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Date: Thu, 6 Nov 2014 11:43:16 +0400
Message-ID: <CA+9eiLtvRU4MTTr8cC2PbLogn3uzM2+oULRJtCkd0r_itoNJ0g@mail.gmail.com>
To: Levi Morrison <morrison.levi@gmail.com>
Cc: Andrea Faulds <ajf@ajf.me>, Stas Malyshev <smalyshev@sugarcrm.com>, 
	internals <internals@lists.php.net>, Nikita Popov <nikita.ppv@gmail.com>
Content-Type: multipart/alternative; boundary=001a113319bc6f0a9605072bd9e7
Subject: Re: [PHP-DEV] [RFC][Vote] Return Types
From: dmitry@zend.com (Dmitry Stogov)

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It's clear, that covariant types are more smart, but not supporting them,
won't prevent access to more specific type properties and methods.
We are not C++ or Java, and we don't need to cast objects to more specific
types.

On the other hand covariance requires all return types to be defined before
class binding.
It may be a serious new problem. For example you won't be able to compile
the following code at all?

<?php
class A {
  function foo(): C {}
}
class B extends A {
  function foo(): C {}
}
class C extends B {
  function foo(): C {}
}
?>

The similar code with argument type hinting works fine.
It's just a first example I could imagine, I believe, we will get more...

Thanks. Dmitry.

On Thu, Nov 6, 2014 at 5:01 AM, Levi Morrison <morrison.levi@gmail.com>
wrote:

> To demonstrate the value of covariance and why `static` alone is not
> sufficient, here is a small example:
>
> interface Enumerable extends \IteratorAggregate {
>     function getIterator(): Enumerator;
> }
>
> class Vector implements Enumerable, \ArrayAccess, \Countable {
>     function getIterator(): VectorEnumerator { /* =E2=80=A6 */ }
> }
>
> class VectorEnumerator implements Enumerator, \Countable {
>     /* =E2=80=A6 */
> }
>
> This shows why covariance is important for two reasons:
> First, it shows that static isn't sufficient. The VectorEnumerator is
> not the calling class, so self and static are not applicable.
> Second, without covariance you could only declare a return type of
> Enumerator for Vector::getIterator(); a calling class couldn't rely on
> the properties of a VectorEnumerator that are unique to it, such as
> \Countable.
>

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