monad-parallel: Parallel execution of monadic computations

[ bsd3, control, library, monads ] [ Propose Tags ]

This package defines classes of monads that can perform multiple executions in parallel and combine their results. For any monad that's an instance of the class, the package re-implements a subset of the Control.Monad interface, but with parallel execution.

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Versions [RSS] [faq] 0.5, 0.5.1, 0.5.2, 0.7, 0.7.1,,,,, 0.7.2,,,,,,
Dependencies base (<5), parallel, transformers (>=0.2 && <0.7), transformers-compat (>=0.3 && <0.7) [details]
License BSD-3-Clause
Copyright (c) 2010-2018 Mario Blazevic
Author Mario Blazevic
Category Control, Monads
Home page
Source repo head: darcs get
Uploaded by MarioBlazevic at 2021-07-31T02:55:08Z
Distributions LTSHaskell:, NixOS:, Stackage:
Downloads 13904 total (138 in the last 30 days)
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Status Hackage Matrix CI
Docs available [build log]
Last success reported on 2021-07-31 [all 1 reports]


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Readme for monad-parallel-

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The low-level libraries available with GHC provide two completely different ways to execute tasks in parallel: par and forkIO. The former can be applied to any pure computation, while the latter works only in the IO monad.

The purpose of the monad-parallel library package is to unify the two approaches. For this purpose, it defines classes of monads that are capable of:

  • splitting or forking monadic computations to be performed in parallel, and
  • combining the results of those computations when they complete.

The way these classes are implemented by the IO monad (as well as any MonadIO instance) is different from the way they're implemented by stateless monads, but the classes abstract these implementation details away.

Class MonadFork

A monad that's an instance of the MonadFork class supports method

forkExec :: MonadFork m => m a -> m (m a)

This function launches the argument computation in a parallel thread and returns a handle to it. The main thread is free to perform some other task and then obtain the result of the parallel computation:

task1 :: MonadFork m => m Int
task2 :: MonadFork m => m Int
task3 :: MonadFork m => m Int

example = do handle1 <- forkExec task1
             handle2 <- forkExec task2
             result3 <- task3
             result1 <- handle1
             result2 <- handle2
             return (result1 + result2 + result3)

Class MonadParallel

A monad that's an instance of the MonadParallel class supports method

bindM2 :: MonadParallel m => (a -> b -> m c) -> m a -> m b -> m c

This method executes its m a and m b arguments in parallel. Once they're both finished, bindM2 runs their results through its first argument and returns the final result. Note that bindM2 can be implemented using forkExec, but not vice versa.

Based on this foundation, the library defines and exports other useful functions. These replicate the functionality of the Control.Monad module, but they execute their monadic arguments in parallel.

bindM3      :: MonadParallel m => (a -> b -> c -> m d) -> m a -> m b -> m c -> m d
liftM2      :: MonadParallel m => (a -> b -> c) -> m a -> m b -> m c
liftM3      :: MonadParallel m => (a1 -> a2 -> a3 -> r) -> m a1 -> m a2 -> m a3 -> m r
ap          :: MonadParallel m => m (a -> b) -> m a -> m b
sequence    :: MonadParallel m => [m a] -> m [a]
sequence_   :: MonadParallel m => [m a] -> m () 
mapM        :: MonadParallel m => (a -> m b) -> [a] -> m [b]
replicateM  :: MonadParallel m => Int -> m a -> m [a]
replicateM_ :: MonadParallel m => Int -> m a -> m ()