Re: [boost] [boost.async] Some questions

On Fri, Aug 18, 2023, 04:36 Klemens Morgenstern via Boost < boost@lists.boost.org> wrote:

On Fri, Aug 18, 2023 at 6:51 AM Andrzej Krzemienski via Boost wrote:

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czw., 17 sie 2023 o 23:55 Ruben Perez

napisał(a):

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One more question. This interface of async::generator,

taking

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two

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parameters, where one can not only generate values from the generator, but also obtain values: is there a real-life use case for this?

I'd say major languages like Python and JS allow for this, too. So if you're coming from these, it makes sense.

Thanks, but still, could someone show a plausible real-life example of this written in Boost.Asynch? I am not familiar with Python's or JS's coroutines. But do they have an *identical* interface?

Not identical, you need to call `send` in python, instead of operator().

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When I was trying to come up with an example, I found the results surprising:

auto output1 = co_await generator(input1); auto output2 = co_await generator(input2);

I expected that this instruction would mean "take input2, suspend, and

when

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resumed return value computed from input2". But because the implementation in the coroutine has to read:

auto next_input = co_yield compute(input);

The consequence is that the co_awaits actually mean "take input2, suspend, and when resumed return value computed from input1".

generators have this "weird" kind of overlap by their nature. They can be made lazy but then the inner workings get utterly confusing too, because where does the input1 come from before the co_yield?

i.e. in your example:

async::generator client(Task t) { for (int i = 0; ; ++i < 100) { std::cout << "processing: " << t.value << std::endl; t = co_yield Result{ std::format("result-{}-{}", i, t.value) }; } co_return Result{ std::format("result-{}-{}", 100, t.value) }; }

when I do the first co_await g(t) - where does `t` go? You're in the co_yield using the t passed in through the argument list, so it's not clear what's going on either. I actually did this in asio::experimental::coro, and I found it worse.

There might be an option to support this with a runtime_option, e.g.:

async::generator client() { auto t = co_await async::this_coro::initial; // wait for the first co_await & make the generator lazy. for (int i = 0; ; ++i < 100) { std::cout << "processing: " << t.value << std::endl; t = co_yield Result{ std::format("result-{}-{}", i, t.value) }; } co_return Result{ std::format("result-{}-{}", 100, t.value) }; }

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Maybe I am doing something wrong, I would like to be corrected. The argument that other languages have it is not a valid one for me. I would still like to know if this has a use case when implemented as it is with C++ coroutines.

I think you're just looking for a lazy generator and I made it eager. There's no reason I couldn't support both.

There's still one thing I don't understand, thoug. The docs say, the difference between the promise and the task type are in eagerness/laziness. But for generator, in the PR, laziness is enabled "manually" with co_await instruction. Couldn't the same be applied to the promise? Is there a value in encoding the laziness property in the type? If so, why the same argument would not apply to generator? Regards, &rzej;

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I enclose my example, where I tried to model a producer and consumer situation, and concluded that I couldn't.

Regards, &rzej;

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