[project @ 2003-09-05 17:36:40 by ross]

[ghc-base.git] / Control / Monad / ST / Lazy.hs

diff --git a/Control/Monad/ST/Lazy.hs b/Control/Monad/ST/Lazy.hs
index ffbb1af..b91e062 100644 (file)
--- a/Control/Monad/ST/Lazy.hs
+++ b/Control/Monad/ST/Lazy.hs
@@ -20,36 +20,41 @@ module Control.Monad.ST.Lazy (
        runST,
        fixST,
 
-       -- * Unsafe operations
-       unsafeInterleaveST,
-       ST.unsafeIOToST,
+       -- * Converting between strict and lazy 'ST'
+       strictToLazyST, lazyToStrictST,
 
        -- * Converting 'ST' To 'IO'
        RealWorld,
-       ST.stToIO,
+       stToIO,
 
-#ifndef __HUGS__
-       -- * Converting between strict and lazy 'ST'
-       strictToLazyST, lazyToStrictST
-#endif
+       -- * Unsafe operations
+       unsafeInterleaveST,
+       unsafeIOToST
     ) where
 
 import Prelude
 
 import Control.Monad.Fix
 
-#ifdef __GLASGOW_HASKELL__
+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 as ST
+import Hugs.LazyST
 #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)
 
@@ -76,9 +81,15 @@ instance Monad (ST s) where
            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 
@@ -119,3 +130,12 @@ lazyToStrictST (ST m) = GHC.ST.ST $ \s ->
 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 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