-- Portability : non-portable (requires universal quantification for runST)
--
-- This module presents an identical interface to "Control.Monad.ST",
--- but the underlying implementation of the state thread is /lazy/ (in
--- the sense that (@_|_ >> a@ is not necessarily equal to @_|_@).
+-- except that the monad delays evaluation of state operations until
+-- a value depending on them is required.
--
-----------------------------------------------------------------------------
module Control.Monad.ST.Lazy (
- -- * The 'ST' monad
- ST,
- runST,
- fixST,
-
- -- * Unsafe operations
- unsafeInterleaveST,
- unsafeIOToST,
-
- -- * Converting 'ST' To 'IO'
- RealWorld,
- stToIO,
-
-#ifndef __HUGS__
- -- * Converting between strict and lazy 'ST'
- strictToLazyST, lazyToStrictST
-#endif
+ -- * The 'ST' monad
+ ST,
+ runST,
+ fixST,
+
+ -- * Converting between strict and lazy 'ST'
+ strictToLazyST, lazyToStrictST,
+
+ -- * Converting 'ST' To 'IO'
+ RealWorld,
+ stToIO,
+
+ -- * Unsafe operations
+ unsafeInterleaveST,
+ unsafeIOToST
) where
import Prelude
import Control.Monad.Fix
-#ifdef __GLASGOW_HASKELL__
import qualified Control.Monad.ST as ST
+
+#ifdef __GLASGOW_HASKELL__
import qualified GHC.ST
import GHC.Base
-import Control.Monad
#endif
#ifdef __HUGS__
#endif
#ifdef __GLASGOW_HASKELL__
+-- | The lazy 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 'stToIO').
+--
+-- It serves to keep the internal states of different invocations of
+-- 'runST' separate from each other and from invocations of 'stToIO'.
+--
+-- The '>>=' and '>>' operations are not strict in the state. For example,
+--
+-- @'runST' (writeSTRef _|_ v >>= readSTRef _|_ >> return 2) = 2@
newtype ST s a = ST (State s -> (a, State s))
data State s = S# (State# s)
return a = ST $ \ s -> (a,s)
m >> k = m >>= \ _ -> k
- fail s = error s
+ fail s = error s
(ST m) >>= k
= ST $ \ s ->
k_a new_s
{-# NOINLINE runST #-}
+-- | 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 = 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 = _|_@.
fixST :: (a -> ST s a) -> ST s a
fixST m = ST (\ s ->
- let
- ST m_r = m r
- (r,s') = m_r s
- in
- (r,s'))
+ let
+ ST m_r = m r
+ (r,s') = m_r s
+ in
+ (r,s'))
#endif
instance MonadFix (ST s) where
- mfix = fixST
+ mfix = fixST
-- ---------------------------------------------------------------------------
-- Strict <--> Lazy
strictToLazyST :: ST.ST s a -> ST s a
strictToLazyST m = ST $ \s ->
let
- pr = case s of { S# s# -> GHC.ST.liftST m s# }
- r = case pr of { GHC.ST.STret _ v -> v }
- s' = case pr of { GHC.ST.STret s2# _ -> S# s2# }
- in
- (r, s')
+ pr = case s of { S# s# -> GHC.ST.liftST m s# }
+ r = case pr of { GHC.ST.STret _ v -> v }
+ s' = case pr of { GHC.ST.STret s2# _ -> S# s2# }
+ in
+ (r, s')
{-|
Convert a lazy 'ST' computation into a strict one.
unsafeInterleaveST :: ST s a -> ST s a
unsafeInterleaveST = strictToLazyST . ST.unsafeInterleaveST . lazyToStrictST
+#endif
unsafeIOToST :: IO a -> ST s a
unsafeIOToST = strictToLazyST . ST.unsafeIOToST
+-- | A monad transformer embedding lazy state transformers in the 'IO'
+-- monad. The 'RealWorld' parameter indicates that the internal state
+-- used by the 'ST' computation is a special one supplied by the 'IO'
+-- monad, and thus distinct from those used by invocations of 'runST'.
stToIO :: ST RealWorld a -> IO a
stToIO = ST.stToIO . lazyToStrictST
-#endif