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

On Thu, Nov 9, 2023 at 9:41 PM Andrzej Krzemienski via Boost 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-...

Example:

``` promise<string> talk_to_server_1(); promise<string> talk_to_server_2(); promise<string> timeout_with_default();

vector servers = { talk_to_server_1(), talk_to_server_2(), timeout_with_default() }; pair 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 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.