Remove Control.Parallel*, now in package parallel

[haskell-directory.git] / Control / Monad / ST / Lazy.hs

diff --git a/Control/Monad/ST/Lazy.hs b/Control/Monad/ST/Lazy.hs
index ea30d20..5bf1265 100644 (file)
--- a/Control/Monad/ST/Lazy.hs
+++ b/Control/Monad/ST/Lazy.hs
@@ -2,42 +2,71 @@
 -- |
 -- 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)
 --
--- 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 (
+       -- * The 'ST' monad
        ST,
-
        runST,
-       unsafeInterleaveST,
        fixST,
 
-       ST.unsafeIOToST, ST.stToIO,
+       -- * Converting between strict and lazy 'ST'
+       strictToLazyST, lazyToStrictST,
+
+       -- * Converting 'ST' To 'IO'
+       RealWorld,
+       stToIO,
 
-       strictToLazyST, lazyToStrictST
+       -- * Unsafe operations
+       unsafeInterleaveST,
+       unsafeIOToST
     ) where
 
 import Prelude
 
-#ifdef __GLASGOW_HASKELL__
+import Control.Monad.Fix
+
+import Control.Monad.ST (RealWorld)
 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 ->
@@ -61,25 +90,37 @@ instance Monad (ST s) where
            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
+                  (r,s') = m_r s
                in
-                  (r,s))
+                  (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 
@@ -89,10 +130,23 @@ strictToLazyST m = ST $ \s ->
        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