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Date: Tue, 25 Nov 2014 14:43:26 -0700
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To: Marc Bennewitz <dev@mabe.berlin>
Cc: internals <internals@lists.php.net>
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Subject: Re: [PHP-DEV] [RFC][Discussion] Return Type Variance Checking
From: levim@php.net (Levi Morrison)

On Tue, Nov 25, 2014 at 2:07 PM, Marc Bennewitz <dev@mabe.berlin> wrote:
> I think it's required to do the type check on runtime (Option 2) because
> one of the use cases for return type-hint are factories and such often do
> instantiation in base of unknown string values:
>
> class MyFactory {
>     public static function factory($name) : AdapterInterface {
>         $class = 'MyNamespace\Adapter\' . $name;
>         return $class();
>     }
> }

It seems that I did not explain this clearly enough; I apologize. The
variance has to do with the declared type in the function signature
when inheritance is involved, not the type of the value returned by
the function.

For instance, under any of the three options this code will work just fine:

class Foo {}
class Goo  extends Foo {}

class FooFactory {
    function create(): Foo { return new Goo(); }
}

As long as the return value from FooFactory::create returns Foo or a
subtype of Foo (such as Goo), then it will work.

The variance that is under discussion in this thread is about the
declared return type in the signature:

class GooFactory extends FooFactory {
    function create(): Goo {}
}

In this case, GooFactory::create() declares a return type of Goo,
which is a subtype of Foo [the return type of the inherited method
FooFactory::create()]. This is a covariant return type.

If we choose option 3, the only possible return type for
GooFactory::create is Foo.

Hopefully this clarifies the issue.