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 "<<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}
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}
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