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To: "PHP Internals" <internals@lists.php.net>
Date: Tue, 03 Aug 2010 19:12:22 +0100
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Subject: Closures, rebinding and scopes
From: glopes@nebm.ist.utl.pt ("Gustavo Lopes")

Hi

Even though one cannot bind a static closure to an instance, one can  
rebind its scope with the Closure::bind/Closure::bindTo:

class foo {
	static protected $field = "foo_field";
	static function getClosure() {
		return static function () {
			//causes a crash (fbc already freed) in call after rebinding,
			//so let's not use it
			//echo get_called_class(), "\n";
			echo static::$field, "\n";
		};
	}
}

class subFoo {
	static protected $field = "subfoo_field";
}

$c = foo::getClosure();
$c(); //foo_field
$c = $c->bindTo(new subFoo());
$c(); //subfoo_field

It doesn't make sense to require an instance in order to change the scope  
of a static closure, so perhaps better options would be:

1. Change Closure::bind/Closure::bindTo so that they accept a class name  
for static closures and an instance otherwise
2. Provide a method to change simultaneously the (calling) scope and the  
called scope
3. Provide separate methods to change the called scope and the scope

I think either 2 or 3 would be better than 1 because the fact the  
rebinding a closure changes its scope is not a great idea. Consider:

class foo {
	private $field = "foo";
	function getClosure() {
		return function () {
			echo $this->field, "\n";
		};
	}
}
class subFoo extends foo {}

$f = new subFoo();
$g = new subFoo();
$c = $f->getClosure();
$c(); //foo
$c = $c->bindTo($g); //or even $c->bindTo($f)
$c(); //fails

Thoughts?

-- 
Gustavo Lopes