\begin{code}
-{-# OPTIONS -fno-implicit-prelude #-}
+{-# OPTIONS_GHC -fno-implicit-prelude #-}
-----------------------------------------------------------------------------
-- |
-- Module : GHC.ST
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 unstantiated 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'.
newtype ST s a = ST (STRep s a)
type STRep s a = State# s -> (# State# s, a #)
{-# INLINE return #-}
{-# INLINE (>>) #-}
{-# INLINE (>>=) #-}
- return x = ST $ \ s -> (# s, x #)
- m >> k = m >>= \ _ -> k
+ return x = ST (\ s -> (# s, x #))
+ m >> k = m >>= \ _ -> k
(ST m) >>= k
- = ST $ \ s ->
+ = ST (\ s ->
case (m s) of { (# new_s, r #) ->
case (k r) of { ST k2 ->
- (k2 new_s) }}
+ (k2 new_s) }})
data STret s a = STret (State# s) a
(# 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
-- 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 })