Control.Concurrent.Concurrential

The functions sequentially and concurrently inject IO terms into the ConcurrentialAp applicative functor, whose applicative instance will exploit as much concurrency as possible such that all sequentially terms will be run in the order in which they would have been run had they been typical IOs.

Terms of ConcurrentialAp can be transformed into terms of Concurrential, which is a monad. The order of sequential terms is respected even through binds; a sequential term will not be evaluted until all binds appearing syntactically earlier than it have been expanded.

Documentation

data Concurrential t

Description of computation which is composed of sequential and concurrent parts in some monad m.

Instances

Monad Concurrential

Functor Concurrential

Applicative Concurrential

Typeable (* -> *) Concurrential

newtype ConcurrentialAp t

Concurrential without a Monad instance, but an Applicative instance which exploits concurrency.

Constructors

ConcurrentialAp (Concurrential t)

Instances

Functor ConcurrentialAp

Applicative ConcurrentialAp

Typeable (* -> *) ConcurrentialAp

runConcurrential

Arguments

:: Concurrential t

-> (Async t -> IO r)

Similar contract to withAsync; the Async argument is useless outside of this function.

-> IO r

Run a Concurrential term, realizing the effects of the IO terms which compose it.

sequentially :: IO t -> ConcurrentialAp t

Create an effect which must be run sequentially. If a sequentially io appears in a Concurrential t term then it will always be run to completion before any later sequential part of the term is run. Consider the following terms:

    a = someConcurrential *> sequentially io *> someOtherConcurrential
    b = someConcurrential *> concurrently io *> someOtherConcurrential
    c = someConcurrential *> sequentially io *> concurrently otherIo
  

When running the term a, we are guaranteed that io is completed before any sequential part of someOtherConcurrential is begun, but when running the term b, this is not the case; io may be interleaved with or even run after any part of someOtherConcurrential. The term c highlights an important point: concurrently otherIo may be run before, during or after sequentially io! The ordering through applicative combinators is guaranteed only among sequential terms.

concurrently :: IO t -> ConcurrentialAp t

Create an effect which is run concurrently where possible, i.e. whenever it combined applicatively with other terms. For instance:

    a = concurrently io *> someConcurrential
    b = concurrently io >> someConcurrential
  

When running the term a, the IO term io will be run concurrently with someConcurrential, but not so in b, because monadic composition has been used.

concurrentially :: ConcurrentialAp t -> Concurrential t

Inject a ConcurrentialAp into Concurrential, losing the concurrency-enabling Applicative instance but gaining a Monad instance.

wait :: Async a -> IO a

Wait for an asynchronous action to complete, and return its value. If the asynchronous action threw an exception, then the exception is re-thrown by wait.

wait = atomically . waitSTM