Documentation

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).

Compare two Asyncs that may have different types by their ThreadId.

Spawn an asynchronous action in a separate thread.

Like for forkIO, the action may be left running unintentionally (see module-level documentation for details).

Use withAsync style functions wherever you can instead!

Like async but using forkOS internally.

Like async but using forkOn internally.

Like async but using forkIOWithUnmask internally. The child thread is passed a function that can be used to unmask asynchronous exceptions.

Like asyncOn but using forkOnWithUnmask internally. The child thread is passed a function that can be used to unmask asynchronous exceptions.

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 = mask $ \restore -> do
  a <- async (restore action)
  restore (inner a) `finally` uninterruptibleCancel a

This is a useful variant of async that ensures an Async is never left running unintentionally.

Note: a reference to the child thread is kept alive until the call to withAsync returns, so nesting many withAsync calls requires linear memory.

Like withAsync but uses forkOS internally.

Like withAsync but uses forkOn internally.

Like withAsync but uses forkIOWithUnmask internally. The child thread is passed a function that can be used to unmask asynchronous exceptions.

Like withAsyncOn but uses forkOnWithUnmask internally. The child thread is passed a function that can be used to unmask asynchronous exceptions

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

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.

waitCatch = atomically . waitCatchSTM

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 . pollSTM

A version of wait that can be used inside an STM transaction.

A version of waitCatch that can be used inside an STM transaction.

A version of poll that can be used inside an STM transaction.

Cancel an asynchronous action by throwing the AsyncCancelled 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) AsyncCancelled <* waitCatch a

Note that cancel will not terminate until the thread the Async refers to has terminated. This means that cancel will block for as long said thread blocks when receiving an asynchronous exception.

For example, it could block if:

• It's executing a foreign call, and thus cannot receive the asynchronous exception;
• It's executing some cleanup handler after having received the exception, and the handler is blocking.

Cancel multiple asynchronous actions by throwing the AsyncCancelled exception to each of them in turn, then waiting for all the Async threads to complete.

The exception thrown by cancel to terminate a thread.

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 x = throwTo (asyncThreadId a) x

The notes about the synchronous nature of cancel also apply to cancelWith.

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. The input list must be non-empty.

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 an STM transaction.

Since: 2.1.0

Like waitAnyCatch, but also cancels the other asynchronous operations as soon as one has completed.

Wait for any of the supplied Asyncs to complete. If the first to complete throws an exception, then that exception is re-thrown by waitAny. The input list must be non-empty.

If multiple Asyncs complete or have completed, then the value returned corresponds to the first completed Async in the list.

A version of waitAny that can be used inside an STM transaction.

Since: 2.1.0

Like waitAny, but also cancels the other asynchronous operations as soon as one has completed.

Wait for the first of two Asyncs to finish.

A version of waitEitherCatch that can be used inside an STM transaction.

Since: 2.1.0

Like waitEitherCatch, but also cancels both Asyncs before returning.

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 an STM transaction.

Since: 2.1.0

Like waitEither, but the result is ignored.

A version of waitEither_ that can be used inside an STM transaction.

Since: 2.1.0

Like waitEither, but also cancels both Asyncs before returning.

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 an STM transaction.

Since: 2.1.0

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, wrapped in ExceptionInLinkedThread.

link ignores AsyncCancelled exceptions thrown in the other thread, so that it's safe to cancel a thread you're linked to. If you want different behaviour, use linkOnly.

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, wrapped in ExceptionInLinkedThread.

The supplied predicate determines which exceptions in the target thread should be propagated to the source thread.

Link two Asyncs together, such that if either raises an exception, the same exception is re-thrown in the other Async, wrapped in ExceptionInLinkedThread.

link2 ignores AsyncCancelled exceptions, so that it's possible to cancel either thread without cancelling the other. If you want different behaviour, use link2Only.

Link two Asyncs together, such that if either raises an exception, the same exception is re-thrown in the other Async, wrapped in ExceptionInLinkedThread.

The supplied predicate determines which exceptions in the target thread should be propagated to the source thread.

Run two IO 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 b

Like race, but the result is ignored.

Run two IO 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 b

Run two IO actions concurrently. If both of them end with Right, return both results. If one of then ends with Left, interrupt the other action and return the Left.

concurrently, but ignore the result values

Since: 2.1.1

Maps an IO-performing function over any Traversable data type, performing all the IO actions concurrently, and returning the original data structure with the arguments replaced by the results.

If any of the actions throw an exception, then all other actions are cancelled and the exception is re-thrown.

For example, mapConcurrently works with lists:

pages <- mapConcurrently getURL ["url1", "url2", "url3"]

Take into account that async will try to immediately spawn a thread for each element of the Traversable, so running this on large inputs without care may lead to resource exhaustion (of memory, file descriptors, or other limited resources).

forConcurrently is mapConcurrently with its arguments flipped

pages <- forConcurrently ["url1", "url2", "url3"] $ \url -> getURL url

Since: 2.1.0

mapConcurrently_ is mapConcurrently with the return value discarded; a concurrent equivalent of mapM_.

forConcurrently_ is forConcurrently with the return value discarded; a concurrent equivalent of forM_.

Perform the action in the given number of threads.

Since: 2.1.1

Same as replicateConcurrently, but ignore the results.

Since: 2.1.1

A value of type Concurrently a is an IO operation that can be composed with other Concurrently values, using the Applicative and Alternative instances.

Calling runConcurrently on a value of type Concurrently a will execute the IO 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")

A value of type ConcurrentlyE e a is an IO operation that can be composed with other ConcurrentlyE values, using the Applicative instance.

Calling runConcurrentlyE on a value of type ConcurrentlyE e a will execute the IO operations it contains concurrently, before delivering either the result of type a, or an error of type e if one of the actions returns Left.

| @since 2.2.5

Fork a thread that runs the supplied action, and if it raises an exception, re-runs the action. The thread terminates only when the action runs to completion without raising an exception.