Re: [Capy] Review Note

On Sun, Jun 28, 2026 at 11:25 AM Andrzej Krzemienski <[email protected]>
wrote:

> Actually, I do not think I understand this statement. What does it mean
> when you call the concept `IoAwaitable` a protocol?
>

A protocol in this context means the type requirements plus the behavioral
requirements.


> 1. What is this concept for? Are programmers/other libraries
> allowed/expected to provide their own types modelling `IoAwaitable`? Or
> does anyone is required to only use concrete types that come with Capy?
>
There are three tiers of users identified in the companion design document

IoAwaitable for Coroutine-Native Byte-Oriented I/O
https://isocpp.org/files/papers/P4172R1.pdf

1. Application developers
2. Framework authors
3. I/O library authors

Framework authors and I/O library authors would provide their own types
modeling IoAwaitable. Users would simply consume the ones in Capy, or the
ones provided by other libraries.

2. If it is the latter, why are we even hearing about this protocol? If it
> is the former, then I think the "protocol" is way underspecified.
>

Capy serves the three audiences described above.


> 3. I can write my own coroutine return type that has two-argument
> await_suspend, but lacks await_ready and await_resume. It will pass the
> IoAwaitable check, but I cannot use it inside a coroutine with capy::task
> as a return type. Apparently, there are more syntactic requirements here.
>

The IoAwaitable documentation states:

"An awaitable satisfies IoAwaitable if..."

An IoAwaitable must be an awaitable, which requires at a minimum the three
members described.


> 4. And IoAwaitable doesn't mention any semantic requirements. Concepts
> without semantic requirements are not concepts.
>

https://develop.capy.cpp.al/capy/reference/boost/capy/IoAwaitable.html#_semantic_requirements


> 5. Is it enough for my type to satisfy the concept or is there more that I
> have to do? How do I make `co_await this_coro::environment` work? Is it
> even part of the "IoAwaitable protocol"? How is the environment propagated
> down to other people's coroutines (with their own return types) unless I
> manually make it work?
>

"co_await this_coro::environment" is a feature of the task type. It has
nothing to do with the IoAwaitable protocol, hence it is silent on it.

Environments are propagated automatically to any awaitable which implements
the IoAwaitable protocol, that's the point of it.


> 6. What happens with my promise_type's initial_suspend and final_suspend?
> Are they invoked or overridden by capy::tasks's await_transform?
>

I believe initial_suspend and final_suspend are invoked directly by the
compiler and are not affected (Klemens?)


> I hope that questions demonstrate how unclear it is to the reader what
> "IoAwaitable protocol" is.
>

Well... :)

Thanks
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