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Date: Sat, 9 Sep 2017 14:27:44 +0800
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To: Rowan Collins <rowan.collins@gmail.com>
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Subject: Re: [PHP-DEV] add Fiber (sackful coroutine) support
From: i@lvht.net (Haitao Lv)


> On 2 Sep 2017, at 20:19, Rowan Collins <rowan.collins@gmail.com> =
wrote:
>=20
> On 1 September 2017 14:02:29 BST, Haitao Lv <i@lvht.net> wrote:
>>=20
>> Fiber is a lightweight thread. Please see
>> https://en.wikipedia.org/wiki/Fiber_(computer_science)
>> And ruby support Fiber. Please see
>> https://ruby-doc.org/core-2.4.1/Fiber.html
>=20
> Ah, thanks, that makes more sense now.
>=20
> I note that the examples there all implement it not as a keyword, but =
as a library function, which maybe makes more sense: whereas "yield" =
turns a function declaration into a generator declaration, =
"Fiber\yield", as we might call it, is just a function call which can =
happen anywhere and manipulates some global state.

As the Zend Engine does not offer API to pause execution, I cannot =
implement the await without
introducing a new keyword (just like yield).

> The choice of "await" also feels odd: you're not awaiting the thing on =
the right-hand side of the keyword, you're sending it somewhere and =
awaiting something else.

await is chosen because we cannot reuse the `yield` keyword. Maybe we =
can choose a more proper keyword.

>> You can see the await as a resumable return. The value after it
>> will be returned and the function
>> will be paused.
>=20
> Again, this explanation doesn't make sense if you can call foo() =
without any Fiber code. If there isn't an active Fiber to pause, the =
"await" line can't "return" anything; it will presumably throw an error =
of some sort.
>=20
> function foo($x) {
>     $y =3D Fiber\yield($x);
>     return $x + $y;
> }
> echo foo(1);
>=20
> The 1 isn't "returned" to the echo, it's passed off to a global =
function for further processing. This makes much more sense to me than =
implying that foo() is no longer a normal function.

await is not yield. It need a separate vm stack to work. So call the foo =
function without a Fiber will
throw a runtime error exception.

>=20
>>> Your ->resume() doesn't seem to do anything a normal generator =
can't,
>> except that the yield is nested deeper.
>>=20
>> The await is a nested deeper yield!
>=20
> Yes, I understand that, but that seems to be completely unrelated to =
the API design of having resume() act as start, continue, and final =
result all rolled into one. As I say, I didn't entirely follow the =
reasoning for allowing return in generators, but if Fibers are used for =
the same purpose, I would expect the same concerns to arise.
>=20
> However, I think the comparison to generators may be more distracting =
than useful, as they seem to be very different solutions to the same or =
related problems.

If fiber were implemented, the generator would be a special fiber.