+{-# 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 1.2 2001/07/03 11:37:49 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,
-
- runST,
- unsafeInterleaveST,
- fixST,
+ -- * The 'ST' monad
+ ST,
+ runST,
+ fixST,
- STRef.STRef,
- newSTRef, readSTRef, writeSTRef,
-
- STArray.STArray,
- newSTArray, readSTArray, writeSTArray, boundsSTArray,
- thawSTArray, freezeSTArray, unsafeFreezeSTArray,
-#ifdef __GLASGOW_HASKELL__
--- no 'good' reason, just doesn't support it right now.
- unsafeThawSTArray,
-#endif
+ -- * 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
-import qualified Data.STRef as STRef
-import Data.Array
+import Control.Monad.Fix
-#ifdef __GLASGOW_HASKELL__
import qualified Control.Monad.ST as ST
-import qualified GHC.Arr as STArray
+
+#ifdef __GLASGOW_HASKELL__
import qualified GHC.ST
-import GHC.Base ( ($), ()(..) )
-import Control.Monad
-import Data.Ix
-import GHC.Prim
+import GHC.Base
#endif
#ifdef __HUGS__
-import qualified ST
-import Monad
-import Ix
-import Array
-import PrelPrim ( unST
- , mkST
- , PrimMutableArray
- , PrimArray
- , primNewArray
- , primReadArray
- , primWriteArray
- , primUnsafeFreezeArray
- , primSizeMutableArray
- , primSizeArray
- , primIndexArray
- )
+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
-
-#ifdef __HUGS__
-newtype ST s a = ST (s -> (a,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
-#endif
-
-#ifdef __HUGS__
-runST :: (__forall s. ST s a) -> a
-runST st = case st of ST the_st -> let (r,_) = the_st realWorld in r
- where realWorld = error "runST: entered the RealWorld"
-#endif
+-- | 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))
-
--- ---------------------------------------------------------------------------
--- Variables
-
-newSTRef :: a -> ST s (STRef.STRef s a)
-readSTRef :: STRef.STRef s a -> ST s a
-writeSTRef :: STRef.STRef s a -> a -> ST s ()
-
-newSTRef = strictToLazyST . STRef.newSTRef
-readSTRef = strictToLazyST . STRef.readSTRef
-writeSTRef r a = strictToLazyST (STRef.writeSTRef r a)
-
--- --------------------------------------------------------------------------
--- Arrays
-
-newSTArray :: Ix ix => (ix,ix) -> elt -> ST s (STArray.STArray s ix elt)
-readSTArray :: Ix ix => STArray.STArray s ix elt -> ix -> ST s elt
-writeSTArray :: Ix ix => STArray.STArray s ix elt -> ix -> elt -> ST s ()
-boundsSTArray :: Ix ix => STArray.STArray s ix elt -> (ix, ix)
-thawSTArray :: Ix ix => Array ix elt -> ST s (STArray.STArray s ix elt)
-freezeSTArray :: Ix ix => STArray.STArray s ix elt -> ST s (Array ix elt)
-unsafeFreezeSTArray :: Ix ix => STArray.STArray s ix elt -> ST s (Array ix elt)
-
-#ifdef __GLASGOW_HASKELL__
-
-newSTArray ixs init = strictToLazyST (STArray.newSTArray ixs init)
-
-readSTArray arr ix = strictToLazyST (STArray.readSTArray arr ix)
-writeSTArray arr ix v = strictToLazyST (STArray.writeSTArray arr ix v)
-boundsSTArray arr = STArray.boundsSTArray arr
-thawSTArray arr = strictToLazyST (STArray.thawSTArray arr)
-freezeSTArray arr = strictToLazyST (STArray.freezeSTArray arr)
-unsafeFreezeSTArray arr = strictToLazyST (STArray.unsafeFreezeSTArray arr)
-unsafeThawSTArray arr = strictToLazyST (STArray.unsafeThawSTArray arr)
+ let
+ ST m_r = m r
+ (r,s') = m_r s
+ in
+ (r,s'))
#endif
-
-#ifdef __HUGS__
-newSTArray ixs elt = do
- { arr <- strictToLazyST (primNewArray (rangeSize ixs) elt)
- ; return (STArray ixs arr)
- }
-
-boundsSTArray (STArray ixs arr) = ixs
-readSTArray (STArray ixs arr) ix
- = strictToLazyST (primReadArray arr (index ixs ix))
-writeSTArray (STArray ixs arr) ix elt
- = strictToLazyST (primWriteArray arr (index ixs ix) elt)
-freezeSTArray (STArray ixs arr) = do
- { arr' <- strictToLazyST (primFreezeArray arr)
- ; return (Array ixs arr')
- }
-
-unsafeFreezeSTArray (STArray ixs arr) = do
- { arr' <- strictToLazyST (primUnsafeFreezeArray arr)
- ; return (Array ixs arr')
- }
-
-thawSTArray (Array ixs arr) = do
- { arr' <- strictToLazyST (primThawArray arr)
- ; return (STArray ixs arr')
- }
-
-primFreezeArray :: PrimMutableArray s a -> ST.ST s (PrimArray a)
-primFreezeArray arr = do
- { let n = primSizeMutableArray arr
- ; arr' <- primNewArray n arrEleBottom
- ; mapM_ (copy arr arr') [0..n-1]
- ; primUnsafeFreezeArray arr'
- }
- where
- copy arr arr' i = do { x <- primReadArray arr i; primWriteArray arr' i x }
- arrEleBottom = error "primFreezeArray: panic"
-
-primThawArray :: PrimArray a -> ST.ST s (PrimMutableArray s a)
-primThawArray arr = do
- { let n = primSizeArray arr
- ; arr' <- primNewArray n arrEleBottom
- ; mapM_ (copy arr arr') [0..n-1]
- ; return arr'
- }
- where
- copy arr arr' i = primWriteArray arr' i (primIndexArray arr i)
- arrEleBottom = error "primFreezeArray: panic"
-#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
-
-#ifdef __HUGS__
-strictToLazyST :: ST.ST s a -> ST s a
-strictToLazyST m = ST $ \s ->
- let
- pr = unST m s
- r = fst pr
- s' = snd pr
- in
- (r, s')
-
-
-lazyToStrictST :: ST s a -> ST.ST s a
-lazyToStrictST (ST m) = mkST $ m
-#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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