GHC/ST.lhs (original) (raw)

\begin{code}

module GHC.ST ( ST(..), STret(..), STRep, fixST, runST, runSTRep,

    liftST, unsafeInterleaveST
) where

import GHC.Base import GHC.Show

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}

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

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 :: ST s a -> State# s -> STret s a liftST (ST m) = \s -> case m s of (# s', r #) -> STret s' r

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

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 "<>" 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}

runST :: (forall s. ST s a) -> a runST st = runSTRep (case st of { ST st_rep -> st_rep })

runSTRep :: (forall s. STRep s a) -> a runSTRep st_rep = case st_rep realWorld# of (# _, r #) -> r

\end{code}