[project @ 2002-04-24 16:31:37 by simonmar]
[packages/base.git] / Control / Monad / ST.hs
1 -----------------------------------------------------------------------------
2 -- |
3 -- Module : Control.Monad.ST
4 -- Copyright : (c) The University of Glasgow 2001
5 -- License : BSD-style (see the file libraries/core/LICENSE)
6 --
7 -- Maintainer : libraries@haskell.org
8 -- Stability : experimental
9 -- Portability : non-portable (requires universal quantification for runST)
10 --
11 -- $Id: ST.hs,v 1.6 2002/04/24 16:31:38 simonmar Exp $
12 --
13 -- The State Transformer Monad, ST
14 --
15 -----------------------------------------------------------------------------
16
17 module Control.Monad.ST
18 (
19 ST -- abstract, instance of Functor, Monad, Typeable.
20 , runST -- :: (forall s. ST s a) -> a
21 , fixST -- :: (a -> ST s a) -> ST s a
22 , unsafeInterleaveST -- :: ST s a -> ST s a
23
24 , unsafeIOToST -- :: IO a -> ST s a
25
26 , RealWorld -- abstract
27 , stToIO -- :: ST RealWorld a -> IO a
28 ) where
29
30 import Prelude
31
32 import Control.Monad.Fix
33 import Data.Dynamic
34
35 #ifdef __GLASGOW_HASKELL__
36 import GHC.ST
37 import GHC.Base ( unsafeCoerce#, RealWorld )
38 import GHC.IOBase ( IO(..), stToIO )
39
40 -- This relies on IO and ST having the same representation modulo the
41 -- constraint on the type of the state
42 --
43 unsafeIOToST :: IO a -> ST s a
44 unsafeIOToST (IO io) = ST $ \ s -> (unsafeCoerce# io) s
45 #endif
46
47 instance MonadFix (ST s) where
48 mfix = fixST
49
50 -- ---------------------------------------------------------------------------
51 -- Typeable instance
52
53 sTTc :: TyCon
54 sTTc = mkTyCon "ST"
55
56 instance (Typeable a, Typeable b) => Typeable (ST a b) where
57 typeOf st = mkAppTy sTTc [typeOf ((undefined :: ST a b -> a) st),
58 typeOf ((undefined :: ST a b -> b) st)]