:: a typeCtrl KGHC 10.0.0.20260917 · lts/ghc-10.0.x · 8c2f8e8 · 2026-10-19
Moduleconcurrency-1.11.0.3Haskell2010
This module is a version of the
async package. It
provides a set of operations for running MonadConc operations
asynchronously and waiting for their results.
For example, assuming a suitable getURL function, we can fetch
the contents of two web pages at the same time:
withAsync (getURL url1) $ \a1 -> do
withAsync (getURL url2) $ \a2 -> do
page1 <- wait a1
page2 <- wait a2
...The withAsync function starts an operation in a separate thread, and kills it if the inner action finishes before it completes.
Unlike the regular async package, the Alternative instance for
Concurrently uses forever yield in the definition of empty,
rather than forever (threadDelay maxBound).
An asynchronous action spawned by async or withAsync. Asynchronous actions are executed in a separate thread, and operations are provided for waiting for asynchronous actions to complete and obtaining their results (see e.g. wait).
Note that, unlike the "async" package, Async here does not have an Ord instance. This is because MonadConc ThreadIds do not necessarily have one.
Spawn an asynchronous action in a separate thread.
Like async, but using a named thread for better debugging information.
Like asyncBound, but using a named thread for better debugging information.
Like asyncOn but using a named thread for better debugging information.
Like async but using forkWithUnmask internally.
Like asyncWithUnmask but using a named thread for better debugging information.
Like asyncOn but using forkOnWithUnmask internally.
Like asyncOnWithUnmask but using a named thread for better debugging information.
Spawn an asynchronous action in a separate thread, and pass its
Async handle to the supplied function. When the function returns
or throws an exception, uninterruptibleCancel is called on the Async.
withAsync action inner = bracket (async action) uninterruptiblCancel innerThis is a useful variant of async that ensures an Async is
never left running unintentionally.
Since uninterruptibleCancel may block, withAsync may also block; see uninterruptibleCancel for details.
Like withAsync but using a named thread for better debugging information.
Like withAsyncBound but using a named thread for better debugging information.
Like withAsyncOn but using a named thread for better debugging information.
Like withAsync bit uses forkWithUnmask internally.
Like withAsyncWithUnmask but using a named thread for better debugging information.
Like withAsyncOn bit uses forkOnWithUnmask internally.
Like withAsyncOnWithUnmask but using a named thread for better debugging information.
Wait for an asynchronous action to complete, and return its value. If the asynchronous value threw an exception, then the exception is re-thrown by wait.
wait = atomically . waitSTMA version of wait that can be used inside a MonadSTM transaction.
Check whether an Async has completed yet. If it has not
completed yet, then the result is Nothing, otherwise the result
is Just e where e is Left x if the Async raised an
exception x, or Right a if it returned a value a.
poll = atomically . pollSTMA version of poll that can be used inside a MonadSTM transaction.
Wait for an asynchronous action to complete, and return either
Left e if the action raised an exception e, or Right a if it
returned a value a.
A version of waitCatch that can be used inside a MonadSTM transaction.
Cancel an asynchronous action by throwing the ThreadKilled
exception to it, and waiting for the Async thread to quit. Has no
effect if the Async has already completed.
cancel a = throwTo (asyncThreadId a) ThreadKilled <* waitCatch aNote that cancel will not terminate until the thread the Async refers to has terminated. This means that cancel will block for as long as said thread blocks when receiving an asynchronous exception.
An asynchronous cancel can of course be obtained by wrapping cancel itself in async.
Cancel an asynchronous action.
This is a variant of cancel but it is not interruptible.
Cancel an asynchronous action by throwing the supplied exception to it.
cancelWith a e = throwTo (asyncThreadId a) eThe notes about the synchronous nature of cancel also apply to cancelWith.
A version of waitAny that can be used inside a MonadSTM
transaction.
Wait for any of the supplied asynchronous operations to complete. The value returned is a pair of the Async that completed, and the result that would be returned by wait on that Async.
If multiple Asyncs complete or have completed, then the value returned corresponds to the first completed Async in the list.
A version of waitAnyCatch that can be used inside a MonadSTM
transaction.
Like waitAny, but also cancels the other asynchronous operations as soon as one has completed.
Like waitAnyCatch, but also cancels the other asynchronous operations as soon as one has completed.
Wait for the first of two Asyncs to finish. If the Async
that finished first raised an exception, then the exception is
re-thrown by waitEither.
A version of waitEither that can be used inside a MonadSTM
transaction.
Wait for the first of two Asyncs to finish.
A version of waitEitherCatch that can be used inside a
MonadSTM transaction.
Like waitEither, but also cancels both Asyncs before
returning.
Like waitEitherCatch, but also cancels both Asyncs before
returning.
Like waitEither, but the result is ignored.
A version of waitEither_ that can be used inside a MonadSTM
transaction.
Waits for both Asyncs to finish, but if either of them throws
an exception before they have both finished, then the exception is
re-thrown by waitBoth.
A version of waitBoth that can be used inside a MonadSTM
transaction.
Link the given Async to the current thread, such that if the
Async raises an exception, that exception will be re-thrown in
the current thread.
Link two Asyncs together, such that if either raises an
exception, the same exception is re-thrown in the other Async.
Run two MonadConc actions concurrently, and return the first to
finish. The loser of the race is cancelled.
race left right =
withAsync left $ \a ->
withAsync right $ \b ->
waitEither a bLike race, but the result is ignored.
race_ left right =
withAsync left $ \a ->
withAsync right $ \b ->
waitEither_ a bRun two MonadConc actions concurrently, and return both
results. If either action throws an exception at any time, then the
other action is cancelled, and the exception is re-thrown by
concurrently.
concurrently left right =
withAsync left $ \a ->
withAsync right $ \b ->
waitBoth a bconcurrently_ is concurrently but ignores the return values.
Maps a MonadConc-performing function over any Traversable
data type, performing all the MonadConc actions concurrently, and
returning the original data structure with the arguments replaced
by the results.
For example, mapConcurrently works with lists:
pages <- mapConcurrently getURL ["url1", "url2", "url3"]mapConcurrently_ is mapConcurrently with the return value discarded, just like mapM_.
forConcurrently is mapConcurrently with its arguments flipped
pages <- forConcurrently ["url1", "url2", "url3"] $ \url -> getURL urlforConcurrently_ is forConcurrently with the return value discarded, just like forM_.
Perform the action in the given number of threads.
replicateConcurrently_ is replicateConcurrently with the return values discarded.
A value of type Concurrently m a is a MonadConc operation
that can be composed with other Concurrently values, using the
Applicative and Alternative instances.
Calling runConcurrently on a value of type Concurrently m a
will execute the MonadConc operations it contains concurrently,
before delivering the result of type a.
For example
(page1, page2, page3)
<- runConcurrently $ (,,)
<$> Concurrently (getURL "url1")
<*> Concurrently (getURL "url2")
<*> Concurrently (getURL "url3")ConcurrentlyrunConcurrently :: m aMonadConc m => Functor (Concurrently m)Defined in concurrency-1.11.0.3 · Control.Concurrent.Classy.AsyncMonadConc m => Applicative (Concurrently m)Defined in concurrency-1.11.0.3 · Control.Concurrent.Classy.AsyncMonadConc m => Alternative (Concurrently m)Defined in concurrency-1.11.0.3 · Control.Concurrent.Classy.Async(MonadConc m, Semigroup a) => Semigroup (Concurrently m a)Defined in concurrency-1.11.0.3 · Control.Concurrent.Classy.Async(MonadConc m, Monoid a) => Monoid (Concurrently m a)Defined in concurrency-1.11.0.3 · Control.Concurrent.Classy.Async