classy-parallel- Fork of the monad-parallel package using monad-control

Safe HaskellSafe-Infered




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, Parallel and Fork. 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
      result <- child

In this example, the computations expensive and otherStuff would be performed in parallel. When using the Parallel 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 Fork class can also be instance of the Parallel class by the following rule:

 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} — 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.



class Monad m => Parallel m whereSource

Class of types that can perform two computations in parallel and bind their results together.


bindM2 :: (a -> b -> m c) -> m a -> m b -> m cSource

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}


Parallel [] 
Parallel IO 
Parallel Maybe 
Parallel Identity

Any monad that allows the result value to be extracted, such as Identity or Maybe monad, can implement bindM2 by using par.

Parallel ((->) r) 
Parallel m => Parallel (ListT m) 
Parallel (ResourceT IO) 
Parallel m => Parallel (MaybeT m) 
Parallel m => Parallel (IdentityT m) 
(Parallel m, Error e) => Parallel (ErrorT e m) 
Parallel m => Parallel (ReaderT r m) 

class Monad m => Fork m whereSource

Class of monads that can fork a parallel computation.


forkExec :: m a -> m (m a)Source

Fork a child monadic computation to be performed in parallel with the current one.


Fork [] 
Fork IO 
Fork Maybe 
Fork ((->) r) 
Fork (ResourceT IO) 

bindM3 :: Parallel m => (a -> b -> c -> m d) -> m a -> m b -> m c -> m dSource

Perform three monadic computations in parallel; when they are all finished, pass their results to the function.

Control.Monad equivalents

liftM2 :: Parallel m => (a -> b -> c) -> m a -> m b -> m cSource

Like liftM2, but evaluating its two monadic arguments in parallel.

liftM3 :: Parallel m => (a1 -> a2 -> a3 -> r) -> m a1 -> m a2 -> m a3 -> m rSource

Like liftM3, but evaluating its three monadic arguments in parallel.

ap :: Parallel m => m (a -> b) -> m a -> m bSource

Like ap, but evaluating the function and its argument in parallel.

sequence :: Parallel m => [m a] -> m [a]Source

Like sequence, but executing the actions in parallel.

sequence_ :: Fork m => [m a] -> m ()Source

Like sequence_, but executing the actions in parallel.

mapM :: Parallel m => (a -> m b) -> [a] -> m [b]Source

Like mapM, but applying the function to the individual list items in parallel.

replicateM :: Parallel m => Int -> m a -> m [a]Source

Like replicateM, but executing the action multiple times in parallel.

replicateM_ :: Fork m => Int -> m a -> m ()Source

Like replicateM_, but executing the action multiple times in parallel.

Default instances

defaultForkExec :: Monad m => m a -> m (m a)Source

defaultBindM2 :: Monad m => (a -> b -> m c) -> m a -> m b -> m cSource

bindTrans :: (MonadTransControl t, Parallel m, Monad m, Monad (t m)) => (a -> b -> t m c) -> t m a -> t m b -> t m cSource

Common bind pattern for monad transformers

forkTrans :: (MonadTransControl t, Fork m) => t m a -> t m (t m a)Source

parallelIO :: Fork m => (a -> b -> m c) -> m a -> m b -> m cSource

Defines bindM2 in terms of forkExec