Re: [boost.cobalt] A precondition or an error condition?

From: Klemens Morgenstern (klemensdavidmorgenstern_at_[hidden])
Date: 2023-11-09 14:09:08

On Thu, Nov 9, 2023 at 9:41 PM Andrzej Krzemienski via Boost
<boost_at_[hidden]> wrote:
>
> Hi Everyone,
> I would like to discuss one correctness aspect of Boost.COBALT. We have
> this list and the Slack channel, so I am not sure which place is better.
> Let me do it here.
>

I don't see why this is a correctness issue and not a design choice.
When is it correct to use an exception?

> In Boost.Cobalt (formerly Boost.Async) we have function cobalt::race
> (formerly select()) that takes a range of awaitables `p` and returns an
> awaitable capable on awaiting on one of the elements of `p`:
>
> https://www.boost.org/doc/libs/develop/libs/cobalt/doc/html/index.html#race-outline
>
> Example:
>
> ```
> promise<string> talk_to_server_1();
> promise<string> talk_to_server_2();
> promise<string> timeout_with_default();
>
> vector<promise<int>> servers = { talk_to_server_1(), talk_to_server_2(),
> timeout_with_default() };
> pair<size_t, string> ans = co_await race(servers);
> ```
>
> This obviously cannot work when the input range passed to `race` is empty.

This example doesn't help, because this should obviously use the
variadic version.

The reason to use the ranged one is for when you have another
component dictating what elements to listen to.

try
{
    vector<promise<int>> servers;
    for (const auto & server : config.get_servers())
        servers.push_back(talk_to_server(server));

    co_await race(servers);
}
catch(std::exception &)
{
   // handle errors from running any server
}

That means the code using the ranged version is likely unaware of the
actual value, because there's a very high likelihood it's not known at
compile time.