Creates a simple, non-cancelable F[A] instance that executes an asynchronous process on evaluation.

The given function is being injected with a side-effectful callback for signaling the final result of an asynchronous process.

This operation could be derived from asyncF, because:

F.async(k) <-> F.asyncF(cb => F.delay(k(cb)))

As an example of wrapping an impure async API, here's the implementation of Async.shift:

def shift[F[_]](ec: ExecutionContext)(implicit F: Async[F]): F[Unit] =
  F.async { cb =>
    // Scheduling an async boundary (logical thread fork)
    ec.execute(new Runnable {
      def run(): Unit = {
        // Signaling successful completion
        cb(Right(()))
      }
    })
  }

Creates a simple, non-cancelable F[A] instance that executes an asynchronous process on evaluation.

The given function is being injected with a side-effectful callback for signaling the final result of an asynchronous process. And its returned result needs to be a pure F[Unit] that gets evaluated by the runtime.

Note the simpler async variant async can be derived like this:

F.async(k) <-> F.asyncF(cb => F.delay(k(cb)))

For wrapping impure APIs usually you can use the simpler async, however asyncF is useful in cases where impure APIs are wrapped with the help of pure abstractions, such as Ref.

For example here's how a simple, "pure Promise" implementation could be implemented via Ref (sample is for didactic purposes, as you have a far better Deferred available):

import cats.effect.concurrent.Ref

type Callback[-A] = Either[Throwable, A] => Unit

class PurePromise[F[_], A](ref: Ref[F, Either[List[Callback[A]], A]])
  (implicit F: Async[F]) {

  def get: F[A] = F.asyncF { cb =>
    ref.modify {
      case current @ Right(result) =>
        (current, F.delay(cb(Right(result))))
      case Left(list) =>
        (Left(cb :: list), F.unit)
    }
  }

  def complete(value: A): F[Unit] =
    F.flatten(ref.modify {
      case Left(list) =>
        (Right(value), F.delay(list.foreach(_(Right(value)))))
      case right =>
        (right, F.unit)
    })
}

N.B. if F[_] is a cancelable data type (i.e. implementing Concurrent), then the returned F[Unit] can be cancelable, its evaluation hooking into the underlying cancelation mechanism of F[_], so something like this behaves like you'd expect:

def delayed[F[_], A](thunk: => A)
  (implicit F: Async[F], timer: Timer[F]): F[A] = {

  timer.sleep(1.second) *> F.delay(cb(
    try cb(Right(thunk))
    catch { case NonFatal(e) => Left(cb(Left(e))) }
  ))
}

The asyncF operation behaves like Sync.suspend, except that the result has to be signaled via the provided callback.

ERROR HANDLING

As a matter of contract the returned F[Unit] should not throw errors. If it does, then the behavior is undefined.

This is because by contract the provided callback should only be called once. Calling it concurrently, multiple times, is a contract violation. And if the returned F[Unit] throws, then the implementation might have called it already, so it would be a contract violation to call it without expensive synchronization.

In case errors are thrown the behavior is implementation specific. The error might get logged to stderr, or via other mechanisms that are implementations specific.

A generalized version of bracket which uses ExitCase to distinguish between different exit cases when releasing the acquired resource.

Evaluates F[_], with the effect of starting the run-loop being suspended in the SyncIO context.

val io = F.runAsync(fa)(cb)
// Running io results in evaluation of `fa` starting
io.unsafeRunSync

Suspends the evaluation of an F reference.

Equivalent to FlatMap.flatten for pure expressions, the purpose of this function is to suspend side effects in F.

Operation meant for specifying tasks with safe resource acquisition and release in the face of errors and interruption.

This operation provides the equivalent of try/catch/finally statements in mainstream imperative languages for resource acquisition and release.

Alias for suspend that suspends the evaluation of an F reference and implements cats.Defer typeclass.

Lifts any by-name parameter into the F context.

Equivalent to Applicative.pure for pure expressions, the purpose of this function is to suspend side effects in F.

Executes the given finalizer when the source is finished, either in success or in error, or if canceled.

This variant of guaranteeCase evaluates the given finalizer regardless of how the source gets terminated:

• normal completion
• completion in error
• cancelation

This equivalence always holds:

F.guarantee(fa)(f) <-> F.bracket(F.unit)(_ => fa)(_ => f)

As best practice, it's not a good idea to release resources via guaranteeCase in polymorphic code. Prefer bracket for the acquisition and release of resources.

Executes the given finalizer when the source is finished, either in success or in error, or if canceled, allowing for differentiating between exit conditions.

This variant of guarantee injects an ExitCase in the provided function, allowing one to make a difference between:

• normal completion
• completion in error
• cancelation

This equivalence always holds:

F.guaranteeCase(fa)(f) <-> F.bracketCase(F.unit)(_ => fa)((_, e) => f(e))

As best practice, it's not a good idea to release resources via guaranteeCase in polymorphic code. Prefer bracketCase for the acquisition and release of resources.

Inherited from LiftIO, defines a conversion from IO in terms of the Async type class.

N.B. expressing this conversion in terms of Async and its capabilities means that the resulting F is not cancelable. Concurrent then overrides this with an implementation that is.

To access this implementation as a standalone function, you can use Async.liftIO (on the object companion).

Returns a non-terminating F[_], that never completes with a result, being equivalent to async(_ => ())

Executes the given finalizer when the source is canceled.

The typical use case for this function arises in the implementation of concurrent abstractions, which generally consist of operations that perform asynchronous waiting after concurrently modifying some state: in case the user asks for cancelation, we want to interrupt the waiting operation, and restore the state to its previous value.

waitingOp.onCancel(restoreState)

A direct use of bracket is not a good fit for this case as it would make the waiting action uncancelable.

NOTE: This function handles interruption only, you need to take care of the success and error case elsewhere in your code

Convert to an IO[A].

The law is that toIO(liftIO(ioa)) is the same as ioa