    {-# LANGUAGE MagicHash, UnboxedTuples, Rank2Types, FlexibleInstances,
        MultiParamTypeClasses, UndecidableInstances, RecursiveDo #-}
    {- |
       Module      :  Control.Monad.ST.Trans
       Copyright   :  Josef Svenningsson 2008-2010
                      (c) The University of Glasgow, 1994-2000
       License     :  BSD

       Maintainer  :  josef.svenningsson@gmail.com
       Stability   :  experimental
       Portability :  non-portable (GHC Extensions)

       This module provides the implementation of the 'STT' type for those
       occasions where it's needed in order to implement new liftings through
       operations in other monads.

       Warning! This monad transformer should not be used with monads that
       can contain multiple answers, like the list monad. The reason is that
       the will be duplicated across the different answers and this cause
       Bad Things to happen (such as loss of referential transparency). Safe
       monads include the monads State, Reader, Writer, Maybe and
       combinations of their corresponding monad transformers.
    -}
    module Control.Monad.ST.Trans.Internal where

    import GHC.Base

    -- | 'STT' is the monad transformer providing polymorphic updateable references
    newtype STT s m a = STT (State# s -> m (STTRet s a))

    unSTT :: STT s m a -> (State# s -> m (STTRet s a))
    unSTT (STT f) = f

    -- | 'STTRet' is needed to encapsulate the unboxed state token that GHC passes
    --   around. This type is essentially a pair, but an ordinary pair is not
    --   not allowed to contain unboxed types.
    data STTRet s a = STTRet (State# s) a