runST,
fixST,
- -- * Unsafe operations
- unsafeInterleaveST,
- unsafeIOToST,
+ -- * Converting between strict and lazy 'ST'
+ strictToLazyST, lazyToStrictST,
-- * Converting 'ST' To 'IO'
RealWorld,
stToIO,
- -- * Converting between strict and lazy 'ST'
- strictToLazyST, lazyToStrictST
+ -- * Unsafe operations
+ unsafeInterleaveST,
+ unsafeIOToST
) where
import Prelude
#endif
#ifdef __GLASGOW_HASKELL__
+-- | The lazy state-transformer monad.
+-- The first parameter is used solely to keep the states of different
+-- invocations of 'runST' separate from each other and from invocations
+-- of 'Control.Monad.ST.stToIO'. In the first case the type parameter
+-- is not instantiated; in the second it is 'RealWorld'.
newtype ST s a = ST (State s -> (a, State s))
data State s = S# (State# s)
k_a new_s
{-# NOINLINE runST #-}
+-- | Return the value computed by a state transformer computation.
+-- The @forall@ is a technical device to ensure that the state used
+-- by the 'ST' computation is inaccessible to the rest of the program.
runST :: (forall s. ST s a) -> a
runST st = case st of ST the_st -> let (r,_) = the_st (S# realWorld#) in r
+-- | Allow the result of a state transformer computation to be used (lazily)
+-- inside the computation.
+-- Note that if @f@ is strict, @'fixST' f@ will diverge.
fixST :: (a -> ST s a) -> ST s a
fixST m = ST (\ s ->
let
unsafeIOToST :: IO a -> ST s a
unsafeIOToST = strictToLazyST . ST.unsafeIOToST
+-- | A monad transformer embedding lazy state transformers in the 'IO'
+-- monad. The 'RealWorld' parameter is a technical device to keep the
+-- state used by such computations separate from those inside 'runST'.
stToIO :: ST RealWorld a -> IO a
stToIO = ST.stToIO . lazyToStrictST