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Subject: Re: [std-proposals] Making coroutines more explicit.
From: Jeremy Ong (jeremycong_at_[hidden])
Date: 2019-08-21 22:07:50


Can you shed light on why P1485 was rejected? It strikes me as an
exceedingly sensible change

On Wed, Aug 21, 2019 at 9:18 AM Arthur O'Dwyer via Std-Proposals <
std-proposals_at_[hidden]> wrote:

> On Wed, Aug 21, 2019 at 11:11 AM Robert Kubok via Std-Proposals <
> std-proposals_at_[hidden]> wrote:
>
>> Hello,
>> I have a proposal: make corotines more explicit in use, than it is
>> proposed right now. The thing is: a function is treated as coroutine, when
>> a 'co_yield' or 'co_return' keyword is used inside callable object's body.
>> Whether a function is a coroutine or not, is determined by it's body, not
>> by it's definition. In my opinion this is a language flaw. Therefore I
>> suggest creating a new keyword: 'coroutine', which is going to be a
>> type-specifier to a function or lambda expression and then, changing the
>> upcoming keywords related to coroutines ('co_await', 'co_yield',
>> 'co_return' to 'await', 'yield' and 'return' respectively). As always, it
>> is better to see the syntax in action, so there's the example:
>>
>> coroutine auto iota(int n = 0)
>> {
>> while(true)
>> yield n++;
>> }
>>
>> would be equivalent to proposed:
>>
>> auto iota(int n = 0)
>> {
>> while(true)
>> co_yield n++;
>> }
>>
>> Or with lambda expression:
>> auto getFive = []() coroutine // in the same place as 'constexpr' keyword
>> {
>> return 5;
>> };
>>
>> which of course would be equivalent to:
>> auto getFive = []()
>> {
>> co_return 5;
>> }
>>
>> Why 'coroutine' keyword? Firstly, coroutine should be specified by it's
>> *declaration*,
>>
>
> Nit: You mean "definition," not "declaration." Coroutines are already
> designed so that from the outside a coroutine function looks exactly like a
> subroutine function, and in fact can be implemented as either a coroutine
> or a subroutine. The caller doesn't have to know or care. They just see a
> function returning a foo::future or a foo::generator, which is awaitable;
> how the innards of that future or generator work isn't relevant to the
> caller.
>
> Your proposal is basically identical with Antony Polukhin's P1485 "Better
> keywords for coroutines."
> <http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2019/p1485r1.html>
> https://quuxplusone.github.io/blog/2019/06/26/pro-p1485/
>
> Unfortunately, unless WG21 shapes up real fast, it's "too late" for any
> such change to coroutines. You'd have to fork C++ to get a better syntax at
> this point. :(
>
> –Arthur
> --
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>



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