Async/await implementation in Haskell

Disclaimer: This is not an officially supported Google product.

Goals

In this package we explore how to implement the async/await programming pattern such that:

• It allows to write asynchronous computations in synchronous style: use async to schedule computations in other threads and await for them to finish.
• Implement the internals in purely asynchronous way. That is, no thread must ever wait blocked. All threads must either do work or finish.
• Keep the implementation idependent of a particular scheduling/threading implementation.

The second condition ensures that even if there is limited amount of threads, thread starvation won't occur. In particular, computations can be run even in a single-threaded environment and are sequenced appropriately.

Note: This is somewhat different approach from the one taken by the async Haskell package, which relies on Haskell threads and blocking waits on compuations using STM.

Note: Currently exceptions are not dealt with.

Future plans/ideas

• Exceptions. An exception thrown within an async computation should be raised in all others that await on it, rather than dying silently.
• Race. For two or more asynchronous computations return the first available result. Note that such an addition makes the computations indeterministic in the sense that the outcome depends on the order of evaluation even in cases when computations cannot observer each others' side effects. On the other hand, some concurrency applications cannot be implemented without racing.
• Cancellation (if feasible).

Implementation

The continuation monad allows a computation to explicitly access the whole code that follows it. Instead of returning its result, it passes it as an argument to a continuation. Among other things, this allows a computation to suspend itself by storing the continuation somewhere, and later and resume it.

When specialized to (a -> IO ()) -> IO (), we can work with asynchronous IO operations in synchronous style. For example, if we schedule computations in threads, it is possible to write:

do
  startThread <- liftIO myThreadId
  reschedule
  endThread <- liftIO myThreadId

where reschedule suspends the computation and immediately resumes it in a different thread. Hence startThread and endThread will contain different values. See also testThreadScheduler in Async_test.hs.

The same principle is used when waiting for another computation inside await. If the other computation hasn't finished yet, the current continuation is added to its list of callbacks to invoke once the result is available.