Re: [boost] library to support async/await pattern

Subject: Re: [boost] library to support async/await pattern
From: Oliver Kowalke (oliver.kowalke_at_[hidden])
Date: 2013-04-18 14:22:05

How does it compare to boost.fiber (github.com/olk/boost-fiber)?

2013/4/18 Valentin Milea <valentin.milea_at_[hidden]>

> Is there interest for a coroutine-based library to make asynchronous APIs
> easier to deal with?
>
> Typically, for each task there is cruft: chaining callbacks, managing
> intermediate state and error codes (since exceptions don't fit this model).
> The code flow get inverted and becomes difficult to follow.
>
> Recent versions of F# and C# solve this problem. They implement an await
> operator that effectively suspends the executing method until a task
> completes. The compiler takes care of transforming subsequent code into a
> continuation. Everything runs on the main thread, with asynchronous methods
> spending most time awaiting. N3328 proposes resumable functions of this
> kind in C++.
>
> For an immediate solution we could leverage the Boost.Context/Coroutine
> library. The resulting code may look like this:
>
>   try {
>     task = do_a_async(...)
>
>     // yield until task done
>     task.await();
>   } catch (const some_exception& e) {
>      // exceptions arrive in awaiting context
>   }
>
>   // normal code flow
>   for (auto& task : tasks1) {
>       task.await();
>   }
>
>   taskAny = await_any(tasks2);
>   taskAny.await();
>
>   ...
>
> There needs to be a representation for Awaitable tasks (similar to
> std::future but non-blocking). The other requirement is to have a Scheduler
> (run loop) in order to weave between coroutines.
>
> Benefits:
> - normal code flow: plain conditionals, loops, exceptions, RAII
> - algorithm state tracked on coroutine stack
> - async tasks are composable
> - any async API can be wrapped
>
> Cons:
> - must wrap async APIs (e.g. Boost.Asio)
> - needs std::exception_ptr to dispatch exceptions
> - stackful coroutines are sometimes difficult to debug
>
>
> I wrote an open-source library that does this:
> https://github.com/vmilea/CppAwait.
>
> It's far from Boost style but the concept looks sane. For a comparison
> between async patterns please see:
> https://github.com/vmilea/CppAwait/blob/master/Examples/ex_stockClient.cpp
>
> Making a Boost version would involve serious redesign. So is this worth
> pursuing?
>
> Thanks!
>
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