Re: [Async] Peer review report and conditions

From: Klemens Morgenstern (klemensdavidmorgenstern_at_[hidden])
Date: 2023-10-12 00:39:45

> > >
> > > The reason is that we don't have "async for".
> > > I would need to resume the generator on increment,
> > > but if `itr++` is an awaitable expression that needs to be co_awaited,
> > > it's not an iterator.
> > > If there was an async for (or a common pattern) I'd make generator work with it.
> >
> > Y'see I don't see the need for async for. That's separate to this.
> >
> > `std::generator` provides iterators. When you iterate one of its
> > iterators the coroutine gets resumed and it yields a new value.
> >
> > The reason `std::generator` is like this is because it can be called
> > from non-coroutine code, so `co_await` isn't available. Implied in this
> > is iterator implementations must pump the coroutine by hand.
> >
> > This is very straightforward for the simple case, it gets harder if the
> > generator suspends on say i/o. Then your generator iterator
> > implementation needs to pump the event loop too. This isn't a hard to
> > surmount problem to solve for Boost.Async I think. And I think a Boost
> > level library does need to implement what the C++ standard demands, even
> > if it's a bit dumb.
> >
>
> Ah, gotcha I thought you wanted that to be in async, i.e. with a
> co_await somewhere.
> Yeah that could be done & might actually be something useful for
> non-pushable generators.
> I'll look into it.
>

So the earlier versions of N4775 that had `for co_await` would have
made the `++itr` awaitable.
With std::generator the increment also rethrows exceptions.

That means I could in theory have an async_for do a `co_await
async::next(itr)` instead of `co_await ++itr` and have the same
iterator type work.

There is however a problem with "pumping the event-loop". I have the
executor but I might not know the event-loop it's referring to (it
might be a thread_pool) or something custom.
Now I would either need to somehow create an interface to allow the
user to supply a pump function or require that he uses
asio::io_context for that to work.
I don't like either, I think I'll hold off until there's a user asking for it.