    \begin{code}
    {-# OPTIONS_GHC -fno-implicit-prelude #-}
    -----------------------------------------------------------------------------
    -- |
    -- Module      :  GHC.ST
    -- Copyright   :  (c) The University of Glasgow, 1992-2002
    -- License     :  see libraries/base/LICENSE
    --
    -- Maintainer  :  cvs-ghc@haskell.org
    -- Stability   :  internal
    -- Portability :  non-portable (GHC Extensions)
    --
    -- The 'ST' Monad.
    --
    -----------------------------------------------------------------------------

    -- #hide
    module GHC.ST where

    import GHC.Base
    import GHC.Show
    import GHC.Num

    default ()
    \end{code}

    %*********************************************************
    %*                                                      *
    \subsection{The @ST@ monad}
    %*                                                      *
    %*********************************************************

    The state-transformer monad proper.  By default the monad is strict;
    too many people got bitten by space leaks when it was lazy.

    \begin{code}
    -- | The strict state-transformer monad.
    -- A computation of type @'ST' s a@ transforms an internal state indexed
    -- by @s@, and returns a value of type @a@.
    -- The @s@ parameter is either
    --
    -- * an uninstantiated type variable (inside invocations of 'runST'), or
    --
    -- * 'RealWorld' (inside invocations of 'Control.Monad.ST.stToIO').
    --
    -- It serves to keep the internal states of different invocations
    -- of 'runST' separate from each other and from invocations of
    -- 'Control.Monad.ST.stToIO'.
    --
    -- The '>>=' and '>>' operations are strict in the state (though not in
    -- values stored in the state).  For example,
    --
    -- @'runST' (writeSTRef _|_ v >>= f) = _|_@
    newtype ST s a = ST (STRep s a)
    type STRep s a = State# s -> (# State# s, a #)

    instance Functor (ST s) where
        fmap f (ST m) = ST $ \ s ->
          case (m s) of { (# new_s, r #) ->
          (# new_s, f r #) }

    instance Monad (ST s) where
        {-# INLINE return #-}
        {-# INLINE (>>)   #-}
        {-# INLINE (>>=)  #-}
        return x = ST (\ s -> (# s, x #))
        m >> k   = m >>= \ _ -> k

        (ST m) >>= k
          = ST (\ s ->
            case (m s) of { (# new_s, r #) ->
            case (k r) of { ST k2 ->
            (k2 new_s) }})

    data STret s a = STret (State# s) a

    -- liftST is useful when we want a lifted result from an ST computation.  See
    -- fixST below.
    liftST :: ST s a -> State# s -> STret s a
    liftST (ST m) = \s -> case m s of (# s', r #) -> STret s' r

    {-# NOINLINE unsafeInterleaveST #-}
    unsafeInterleaveST :: ST s a -> ST s a
    unsafeInterleaveST (ST m) = ST ( \ s ->
        let
            r = case m s of (# _, res #) -> res
        in
        (# s, r #)
      )

    -- | Allow the result of a state transformer computation to be used (lazily)
    -- inside the computation.
    -- Note that if @f@ is strict, @'fixST' f = _|_@.
    fixST :: (a -> ST s a) -> ST s a
    fixST k = ST $ \ s ->
        let ans       = liftST (k r) s
            STret _ r = ans
        in
        case ans of STret s' x -> (# s', x #)

    instance  Show (ST s a)  where
        showsPrec _ _  = showString "<<ST action>>"
        showList       = showList__ (showsPrec 0)
    \end{code}

    Definition of runST
    ~~~~~~~~~~~~~~~~~~~

    SLPJ 95/04: Why @runST@ must not have an unfolding; consider:
    \begin{verbatim}
    f x =
      runST ( \ s -> let
                        (a, s')  = newArray# 100 [] s
                        (_, s'') = fill_in_array_or_something a x s'
                      in
                      freezeArray# a s'' )
    \end{verbatim}
    If we inline @runST@, we'll get:
    \begin{verbatim}
    f x = let
            (a, s')  = newArray# 100 [] realWorld#{-NB-}
            (_, s'') = fill_in_array_or_something a x s'
          in
          freezeArray# a s''
    \end{verbatim}
    And now the @newArray#@ binding can be floated to become a CAF, which
    is totally and utterly wrong:
    \begin{verbatim}
    f = let
        (a, s')  = newArray# 100 [] realWorld#{-NB-} -- YIKES!!!
        in
        \ x ->
            let (_, s'') = fill_in_array_or_something a x s' in
            freezeArray# a s''
    \end{verbatim}
    All calls to @f@ will share a {\em single} array!  End SLPJ 95/04.

    \begin{code}
    {-# INLINE runST #-}
    -- The INLINE prevents runSTRep getting inlined in *this* module
    -- so that it is still visible when runST is inlined in an importing
    -- module.  Regrettably delicate.  runST is behaving like a wrapper.

    -- | Return the value computed by a state transformer computation.
    -- The @forall@ ensures that the internal state used by the 'ST'
    -- computation is inaccessible to the rest of the program.
    runST :: (forall s. ST s a) -> a
    runST st = runSTRep (case st of { ST st_rep -> st_rep })

    -- I'm only letting runSTRep be inlined right at the end, in particular *after* full laziness
    -- That's what the "INLINE [0]" says.
    --              SLPJ Apr 99
    -- {-# INLINE [0] runSTRep #-}

    -- SDM: further to the above, inline phase 0 is run *before*
    -- full-laziness at the moment, which means that the above comment is
    -- invalid.  Inlining runSTRep doesn't make a huge amount of
    -- difference, anyway.  Hence:

    {-# NOINLINE runSTRep #-}
    runSTRep :: (forall s. STRep s a) -> a
    runSTRep st_rep = case st_rep realWorld# of
                            (# _, r #) -> r
    \end{code}