Newsgroups: php.internals
Path: news.php.net
Xref: news.php.net php.internals:93281
Mailing-List: contact internals-help@lists.php.net; run by ezmlm
Received-SPF: error (pb1.pair.com: domain heigl.org from 195.191.240.18 cause and error)
To: Marco Perone <m.perone@mvlabs.it>, internals@lists.php.net
References: <5734B148.40602@mvlabs.it>
Message-ID: <c2eecf0a-534d-45cf-2de9-488f64c56e14@heigl.org>
Date: Thu, 12 May 2016 18:47:09 +0200
MIME-Version: 1.0
In-Reply-To: <5734B148.40602@mvlabs.it>
Content-Type: multipart/signed; protocol="application/pkcs7-signature"; micalg=sha-256; boundary="------------ms070401090701090206070208"
Subject: Re: [PHP-DEV] Functorial interfaces
From: andreas@heigl.org (Andreas Heigl)

--------------ms070401090701090206070208
Content-Type: text/plain; charset=utf-8
Content-Transfer-Encoding: quoted-printable

Am 12.05.16 um 18:37 schrieb Marco Perone:
> Hi everybody,
> I hope this is the right place for asking such a question. If that's no=
t
> the case, please excuse me; I'd appreciate if you could redirect me to
> the appropriate place.
>=20
> The PHP manual says, regarding Interfaces, that the class implementing
> the interface must use the exact same method signatures as are defined
> in the interface. Not doing so will result in a fatal error.
>=20
> I don't understand why this needs to be so. Consider for example the
> following case: I have an interface
>=20
> interface Foo
> {
>     public function foo(SpecialClass $object);
> }
>=20
> and a class
>=20
> class Bar
> {
>     public function foo(BaseClass $object)
>     {
>         // do something with BaseClass $object
>     }
> }
>=20
> that has the same signature of Foo except for the fact that the method
> foo takes a BaseClass, where SpecialClass extends BaseClass.
>=20
> From a theoretical point of view, saying that a class satisfies the
> interface Foo means that it has a method foo that is able to deal with =
a
> SpecialClass object. So Bar satisfies the contract imposed by Foo since=

> it has a method foo that is able to deal with a BaseClass, which means
> that it will also be able to deal with SpecialClass.
>=20
> Hence it would seem natural to say that Bar implements Foo. But, as the=

> documentation says, this produces a fatal error.
>=20
> What is the reason for this fatal error? Is there a specific reason to
> do things this way? It this is the case, could you please provide an
> example that proves why my approach creates problems.
> Thank you
>=20
Hey Marco.

It's the other way around.

The interface creates a contract that ensures that you can use ALL
methods available in your SpecialClass. But that might be more, never
less methods than in BaseClass. So when you try to call methods of the
SpecialClass while executing Bar::foo() (which you have a contract on)
it might fail as BaseClass might not have those methods.

class BaseClass {
    public function a(){}
}

class SpecialClass extends BaseClass{
    public function b(){}
}

Think about the implementation of your class Bar:

class Bar implements Foo {
    public function foo($x){
	$x->b();
    }
}

So calling (new Bar())->foo(new BaseClass()) will fail as the method 'b'
isn't implemented, but it should be according to the interface...

Hope that helps ;)

Cheers

Andreas

--=20
                                                              ,,,
                                                             (o o)
+---------------------------------------------------------ooO-(_)-Ooo-+
| Andreas Heigl                                                       |
| mailto:andreas@heigl.org                  N 50=C2=B022'59.5" E 08=C2=B0=
23'58" |
| http://andreas.heigl.org                       http://hei.gl/wiFKy7 |
+---------------------------------------------------------------------+
| http://hei.gl/root-ca                                               |
+---------------------------------------------------------------------+


--------------ms070401090701090206070208
Content-Type: application/pkcs7-signature; name="smime.p7s"
Content-Transfer-Encoding: base64
Content-Disposition: attachment; filename="smime.p7s"
Content-Description: S/MIME Cryptographic Signature
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--------------ms070401090701090206070208--