Re: [boost.async] Some feedback

From: Andrzej Krzemienski (akrzemi1_at_[hidden])
Date: 2023-08-29 23:13:05

sob., 19 sie 2023 o 13:48 Matthijs Möhlmann <matthijs_at_[hidden]>
napisał(a):

> On 19/08/2023 00:05, Andrzej Krzemienski via Boost wrote:
> > Hi Everyone,
> > I will not be able to write a decent review due to the lack of time (I am
> > now on vacation), but I wanted to share some partial feedback.
> >
> > I am not well familiar with C++ coroutines. I wrote some toy examples of
> an
> > async TCP echo server in ASIO in the past, to try to learn how coroutines
> > work, but my knowledge is very superficial. I hoped to understand
> > coroutines better from studying  Boost.Async's docs.
> >
> > When I discovered Boost some 18 years ago, I found it useful not only
> > because of the high quality libraries but also because of extraordinary
> > documentation that often provided a very deep insight into the problem
> > domain, a lot of rationale, design goals, and the documentation of
> > engineering trade-offs of different sorts. I do not know how people feel
> > about the importance of documentation nowadays. For me, however, it is
> the
> > primary focus. I devoted like 10 hours in total for studying the docs and
> > running some toy examples. Because I am not very good at C++ coroutines,
> > you get a typical, I would think, feedback from a user that wants to
> learn
> > how coroutines are used, what problems they solve, and what problems they
> > do not solve.
> >
> > Regarding the *really* asynchronous part, promises and tasks, I guess
> they
> > would work fine. By "really asynchronous" I mean the situations where a
> > coroutine is resumed by events unpredictable by the programmer (when data
> > arrives, when certain time passes, when an external connection is
> > requested). I also guess that when Marcelo and Ruben say they
> successfully
> > integrated Boost.Async into their projects, they meant types `task` and
> > `promise`, rather than `generator`.
> >
> > My investigation was interrupted by `generator` which I cannot understand
> > or appreciate. It feels like a type quite separate from the rest of the
> > library. For instance, through all the simple examples in the docs, only
> > the examples for `generator` never use the trick with `co_await
> > async::this_coro::executor`.
> >
> > I needed to ask a lot of questions, and even though Klemens is very
> > responsive and very helpful, I still do not understand how
> > `async::generator` solves real problems. This may indicate that either I
> am
> > a very slow learner, or that there is something wrong with the design of
> > `generator`. But even if it is the former, I think I represent the
> average
> > user of this library.
> >
> > When I try to use a C+23 std::generator, the usage is intuitive and
> > straightforward to me. I know how to easily yield a sequence of values,
> and
> > on the caller side how to easily check if I can still obtain the next
> value
> > from the generator:
> >
> > ```
> > std::generator<int> seq(int from, int to)
> > {
> >      for (int i = from; i <= to; ++i)
> >          co_yield i;
> > }
> >
> > int main()
> > {
> >      for(int i : seq(1, 4))
> >          std::cout << i << std::endl;
> > }
> > ```
> > https://godbolt.org/z/PTvz3v4Y6
> >
> > But when I try what I believe to be an analogous example in Boost.Async,
> I
> > get an exception:
> >
> > ```
> > async::generator<int> seq(int from, int to)
> > {
> >      for (int i = from; i <= to; ++i)
> >          co_yield i;
> > }
> >
> > async::main co_main(int argc, char* argv[])
> > {
> >      auto s = seq(1, 4);
> >      while (s) {
> >          std::cout << co_await s << std::endl;
> >      }
> > }
> > ```
> >
> > And the docs or examples don't help me understand why this use case
> doesn't
> > work.
> > The case with the eager generator that can be injected values is even
> > weirder. I think it is wrong. Klemens thinks otherwise, but the docs do
> not
> > give any convincing example of where and how it could be used.
> >
> > The lack of examples or description can be just that, but it could also
> > indicate that the design for generators is missing something.
> >
> > In this feedback, I am only mentioning generators, because this is the
> > place where I stuck in my review. I guess that other parts of the library
> > do not have this problem.
> >
> > I am pretty sure that this library will be accepted into Boost at some
> > point. It addresses a real need (although not necessarily the
> generators),
> > and the feedback from Ruben and Marcelo are strong testimony to the
> quality
> > of the library.
> >
> > But at this point, I am not comfortable with adding a library whose
> design
> > for generators cannot be explained in the docs to the extent that I
> > consider satisfactory.
> >
> > Would this library lose in usability if generators were removed?
> >
> > I know it is not a fair review. I only covered one subject, and my
> feeling
> > is that it is not the most relevant one. So please consider it as "just
> > feedback" rather than a review.
> >
> > Again, thanks Klemens for writing and sharing this library. I hope that
> > this message does not come across as negative. I want to think that I am
> > contributing to the quality of Boost.Async.
> >
> > Regards,
> > &rzej;
> >
> > _______________________________________________
> > Unsubscribe & other changes:
> http://lists.boost.org/mailman/listinfo.cgi/boost
>
> Just a question, did you also look at the ticker example? The generator is
> used there.
>

Thanks!
I missed that one.

Regards,
&rzej;

>
> Regards, Matthijs
>
>