5ea105224babbd25198d67fa10cbb38cd154f694
[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 #include "Typeable.h"
39
40 #ifdef __HUGS__
41 import Hugs.ST
42 import qualified Hugs.LazyST as LazyST
43
44 fixST :: (a -> ST s a) -> ST s a
45 fixST f = LazyST.lazyToStrictST (LazyST.fixST (LazyST.strictToLazyST . f))
46
47 unsafeInterleaveST :: ST s a -> ST s a
48 unsafeInterleaveST =
49 LazyST.lazyToStrictST . LazyST.unsafeInterleaveST . LazyST.strictToLazyST
50 #endif
51
52 #ifdef __GLASGOW_HASKELL__
53 import GHC.ST
54 import GHC.Base ( unsafeCoerce#, RealWorld )
55 import GHC.IOBase ( IO(..), stToIO )
56
57 -- This relies on IO and ST having the same representation modulo the
58 -- constraint on the type of the state
59 --
60 unsafeIOToST :: IO a -> ST s a
61 unsafeIOToST (IO io) = ST $ \ s -> (unsafeCoerce# io) s
62 #endif
63
64 instance MonadFix (ST s) where
65 mfix = fixST
66
67 -- ---------------------------------------------------------------------------
68 -- Typeable instance
69
70 INSTANCE_TYPEABLE2(ST,sTTc,"ST")