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Subject: Re: [PHP-DEV] [RFC] Comparable: the revenge
From: dennis@birkholz.biz (Dennis Birkholz)

Hi Adam,

Am 19.02.2015 um 22:40 schrieb Adam Harvey:
> RFC: https://wiki.php.net/rfc/comparable
> PR: https://github.com/php/php-src/pull/1097

I see a little problem here by allowing any comparison. Comparing is
used for ordering, but you can easily construct ordering mechanisms that
simply don't work:

class A implements Comparable {
	public $value;
	
	function compareTo($other) {
		if ($other instanceof A) {
			return $this->value <=> $other->value;
		} else {
			return -1;
		}
	}
}

class B implements Comparable {
	public $value;
	
	function compareTo($other) {
		if ($other instanceof B) {
			return $this->value <=> $other->value;
		} else {
			return -1;
		}
	}
}

$a = new A();
$a->value = 1;

$b = new B();
$b->value = 2;

$a->compareTo($b); // -1
$b->compareTo($a); // -1

If you now have a mixed list of objects of A and B, the actual order in
the list completely depends whether A's compareTo() method is called or
B's compareTo() method.
I think comparison should always be symmetric:
$a < $b <=> $b > $a
Otherwise the behavior is nearly unpredictable/random.

As we have no method overloading, the only viable option I see is to
only allow comparison of objects of the same type.

Thanks
Dennis