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Date: Tue, 03 Mar 2015 12:27:48 +0000
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Subject: Re: [PHP-DEV] [RFC Discuss] Generator Delegation
From: rowan.collins@gmail.com (Rowan Collins)

Bob Weinand wrote on 03/03/2015 12:08:
>> Am 03.03.2015 um 12:34 schrieb Rowan Collins <rowan.collins@gmail.com>:
>>
>> Niklas Keller wrote on 03/03/2015 10:55:
>>>     Gr, top-posting...
>>>
>>>
>>> Sorry, was on mobile. ;-)
>>>
>>>     However, since the existence of the word "yield" is the only thing
>>>     that
>>>     marks a coroutine now, how about using a variant of that for the final
>>>     value, e.g. "yield final $foo"?
>>>
>>>
>>> What's the final value? The last "yield"ed value or a return?
>> "yield final" would mark the final result of the coroutine, as opposed to the intermediate values passed out with a normal "yield".
>>
>>
>>> Just to give you some real world example:
>>> If you're using "return", it would look like that:
>>>
>>> public function getSession ($sessionId) {
>>>     $result = yield $this->redis->get("session.{$sessionId}")); // We're waiting here until redis responded.
>>>     return json_decode($result);
>>> }
>> My suggestion is simply to change the keyword:
>>
>> public function getSession ($sessionId) {
>>     $result = yield $this->redis->get("session.{$sessionId}")); // We're waiting here until redis responded.
>>     yield final json_decode($result);
>> }
>>
>>
>> The reasoning being that when you run getSession(42), it *doesn't* return the result of json_decode(), it returns a pointer to the coroutine's state, which you can resume later.
>>
>> Actually, I don't think that example makes sense, because JSON gets sent out at the first yield, and then sent back in, so the caller would look something like this:
>>
>> $foo = getSession(42);
>> $json_data = $foo->current();
>> $foo->send($json_data);
>> $decoded = $foo->getReturn();
>>
>> But never mind, I think we both get the idea.
>>
>> I understand the desire for a "final result", but I don't like reusing the word "return", because it's never "returned" as the result of running the function, it's just made available through some specific method/syntax.
>>
>> Regards,
>> -- 
>> Rowan Collins
>> [IMSoP]
> Hey,
>
> We currently already have "return;" (without value) as allowed syntax to terminate Generator execution. Thus, the logical consequence is just adding a value to that return.

Hm, I didn't realise that. I suppose it makes sense that you'd want a 
way to do that, but it does make it very hard to see if something's 
going to behave as a function or a coroutine if they use the same 
keywords to mean different things.

> When we want to *return*, it should be also a real *return". Yes, it is just accessible through special syntax (or a method), but it's still *return*ing into the calling frame.

Well, so is "yield" - it just leaves the Generator in a state that can 
be resumed, whereas "return"/"yield final" leaves it in a final, 
non-resumable state.

> Why should the word "return" be unique to methods or functions?

It just doesn't feel like the same thing as a return value to me, for 
the same reason a generator doesn't feel like the same thing as a 
function. In "function foo() { return 42; }", "return" means "this is 
what you'll get when you run foo()"; in "function foo() { return 42; 
yield; }", what you get when you run foo() is a pointer to the resumable 
state, and return means "this is what you'll get if you ask the 
generator/coroutine instance you get by running foo() for its final result".

Regards,
-- 
Rowan Collins
[IMSoP]