1 -----------------------------------------------------------------------------
3 -- Module : Control.Monad.ST
4 -- Copyright : (c) The University of Glasgow 2001
5 -- License : BSD-style (see the file libraries/base/LICENSE)
7 -- Maintainer : libraries@haskell.org
8 -- Stability : experimental
9 -- Portability : non-portable (requires universal quantification for runST)
11 -- This library provides support for /strict/ state threads, as
12 -- described in the PLDI \'94 paper by John Launchbury and Simon Peyton
13 -- Jones /Lazy State Threads/.
15 -----------------------------------------------------------------------------
17 module Control.Monad.ST
20 ST, -- abstract, instance of Functor, Monad, Typeable.
21 runST, -- :: (forall s. ST s a) -> a
22 fixST, -- :: (a -> ST s a) -> ST s a
24 -- * Unsafe operations
25 unsafeInterleaveST, -- :: ST s a -> ST s a
26 unsafeIOToST, -- :: IO a -> ST s a
28 -- * Converting 'ST' to 'IO'
29 RealWorld, -- abstract
30 stToIO -- :: ST RealWorld a -> IO a
35 import Control.Monad.Fix
38 #ifdef __GLASGOW_HASKELL__
40 import GHC.Base ( unsafeCoerce#, RealWorld )
41 import GHC.IOBase ( IO(..), stToIO )
43 -- This relies on IO and ST having the same representation modulo the
44 -- constraint on the type of the state
46 unsafeIOToST :: IO a -> ST s a
47 unsafeIOToST (IO io) = ST $ \ s -> (unsafeCoerce# io) s
50 instance MonadFix (ST s) where
53 -- ---------------------------------------------------------------------------
59 instance (Typeable a, Typeable b) => Typeable (ST a b) where
60 typeOf st = mkAppTy sTTc [typeOf ((undefined :: ST a b -> a) st),
61 typeOf ((undefined :: ST a b -> b) st)]