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Subject: Re: [boost] Futures
From: Thomas Heller (thom.heller_at_[hidden])
Date: 2015-01-04 07:37:22

On Sunday, January 04, 2015 12:46:26 Bjorn Reese wrote:
> On 01/04/2015 11:25 AM, Thomas Heller wrote:
> > What's the difference between async_result and a future? I am unable to
> > find that in the ASIO documentation.
> async_result is like a return type trait. In its generalized form in
> Asio it supports callbacks, but has be specialized for futures and
> coroutines. Other mechanisms can be added; for instance, Boost.Fiber
> comes with an async_result specialization.
> The important point with that design is that with async_result the
> user of an asynchronous function gets to decide which mechanism to use.
> Examples:
> // Callback
> socket.async_send(buffer, [] (error_code, size_t) { });
> // Future
> auto myfuture = socket.async_send(buffer, use_future);
> // Coroutine
> socket.async_send(buffer, yield);

Interesting. Let me try to explain what I am seeing in the future concept first
and try to translate that async_result:
For me, the concept behind futures is twofold, you have the future<T> on the
*receiving* side which is, apart from the return type of the async operation,
a completely type erased (as in type of asynchronous operation) handle to a
future result. Things like promise, packaged_task or whatnot represent the
*sending* side of the operation. That means, the only difference here is how
the future result is being computed (locally, remotely or on a OpenCL device).
This mechanism is completely hidden from the user, and currently a
implementation detail of the various different libraries (with HPX being the
exception here). And I believe this is a good thing. async_result seems to let
the user decide how she would like to have the result produced, at least it
looks like it ... after looking at the implementation it's "just" a trait from
ASIO's async model to whichever you prefer. That could be done with different
future islands just as well. The important part, for me, *how* the is being
produced is not covered. In addition, it doesn't solve the problem of how to
compose different future-like types or async concepts.

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