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To: "Etienne Kneuss" <etienne@immomigsa.ch>, "internals@lists.php.net"
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Date: Sat, 07 Aug 2010 04:25:38 +0100
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Subject: Re: [PHP-DEV] ArrayAccess::offsetGet not returning a ref
From: glopes@nebm.ist.utl.pt ("Gustavo Lopes")

On Sat, 07 Aug 2010 03:55:25 +0100, Etienne Kneuss <etienne@immomigsa.ch>  
wrote:

>>
>> Returning a ref is an additional constraint, meaning that it would
>> actually be covariance, and not contravariance. But yes, it makes
>> sense.
>

Yes, you are right. To put it another way: a subclass should be able to
* Relax the preconditions
* Strengthen the post-conditions

I actually noticed this shortly after sending the e-mail, but hoped no one  
else would :p

> Actually, after rereading what you said and the proposed diff, I'm not  
> sure it makes sense. It should _not_ be consistent with arguments.
>
> A method subtype may relax the parent prototype by not requiring a
> ref for an argument. This is fine and it's indeed contra-variant.

Again, my mistake. The if before was indeed about arguments...

> However, the return value of a method subtype should be able to return a
> ref even if the parent prototype doesnt. However, if the parent requires
> a ref, the child should too. Which is covariant.
>

I'm confused, this was what exactly what I suggested.

The condition I suggested is this (proto is the parent, fe the child):

if (proto->common.return_reference && !fe->common.return_reference) {
	return 0;
}

So it returns an error condition if the parent returns a reference and the  
child doesn't.

It's the check on the arguments that's wrong in this sense. However, I  
think invariance should be required here. The thing is, passing an  
argument by reference is actually also a post-condition -- if we pass an  
argument by reference, we're going to do something with it.

In fact, invariance is required for the non-"rest" part of the arguments:

if (fe->common.arg_info[i].pass_by_reference !=  
proto->common.arg_info[i].pass_by_reference) {

Again, sorry for the confusion.

-- 
Gustavo Lopes