM,
            catchSTM
      ) where

    #ifdef __GLASGOW_HASKELL__
    #if ! (MIN_VERSION_base(4,3,0))
    import GHC.Conc hiding (catchSTM)
    import Control.Monad    ( MonadPlus(..) )
    import Control.Exception
    #else
    import GHC.Conc
    #endif
    import GHC.Exts
    import Control.Monad.Fix
    #else
    import Control.Sequential.STM
    #endif

    #ifdef __GLASGOW_HASKELL__
    #if ! (MIN_VERSION_base(4,3,0))
    import Control.Applicative
    import Control.Monad (ap)
    #endif
    #endif

    #ifdef __GLASGOW_HASKELL__
    #if ! (MIN_VERSION_base(4,3,0))
    instance MonadPlus STM where
      mzero = retry
      mplus = orElse

    instance Applicative STM where
      pure = return
      (<*>) = ap

    instance Alternative STM where
      empty = retry
      (<|>) = orElse
    #endif

    check :: Bool -> STM ()
    check b = if b then return () else retry
    #endif

    #if ! (MIN_VERSION_base(4,3,0))
    -- |Exception handling within STM actions.
    catchSTM :: Exception e => STM a -> (e -> STM a) -> STM a
    catchSTM (STM m) handler = STM $ catchSTM# m handler'
        where
          handler' e = case fromException e of
                         Just e' -> case handler e' of STM m' -> m'
                         Nothing -> raiseIO# e

    -- | A variant of 'throw' that can only be used within the 'STM' monad.
    --
    -- Throwing an exception in @STM@ aborts the transaction and propagates the
    -- exception.
    --
    -- Although 'throwSTM' has a type that is an instance of the type of 'throw', the
    -- two functions are subtly different:
    --
    -- > throw e    `seq` x  ===> throw e
    -- > throwSTM e `seq` x  ===> x
    --
    -- The first example will cause the exception @e@ to be raised,
    -- whereas the second one won\'t.  In fact, 'throwSTM' will only cause
    -- an exception to be raised when it is used within the 'STM' monad.
    -- The 'throwSTM' variant should be used in preference to 'throw' to
    -- raise an exception within the 'STM' monad because it guarantees
    -- ordering with respect to other 'STM' operations, whereas 'throw'
    -- does not.
    throwSTM :: Exception e => e -> STM a
    throwSTM e = STM $ raiseIO# (toException e)
    #endif

    data STMret a = STMret (State# RealWorld) a

    liftSTM :: STM a -> State# RealWorld -> STMret a
    liftSTM (STM m) = \s -> case m s of (# s', r #) -> STMret s' r

    instance MonadFix STM where
      mfix k = STM $ \s ->
        let ans        = liftSTM (k r) s
            STMret _ r = ans
        in case ans of STMret s' x -> (# s', x #)