------------------------------------------------------------------------------
--- |
--- Module : Control.Monad.ST.Lazy
--- Copyright : (c) The University of Glasgow 2001
--- License : BSD-style (see the file libraries/base/LICENSE)
---
--- Maintainer : libraries@haskell.org
--- Stability : provisional
--- Portability : non-portable (requires universal quantification for runST)
---
--- This module presents an identical interface to "Control.Monad.ST",
--- except that the monad delays evaluation of state operations until
--- a value depending on them is required.
---
------------------------------------------------------------------------------
-
-module Control.Monad.ST.Lazy (
- -- * The 'ST' monad
- ST,
- runST,
- fixST,
-
- -- * Converting between strict and lazy 'ST'
- strictToLazyST, lazyToStrictST,
-
- -- * Converting 'ST' To 'IO'
- RealWorld,
- stToIO,
-
- -- * Unsafe operations
- unsafeInterleaveST,
- unsafeIOToST
- ) where
-
-import Prelude
-
-import Control.Monad.Fix
-
-import Control.Monad.ST (RealWorld)
-import qualified Control.Monad.ST as ST
-
-#ifdef __GLASGOW_HASKELL__
-import qualified GHC.ST
-import GHC.Base
-import Control.Monad
-#endif
-
-#ifdef __HUGS__
-import Hugs.LazyST
-#endif
-
-#ifdef __GLASGOW_HASKELL__
--- | The lazy state-transformer monad.
--- A computation of type @'ST' s a@ transforms an internal state indexed
--- by @s@, and returns a value of type @a@.
--- The @s@ parameter is either
---
--- * an unstantiated type variable (inside invocations of 'runST'), or
---
--- * 'RealWorld' (inside invocations of 'stToIO').
---
--- It serves to keep the internal states of different invocations of
--- 'runST' separate from each other and from invocations of 'stToIO'.
---
--- The '>>=' and '>>' operations are not strict in the state. For example,
---
--- @'runST' (writeSTRef _|_ v >>= readSTRef _|_ >> return 2) = 2@
-newtype ST s a = ST (State s -> (a, State s))
-data State s = S# (State# s)
-
-instance Functor (ST s) where
- fmap f m = ST $ \ s ->
- let
- ST m_a = m
- (r,new_s) = m_a s
- in
- (f r,new_s)
-
-instance Monad (ST s) where
-
- return a = ST $ \ s -> (a,s)
- m >> k = m >>= \ _ -> k
- fail s = error s
-
- (ST m) >>= k
- = ST $ \ s ->
- let
- (r,new_s) = m s
- ST k_a = k r
- in
- k_a new_s
-
-{-# NOINLINE runST #-}
--- | Return the value computed by a state transformer computation.
--- The @forall@ ensures that the internal state used by the 'ST'
--- computation is inaccessible to the rest of the program.
-runST :: (forall s. ST s a) -> a
-runST st = case st of ST the_st -> let (r,_) = the_st (S# realWorld#) in r
-
--- | Allow the result of a state transformer computation to be used (lazily)
--- inside the computation.
--- Note that if @f@ is strict, @'fixST' f = _|_@.
-fixST :: (a -> ST s a) -> ST s a
-fixST m = ST (\ s ->
- let
- ST m_r = m r
- (r,s') = m_r s
- in
- (r,s'))
-#endif
-
-instance MonadFix (ST s) where
- mfix = fixST
-
--- ---------------------------------------------------------------------------
--- Strict <--> Lazy
-
-#ifdef __GLASGOW_HASKELL__
-{-|
-Convert a strict 'ST' computation into a lazy one. The strict state
-thread passed to 'strictToLazyST' is not performed until the result of
-the lazy state thread it returns is demanded.
--}
-strictToLazyST :: ST.ST s a -> ST s a
-strictToLazyST m = ST $ \s ->
- let
- pr = case s of { S# s# -> GHC.ST.liftST m s# }
- r = case pr of { GHC.ST.STret _ v -> v }
- s' = case pr of { GHC.ST.STret s2# _ -> S# s2# }
- in
- (r, s')
-
-{-|
-Convert a lazy 'ST' computation into a strict one.
--}
-lazyToStrictST :: ST s a -> ST.ST s a
-lazyToStrictST (ST m) = GHC.ST.ST $ \s ->
- case (m (S# s)) of (a, S# s') -> (# s', a #)
-
-unsafeInterleaveST :: ST s a -> ST s a
-unsafeInterleaveST = strictToLazyST . ST.unsafeInterleaveST . lazyToStrictST
-#endif
-
-unsafeIOToST :: IO a -> ST s a
-unsafeIOToST = strictToLazyST . ST.unsafeIOToST
-
--- | A monad transformer embedding lazy state transformers in the 'IO'
--- monad. The 'RealWorld' parameter indicates that the internal state
--- used by the 'ST' computation is a special one supplied by the 'IO'
--- monad, and thus distinct from those used by invocations of 'runST'.
-stToIO :: ST RealWorld a -> IO a
-stToIO = ST.stToIO . lazyToStrictST