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Date: Wed, 5 Nov 2014 19:01:12 -0700
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To: Andrea Faulds <ajf@ajf.me>
Cc: Stas Malyshev <smalyshev@sugarcrm.com>, Dmitry Stogov <dmitry@zend.com>, 
	internals <internals@lists.php.net>, Nikita Popov <nikita.ppv@gmail.com>
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Subject: Re: [PHP-DEV] [RFC][Vote] Return Types
From: morrison.levi@gmail.com (Levi Morrison)

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.