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Date: Thu, 07 Jun 2012 10:46:16 +0200
To: Nikita Popov <nikita.ppv@googlemail.com>
Cc: internals PHP <internals@lists.php.net>
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Subject: Re: [PHP-DEV] Generators in PHP
From: glopes@nebm.ist.utl.pt (Gustavo Lopes)

(Sorry, I pressed something that sent the message prematurely)

On Thu, 7 Jun 2012 01:10:52 +0200, Nikita Popov wrote:

>> current() and key() should return false when !valid()
>
> Are you sure about that? The Iterator::current() docs don't specify
> anything, but the Iterator::key() docs say that it should return NULL
> on failure. Checking on the first spl class that came to my mind
> SplFixedArray also returns NULL when it is out of elements.

My bad. I was under the impression the semantics were similar to those 
of next(), key(), etc. Instead the docs say under current() that the 
function can return anything, under key) that it returns NULL on failure 
(and issues an E_NOTICE) and for next() that "the return value is 
ignored" -- whatever that means; I'll interpret it as saying anything 
can be returned. I'm not sure how correct this documentation is, though.

>> next() is underspecified.
> Not sure what exactly you mean. next() really doesn't do much more
> than resuming the generator. What should I add additionally about it?

Sorry for not having been clearer. I mean you say "Resumes the 
generator (unless the generator is already closed)", but you don't 
specify what it returns (though apparently Iterator does not require you 
to specify anything specific) and you don't say what happens when 
theiterator is already closed.

>> In fact, if you can't implement Iterator in full, you should 
>> implement Traversable instead.
> Not sure whether the lack of rewinding behavior really disqualifies 
> it
> to use the Iterator interface. I think having the internal iteration
> methods exposed is quite handy, especially if you consider their use
> as coroutines, where you often want to call the iteration interface
> manually and not using a foreach loop.
>

Well, the Iterator::rewind() docs say that rewind() rewinds the 
iterator. If this is not possible, then Iterator should not be 
implemented. See 
http://www.parashift.com/c++-faq-lite/proper-inheritance.html#faq-21.8

In the same order, your options are:

1. As changing Iterator is not possible, have a new interface, say, 
BasicIterator from which Iterator will inherit without rewind().
2. Make all the Generators rewindable (possibly by fetching a new 
generator each time).
3. Do not implement Iterator.

(By the way, if you implement Traversable, one you can always use the 
unsafe IteratorIterator to get access to the methods)

>  * Exceptions can be thrown from generators as always.
>  * Not sure what you mean by "generator returns function". What would
> be that problem with that?

I mean how do you deal with "return $foo" in the body of the generator? 
Does it work like a final yield? Is the return value ignored?

>  * Generators can be methods, as shown in the example in the
> Introduction section. Closures can also be generators.

Can other callable classes be generators?

Are you planning on any internal API for functions to implement 
generators (though I really haven't thought how that would work)?

-- 
Gustavo Lopes