1 -----------------------------------------------------------------------------
3 -- Module : Control.Monad.ST.Lazy
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 : provisional
9 -- Portability : non-portable (requires universal quantification for runST)
11 -- This module presents an identical interface to "Control.Monad.ST",
12 -- but the underlying implementation of the state thread is /lazy/ (in
13 -- the sense that (@_|_ >> a@ is not necessarily equal to @_|_@).
15 -----------------------------------------------------------------------------
17 module Control.Monad.ST.Lazy (
23 -- * Unsafe operations
27 -- * Converting 'ST' To 'IO'
32 -- * Converting between strict and lazy 'ST'
33 strictToLazyST, lazyToStrictST
39 import Control.Monad.Fix
41 #ifdef __GLASGOW_HASKELL__
42 import qualified Control.Monad.ST as ST
43 import qualified GHC.ST
49 import Hugs.LazyST as ST
52 #ifdef __GLASGOW_HASKELL__
53 newtype ST s a = ST (State s -> (a, State s))
54 data State s = S# (State# s)
56 instance Functor (ST s) where
57 fmap f m = ST $ \ s ->
64 instance Monad (ST s) where
66 return a = ST $ \ s -> (a,s)
67 m >> k = m >>= \ _ -> k
78 {-# NOINLINE runST #-}
79 runST :: (forall s. ST s a) -> a
80 runST st = case st of ST the_st -> let (r,_) = the_st (S# realWorld#) in r
82 fixST :: (a -> ST s a) -> ST s a
91 instance MonadFix (ST s) where
94 -- ---------------------------------------------------------------------------
97 #ifdef __GLASGOW_HASKELL__
99 Convert a strict 'ST' computation into a lazy one. The strict state
100 thread passed to 'strictToLazyST' is not performed until the result of
101 the lazy state thread it returns is demanded.
103 strictToLazyST :: ST.ST s a -> ST s a
104 strictToLazyST m = ST $ \s ->
106 pr = case s of { S# s# -> GHC.ST.liftST m s# }
107 r = case pr of { GHC.ST.STret _ v -> v }
108 s' = case pr of { GHC.ST.STret s2# _ -> S# s2# }
113 Convert a lazy 'ST' computation into a strict one.
115 lazyToStrictST :: ST s a -> ST.ST s a
116 lazyToStrictST (ST m) = GHC.ST.ST $ \s ->
117 case (m (S# s)) of (a, S# s') -> (# s', a #)
119 unsafeInterleaveST :: ST s a -> ST s a
120 unsafeInterleaveST = strictToLazyST . ST.unsafeInterleaveST . lazyToStrictST