[project @ 2002-07-16 16:08:58 by ross]
[packages/old-locale.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/base/LICENSE)
6 --
7 -- Maintainer : libraries@haskell.org
8 -- Stability : experimental
9 -- Portability : non-portable (requires universal quantification for runST)
10 --
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/.
14 --
15 -----------------------------------------------------------------------------
16
17 module Control.Monad.ST
18 (
19 -- * The 'ST' Monad
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
23
24 -- * Unsafe operations
25 unsafeInterleaveST, -- :: ST s a -> ST s a
26 unsafeIOToST, -- :: IO a -> ST s a
27
28 -- * Converting 'ST' to 'IO'
29 RealWorld, -- abstract
30 stToIO -- :: ST RealWorld a -> IO a
31 ) where
32
33 import Prelude
34
35 import Control.Monad.Fix
36 import Data.Dynamic
37
38 #ifdef __HUGS__
39 import Hugs.ST
40 #endif
41
42 #ifdef __GLASGOW_HASKELL__
43 import GHC.ST
44 import GHC.Base ( unsafeCoerce#, RealWorld )
45 import GHC.IOBase ( IO(..), stToIO )
46
47 -- This relies on IO and ST having the same representation modulo the
48 -- constraint on the type of the state
49 --
50 unsafeIOToST :: IO a -> ST s a
51 unsafeIOToST (IO io) = ST $ \ s -> (unsafeCoerce# io) s
52 #endif
53
54 instance MonadFix (ST s) where
55 mfix = fixST
56
57 -- ---------------------------------------------------------------------------
58 -- Typeable instance
59
60 sTTc :: TyCon
61 sTTc = mkTyCon "ST"
62
63 instance (Typeable a, Typeable b) => Typeable (ST a b) where
64 typeOf st = mkAppTy sTTc [typeOf ((undefined :: ST a b -> a) st),
65 typeOf ((undefined :: ST a b -> b) st)]