Newsgroups: php.internals
Path: news.php.net
Xref: news.php.net php.internals:60763
Mailing-List: contact internals-help@lists.php.net; run by ezmlm
Received-SPF: pass (pb1.pair.com: domain googlemail.com designates 209.85.217.170 as permitted sender)
MIME-Version: 1.0
In-Reply-To: <CAEPJdu=gVkZ1KxxsJLQPaj=gRqLsyBsznVjY0nRLPqPz0VMhjQ@mail.gmail.com>
References: <CAF+90c_47HtYHnMztUUb7Gd5_gKhpJYJN=G-KJFSmSt_=w2+Lw@mail.gmail.com>
	<CAEPJdu=gVkZ1KxxsJLQPaj=gRqLsyBsznVjY0nRLPqPz0VMhjQ@mail.gmail.com>
Date: Wed, 6 Jun 2012 19:54:36 +0200
Message-ID: <CAF+90c_znWL__ebs5zy7KMebuRyv4JixuZr_nxhs6jn8P4OtMQ@mail.gmail.com>
To: Benjamin Eberlei <kontakt@beberlei.de>
Cc: PHP internals <internals@lists.php.net>
Content-Type: text/plain; charset=ISO-8859-1
Subject: Re: [PHP-DEV] Generators in PHP
From: nikita.ppv@googlemail.com (Nikita Popov)

On Tue, Jun 5, 2012 at 8:52 PM, Benjamin Eberlei <kontakt@beberlei.de> wrote:
> This is really awesome. Some remarks:
>
> 1. The * looks a bit hidden. How about reusing the yield keyword? "function
> yield getLines()"
I mainly chose the * modifier because this is how JavaScript
(ECMAScript Harmony) does it. It also seemed to convey the idea of
returning multiple values well (Cleene star). But I wouldn't have much
of a problem using some other keyword. As already written in the RFC
"generator" is problematic as it is not an uncommon word. I wouldn't
like reusing "yield" as it doesn't make much sense semantically there
(if you read "function yield xyz()" out). What seems plausible is
using a separate keyword "yielding", so you have a "yielding function
xyz()".

> 2. Any comments about iterator vs closure?
>
> function getLinesFromFile($fileName, Closure $yield) {
> //..
>
>     while (false !== $line = fgets($fileHandle)) {
>         $yield($line);
>     }
> // ..
> }
>

Yes, using callbacks is indeed a common approach to work around the
lack of generators. The problems with the callback approach (and
several other alternatives) is outlined very well in the first few
paragraphs of PEP 255 (http://www.python.org/dev/peps/pep-0255/).
Basically the issue is that without using generators either the
producer (here getLinesFromFile) or the consumer (here the main code)
has to manually keep track of it's current state. Depending on the
complexity of the producer/consumer this can become very hard, which
can be especially well seen in the tokenizer/parser examples given in
the aforementioned PEP. When using generators on the other hand the
state is implicitly maintained by the VM, so the programmer doesn't
have to worry about that anymore.

So basically, yes, callbacks can be used for the simpler applications
of generators, but with more complex problems they can quickly become
very messy.

Nikita