+{-# LANGUAGE CPP, MagicHash, UnboxedTuples, Rank2Types #-}
+
-----------------------------------------------------------------------------
---
+-- |
-- Module : Control.Monad.ST.Lazy
-- Copyright : (c) The University of Glasgow 2001
--- License : BSD-style (see the file libraries/core/LICENSE)
+-- License : BSD-style (see the file libraries/base/LICENSE)
--
-- Maintainer : libraries@haskell.org
-- Stability : provisional
-- Portability : non-portable (requires universal quantification for runST)
--
--- $Id: Lazy.hs,v 1.4 2002/01/02 14:40:09 simonmar Exp $
---
--- This module presents an identical interface to Control.Monad.ST,
--- but the underlying implementation of the state thread is lazy.
+-- This module presents an identical interface to "Control.Monad.ST",
+-- except that the monad delays evaluation of state operations until
+-- a value depending on them is required.
--
-----------------------------------------------------------------------------
module Control.Monad.ST.Lazy (
- ST,
+ -- * The 'ST' monad
+ ST,
+ runST,
+ fixST,
- runST,
- unsafeInterleaveST,
- fixST,
+ -- * Converting between strict and lazy 'ST'
+ strictToLazyST, lazyToStrictST,
- ST.unsafeIOToST, ST.stToIO,
+ -- * Converting 'ST' To 'IO'
+ RealWorld,
+ stToIO,
- strictToLazyST, lazyToStrictST
+ -- * Unsafe operations
+ unsafeInterleaveST,
+ unsafeIOToST
) where
import Prelude
-#ifdef __GLASGOW_HASKELL__
+import Control.Monad.Fix
+
import qualified Control.Monad.ST as ST
+
+#ifdef __GLASGOW_HASKELL__
import qualified GHC.ST
import GHC.Base
-import Control.Monad
+#endif
+
+#ifdef __HUGS__
+import Hugs.LazyST
#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)
-#endif
instance Functor (ST s) where
fmap f m = ST $ \ s ->
return a = ST $ \ s -> (a,s)
m >> k = m >>= \ _ -> k
- fail s = error s
+ fail s = error s
(ST m) >>= k
= ST $ \ s ->
in
k_a new_s
-
-#ifdef __GLASGOW_HASKELL__
{-# 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
+
-- ---------------------------------------------------------------------------
-- Strict <--> Lazy
#ifdef __GLASGOW_HASKELL__
+{-|
+Convert a strict 'ST' computation into a lazy one. The strict state
+thread passed to 'strictToLazyST' is not performed until the result of
+the lazy state thread it returns is demanded.
+-}
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.
+-}
lazyToStrictST :: ST s a -> ST.ST s a
lazyToStrictST (ST m) = GHC.ST.ST $ \s ->
case (m (S# s)) of (a, S# s') -> (# s', a #)
-#endif
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
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