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

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

diff --git a/Control/Monad/ST.hs b/Control/Monad/ST.hs
index 7817add..705787e 100644 (file)
--- a/Control/Monad/ST.hs
+++ b/Control/Monad/ST.hs
@@ -1,45 +1,67 @@
 -----------------------------------------------------------------------------
--- 
+-- |
 -- Module      :  Control.Monad.ST
 -- 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   :  experimental
--- Portability :  non-portable
---
--- $Id: ST.hs,v 1.2 2001/06/29 09:41:37 simonmar Exp $
+-- Portability :  non-portable (requires universal quantification for runST)
 --
--- The State Transformer Monad, ST
+-- This library provides support for /strict/ state threads, as
+-- described in the PLDI \'94 paper by John Launchbury and Simon Peyton
+-- Jones /Lazy State Threads/.
 --
 -----------------------------------------------------------------------------
 
 module Control.Monad.ST
-      (
-       ST                  -- abstract, instance of Functor, Monad, Typeable.
-      , runST              -- :: (forall s. ST s a) -> a
-      , fixST              -- :: (a -> ST s a) -> ST s a
-      , unsafeInterleaveST  -- :: ST s a -> ST s a
+  (
+       -- * The 'ST' Monad
+       ST,             -- abstract, instance of Functor, Monad, Typeable.
+       runST,          -- :: (forall s. ST s a) -> a
+       fixST,          -- :: (a -> ST s a) -> ST s a
 
-      , unsafeIOToST       -- :: IO a -> ST s a
+       -- * Converting 'ST' to 'IO'
+       RealWorld,              -- abstract
+       stToIO,                 -- :: ST RealWorld a -> IO a
 
-      , RealWorld          -- abstract
-      , stToIO             -- :: ST RealWorld a -> IO a
+       -- * Unsafe operations
+       unsafeInterleaveST,     -- :: ST s a -> ST s a
+       unsafeIOToST            -- :: IO a -> ST s a
       ) where
 
 import Prelude
 
-import Data.Dynamic
+import Control.Monad.Fix
+import Data.Typeable
+
+#ifdef __HUGS__
+import Hugs.ST
+import qualified Hugs.LazyST as LazyST
+
+fixST :: (a -> ST s a) -> ST s a
+fixST f = LazyST.lazyToStrictST (LazyST.fixST (LazyST.strictToLazyST . f))
+
+unsafeInterleaveST :: ST s a -> ST s a
+unsafeInterleaveST =
+    LazyST.lazyToStrictST . LazyST.unsafeInterleaveST . LazyST.strictToLazyST
+#endif
 
 #ifdef __GLASGOW_HASKELL__
 import GHC.ST
-import GHC.Prim                ( unsafeCoerce#, RealWorld )
+import GHC.Base                ( unsafeCoerce#, RealWorld )
 import GHC.IOBase      ( IO(..), stToIO )
 
+-- This relies on IO and ST having the same representation modulo the
+-- constraint on the type of the state
+--
 unsafeIOToST        :: IO a -> ST s a
 unsafeIOToST (IO io) = ST $ \ s -> (unsafeCoerce# io) s
 #endif
 
+instance MonadFix (ST s) where
+       mfix = fixST
+
 -- ---------------------------------------------------------------------------
 -- Typeable instance