Re: Capy Review, take 2

On Tuesday, June 30th, 2026 at 2:45 PM, Rainer Deyke via Boost <[email protected]> wrote:

> The strength of coroutines is that they allow chains of callbacks to be
> written as if they were linear functions.  Using capy::run reads like a
> callback.  Using resume_on reads like a linear function.
> 
> Callback version:
> 
>    task<> f() {
>      auto image = co_await load_image();
>      main_thread_callbacks.push_back([image = std::move(image)] {
>        upload_texture_to_opengl(image);
>      });
>    }
> 
> capy::run version:
> 
>    task<> f() {
>      auto image = co_await load_image();
>      capy::run(main_thread_executor)([image = std::move(image)] {
>        upload_texture_to_opengl(image);
>      });
>    }
> 
> resume_on version:
> 
>    task<> f() {
>      auto image = co_await load_image();
>      resume_on(main_thread_executor);
>      upload_texture_to_opengl(image);
>    }
> 
> Can you see the similarity of the first two, and how the third one differs?

I found your argument compelling which led to me to think more about resume_on(). I also injected my own assumption that you meant "co_await resume_on(ex);" -- correct me if I'm wrong.

Let's explore another example:

task<> task_B() {
  co_await resume_on( other_ex );
}

task<> task_A() {
    for(;;)
      co_await task_B();
}

I think this may illustrate the point of contention. If you launch task_A on a certain executor, we currently can make that guarantee to the user that task_A will run on said executor.

I suspect the implementation you desire for resume_on(ex) is essentially a cheap/efficient executor switch. If we did that, task_B can switch the executor that task_A resumes on which breaks what we're trying to do here. If that implementation is not correct, then it needs to move closer to what run() already does.
_______________________________________________
Boost mailing list -- [email protected]
To unsubscribe send an email to [email protected]
https://lists.boost.org/mailman3/lists/boost.lists.boost.org/
Archived at: https://lists.boost.org/archives/list/[email protected]/message/TFRN3TBTLL6CFHAL66JY2LE4JIWV3MUZ/