when_any support

[Timmmm]

Would it be possible to have a when_any function, similar to when_all but it returns when the first awaitable is ready instead of waiting for all of them? Similar to Promise.race() in JS or select_all() in Rust.

[jbaldwin]

Hi @Timmmm

I had an issue for a long time considering adding it #153 , and I think its still a good idea. The trouble I ran into is that I'd probably need to introduce some type of cancellation token (or std::stop_token?) like behavior to every task. It would also be on the user to cancel the task if a stop was requested. So its possibly a large API change, or maybe it can be sorta shoe-horned in. But I haven't tried yet.

[Timmmm]

Wouldn't it just stop resuming the other tasks once one of them has finished? That would be fine for my use case at least. I'm not using the normal threading runtime though so I don't know if that's ok there...

You could return the tasks that didn't complete to the caller of when_any() so that it could resume them if it wants. Something like this (conceptually; obviously you don't want this actual API):

std::set<task<T>> tasks;

while (!tasks.empty()) {
  std::set<task<T>> remaining_tasks;
  task<T> completed_task;
  T return_value = co_await when_all(tasks, &remaining_tasks);
  tasks = remaining_tasks;
}

(Sorry if that's way off base; I'm a coroutine noob.)