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Date: Fri, 21 Dec 2012 15:54:40 +0100
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To: Stas Malyshev <smalyshev@sugarcrm.com>
Cc: PHP internals <internals@lists.php.net>
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Subject: Re: [PHP-DEV] Adding Generator::throw()
From: nikita.ppv@gmail.com (Nikita Popov)

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On Mon, Dec 17, 2012 at 11:45 PM, Nikita Popov <nikita.ppv@gmail.com> wrote:

> On Mon, Dec 17, 2012 at 11:03 PM, Stas Malyshev <smalyshev@sugarcrm.com>wrote:
>
>> Hi!
>>
>> > Basically the method allows you to do delegate error handling to the
>> > coroutine, rather than doing it yourself (as you are not always able to
>> do
>> > it). It is particularly useful in more complicated settings, e.g. if you
>> > are doing task scheduling through coroutines. For a small sample of how
>>
>> Could you expand on this point a bit more? It sounds like using
>> exceptions for flow control, which is usually a very bad idea.
>>
>> > this looks like see http://taskjs.org/. What the ->throw() method
>> would do
>> > in these examples is that it allows to check for errors by try/catching
>> the
>> > yield statement (rather than going for some odd solution with error
>> > callbacks).
>>
>> Could you point to some specific example?
>>
> The basic idea behind this kind of task management is that you can do
> asynchronous operations just like you would synchronous ones, i.e. without
> the need for callbacks or wait-loops. Whenever the functions wants to
> perform some async action it doesn't directly do it, rather it yields the
> operation that does it. The scheduler then waits until that operation is
> finished (running other tasks in the meantime) and only resumes the
> coroutine once it is finished. This way you can write async code without
> the ugliness that is usually involved with async code.
>
> Here is an example to get a rough idea of how the use looks like:
>
> function server() {
>     // ...
>     $stuff = yield $socket->recv();
>     // ...
>     yield $socket->send($response);
>     // ...
> }
>
> The throw() method comes in when you want to handle errors on those
> asynchronous operations. Without it you would be forced to use error codes.
> throw() allows you to do the error handling just like you would normally
> do. E.g. consider that $socket->send() were a fallible operation. Then you
> could catch errors like this:
>
> function server() {
>     // ...
>     try {
>         yield $socket->send($response);
>     } catch (SocketException $e) {
>         // do some error handling
>     }
>     // ...
> }
>
> In this case the outside code (where the exception comes from) can't know
> what it should do in case of an error. Only the code in the coroutine knows
> that. That's why you need some possibility to throw the error into the
> context that knows how to deal with it.
>
> I hope that this makes it a bit more clear.
>
> Nikita
>

If there are no further objections I'll commit this tomorrow.

Nikita

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