Copyright 2010 Mario Blazevic

    This file is part of the Streaming Component Combinators (SCC) project.

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-- | This module defines classes of monads that can perform multiple computations in parallel and, more importantly,
-- combine the results of those parallel computations.
-- There are two classes exported by this module, 'MonadParallel' and 'MonadFork'. The former is more generic, but the
-- latter is easier to use: when invoking any expensive computation that could be performed in parallel, simply wrap the
-- call in 'forkExec'. The function immediately returns a handle to the running computation. The handle can be used to
-- obtain the result of the computation when needed:
-- @
--   do child <- forkExec expensive
--      otherStuff
--      result <- child
-- @
-- In this example, the computations /expensive/ and /otherStuff/ would be performed in parallel. When using the
-- 'MonadParallel' class, both parallel computations must be specified at once:
-- @
--   bindM2 (\\ childResult otherResult -> ...) expensive otherStuff
-- @
-- In either case, for best results the costs of the two computations should be roughly equal.
-- Any monad that is an instance of the 'MonadFork' class is also an instance of the 'MonadParallel' class, and the
-- following law should hold:
-- @ bindM2 f ma mb = do {a' <- forkExec ma; b <- mb; a <- a'; f a b} @ 
-- When operating with monads free of side-effects, such as 'Identity' or 'Maybe', 'forkExec' is equivalent to 'return'
-- and 'bindM2' is equivalent to @ \\ f ma mb -> do {a <- ma; b <- mb; f a b} @ &#x2014; the only difference is in the
-- resource utilisation. With the 'IO' monad, on the other hand, there may be visible difference in the results because
-- the side effects of /ma/ and /mb/ may be arbitrarily reordered.

{-# LANGUAGE ScopedTypeVariables #-}

module Control.Monad.Parallel
    -- * Classes
    MonadParallel(..), MonadFork(..),
    -- * Control.Monad equivalents
    liftM2, liftM3, ap, sequence, sequence_, mapM, replicateM, replicateM_

import Prelude ()
import Control.Concurrent (forkIO)
import Control.Concurrent.MVar (newEmptyMVar, putMVar, takeMVar)
import Control.Monad (Monad, (>>=), return)
import Control.Parallel (par, pseq)
import Data.Functor.Identity (Identity)
import Data.Int (Int)
import Data.List (foldr, map, replicate)
import Data.Maybe (Maybe)
import System.IO (IO)

-- | Class of monads that can perform two computations in parallel and bind their results together.
class Monad m => MonadParallel m where
   -- | Perform two monadic computations in parallel; when they are both finished, pass the results to the function.
   -- Apart from the possible ordering of side effects, this function is equivalent to
   -- @\\f ma mb-> do {a <- ma; b <- mb; f a b}@
   bindM2 :: (a -> b -> m c) -> m a -> m b -> m c
   bindM2 f ma mb = let ma' = ma >>= return
                        mb' = mb >>= return
                    in ma' `par` (mb' `pseq` do {a <- ma'; b <- mb'; f a b})

-- | Class of monads that can fork a parallel computation.
class MonadParallel m => MonadFork m where
   -- | Fork a child monadic computation to be performed in parallel with the current one.
   forkExec :: m a -> m (m a)
   forkExec e = let result = e >>= return
                in result `par` (return result)

-- | Perform three monadic computations in parallel; when they are all finished, pass their results to the function.
bindM3 :: MonadParallel m => (a -> b -> c -> m d) -> m a -> m b -> m c -> m d
bindM3 f ma mb mc = bindM2 (\f' c-> f' c) (liftM2 f ma mb) mc

-- | Like 'Control.Monad.liftM2', but evaluating its two monadic arguments in parallel.
liftM2 :: MonadParallel m => (a -> b -> c) -> m a -> m b -> m c
liftM2 f m1 m2 = bindM2 (\a b-> return (f a b)) m1 m2

-- | Like 'Control.Monad.liftM3', but evaluating its three monadic arguments in parallel.
liftM3  :: (MonadParallel m) => (a1 -> a2 -> a3 -> r) -> m a1 -> m a2 -> m a3 -> m r
liftM3 f m1 m2 m3 = bindM3 (\a b c-> return (f a b c)) m1 m2 m3

-- | Like 'Control.Monad.ap', but evaluating the function and its argument in parallel.
ap :: MonadParallel m => m (a -> b) -> m a -> m b
ap mf ma = bindM2 (\f a-> return (f a)) mf ma

-- | Like 'Control.Monad.sequence', but executing the actions in parallel.
sequence :: MonadParallel m => [m a] -> m [a]
sequence ms = foldr k (return []) ms where
   k m m' = liftM2 (:) m m'

-- | Like 'Control.Monad.sequence_', but executing the actions in parallel.
sequence_ :: MonadParallel m => [m a] -> m () 
sequence_ ms = foldr (liftM2 (\ _ _ -> ())) (return ()) ms

-- | Like 'Control.Monad.mapM', but applying the function to the individual list items in parallel.
mapM :: MonadParallel m => (a -> m b) -> [a] -> m [b]
mapM f list = sequence (map f list)

-- | Like 'Control.Monad.replicateM', but executing the action multiple times in parallel.
replicateM :: MonadParallel m => Int -> m a -> m [a]
replicateM n action = sequence (replicate n action)

-- | Like 'Control.Monad.replicateM_', but executing the action multiple times in parallel.
replicateM_ :: MonadParallel m => Int -> m a -> m ()
replicateM_ n action = sequence_ (replicate n action)

-- | Any monad that allows the result value to be extracted, such as `Identity` or `Maybe` monad, can implement
-- `bindM2` by using `par`.
instance MonadParallel Identity
instance MonadParallel Maybe
instance MonadParallel []

-- | IO is parallelizable by `forkIO`.
instance MonadParallel IO where
   bindM2 f ma mb = do vb <- newEmptyMVar
                       _ <- forkIO (mb >>= putMVar vb)
                       a <- ma
                       b <- takeMVar vb
                       f a b

instance MonadFork Maybe
instance MonadFork []

-- | IO is forkable by `forkIO`.
instance MonadFork IO where
   forkExec ma = do v <- newEmptyMVar
                    _ <- forkIO (ma >>= putMVar v)
                    return (takeMVar v)