[project @ 2003-09-05 17:36:40 by ross]
[packages/old-time.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 -- * Converting 'ST' to 'IO'
25 RealWorld, -- abstract
26 stToIO, -- :: ST RealWorld a -> IO a
27
28 -- * Unsafe operations
29 unsafeInterleaveST, -- :: ST s a -> ST s a
30 unsafeIOToST -- :: IO a -> ST s a
31 ) where
32
33 import Prelude
34
35 import Control.Monad.Fix
36 import Data.Typeable
37
38 #ifdef __HUGS__
39 import Hugs.ST
40 import qualified Hugs.LazyST as LazyST
41
42 fixST :: (a -> ST s a) -> ST s a
43 fixST f = LazyST.lazyToStrictST (LazyST.fixST (LazyST.strictToLazyST . f))
44
45 unsafeInterleaveST :: ST s a -> ST s a
46 unsafeInterleaveST =
47 LazyST.lazyToStrictST . LazyST.unsafeInterleaveST . LazyST.strictToLazyST
48 #endif
49
50 #ifdef __GLASGOW_HASKELL__
51 import GHC.ST
52 import GHC.Base ( unsafeCoerce#, RealWorld )
53 import GHC.IOBase ( IO(..), stToIO )
54
55 -- This relies on IO and ST having the same representation modulo the
56 -- constraint on the type of the state
57 --
58 unsafeIOToST :: IO a -> ST s a
59 unsafeIOToST (IO io) = ST $ \ s -> (unsafeCoerce# io) s
60 #endif
61
62 instance MonadFix (ST s) where
63 mfix = fixST
64
65 -- ---------------------------------------------------------------------------
66 -- Typeable instance
67
68 sTTc :: TyCon
69 sTTc = mkTyCon "ST"
70
71 instance (Typeable a, Typeable b) => Typeable (ST a b) where
72 typeOf st = mkAppTy sTTc [typeOf ((undefined :: ST a b -> a) st),
73 typeOf ((undefined :: ST a b -> b) st)]