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Embed ST actions in Eval #41

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27 changes: 27 additions & 0 deletions Control/Parallel/Strategies.hs
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Original file line number Diff line number Diff line change
@@ -1,5 +1,6 @@
{-# LANGUAGE BangPatterns, CPP, MagicHash, UnboxedTuples #-}
{-# LANGUAGE GeneralizedNewtypeDeriving #-}
{-# LANGUAGE RankNTypes #-}
-----------------------------------------------------------------------------
-- |
-- Module : Control.Parallel.Strategies
Expand Down Expand Up @@ -118,6 +119,7 @@ module Control.Parallel.Strategies (
, Eval -- instances: Monad, Functor, Applicative
, runEval -- :: Eval a -> a
, runEvalIO -- :: Eval a -> IO a
, stToEval -- :: (forall s. ST s a) -> Eval a
,

-- * API History
Expand Down Expand Up @@ -150,6 +152,12 @@ import Control.Applicative
import Control.Parallel
import Control.DeepSeq (NFData(rnf))

#if __GLASGOW_HASKELL__ >= 702
import Control.Monad.ST.Strict (ST, stToIO)
#else
import Control.Monad.ST.Strict (ST, runST)
#endif

#if MIN_VERSION_base(4,4,0)
import System.IO.Unsafe (unsafeDupablePerformIO)
import Control.Exception (evaluate)
Expand Down Expand Up @@ -224,6 +232,16 @@ runEval = unsafePerformIO . unEval_
runEvalIO :: Eval a -> IO a
runEvalIO = unEval_

-- | Run an 'ST' computation in the 'Eval' monad.
--
-- > stToEval m = runST (pure <$> m)
stToEval :: (forall s. ST s a) -> Eval a
-- Do not be tempted to allow non-closed ST computations (i.e., to
-- drop the forall). ST computations using the same mutable
-- references and arrays could end up running in parallel, stepping
-- on each other's toes.
stToEval m = Eval (stToIO m)

#else

data Eval a = Done a
Expand All @@ -237,6 +255,15 @@ runEval (Done x) = x
runEvalIO :: Eval a -> IO a
runEvalIO (Done x) = return x

-- | Run an 'ST' computation in the 'Eval' monad.
--
-- > stToEval m = runST (pure <$> m)
stToEval :: (forall s. ST s a) -> Eval a
-- It must be runST (Eval <$> m) and not Eval (runST m). The
-- latter will delay the ST computation until its result is
-- forced to WHNF. We want to run it immediately.
stToEval m = runST (pure <$> m)

instance Functor Eval where
fmap = liftM

Expand Down