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Date: Wed, 06 Aug 2025 13:41:14 +0200
To: "Hans Krentel" <hanskrentel@yahoo.de>
Cc: internals@lists.php.net
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 <1754476976305.2190696371.1573579683@yahoo.de>
Subject: Re: [PHP-DEV] Protected inheritance hierarchies
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From: rob@bottled.codes ("Rob Landers")

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On Wed, Aug 6, 2025, at 13:26, Hans Krentel wrote:
>=20
>=20
>=20
> On Sunday 03 August 2025 11:30:13 (+02:00), Rob Landers wrote:
>=20
> > I'm not sure that this is a bug. You can redeclare the same type and=
 add=20
> hooks (or change them), which breaks all assumptions about=20
> substitutability.
>=20
> Rob, maybe you can lecture me a bit: Isn't substitutability on the pub=
lic=20
> interface / protocol only? What am I not seeing in your example?

From a child class's perspective, the "public interface" is both protect=
ed + public of the parent. If you change or misuse a parent's implementa=
tion or invariants, it violates LSP, even if it doesn't affect external =
clients immediately.

Take for example:

class Fruit {
  public string $kind { get =3D> "fruit" }
}

class Vegetable extends Fruit {
  public string $kind { get =3D> "vegetable" }
}

function foo(Fruit $fruit) {}

foo(new Vegetable); // hmmm

This is a "soft" violation that only makes sense to us humans, but PHP a=
llows it. It requires us humans to realize we are performing an LSP viol=
ation and refactor the code so that we don't pass a Carrot to someone ex=
pecting a Mango.

This can be done through protected means as well (simply replace the pro=
perties above as protected properties used internally), and it won't be =
as obvious to consumers outside the class, but still there, nonetheless.

>=20
> Btw, I found the fatal error as well (and then useful, because moving =
a=20
> protected property upwards, PHP tells where duplicates are downwards i=
n the=20
> hierarchy IIRC), however I'm also looking for something more internal =
for=20
> user land class and objects trees and the subject caught my attention.
>=20
> -- hakre
>=20

=E2=80=94 Rob
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<!DOCTYPE html><html><head><title></title></head><body><div><br></div><d=
iv><br></div><div>On Wed, Aug 6, 2025, at 13:26, Hans Krentel wrote:</di=
v><blockquote type=3D"cite" id=3D"qt" style=3D""><div><br></div><div><br=
></div><div><br></div><div>On Sunday 03 August 2025 11:30:13 (+02:00), R=
ob Landers wrote:</div><div><br></div><div>&gt; I'm not sure that this i=
s a bug. You can redeclare the same type and add&nbsp;</div><div>hooks (=
or change them), which breaks all assumptions about&nbsp;</div><div>subs=
titutability.</div><div><br></div><div>Rob, maybe you can lecture me a b=
it: Isn't substitutability on the public&nbsp;</div><div>interface / pro=
tocol only? What am I not seeing in your example?</div></blockquote><div=
><br></div><div>From a child class's perspective, the "public interface"=
 is both protected + public of the parent. If you change or misuse a par=
ent's implementation or invariants, it violates LSP, even if it doesn't =
affect external clients immediately.</div><div><br></div><div>Take for e=
xample:</div><div><br></div><div>class Fruit {</div><div>&nbsp; public s=
tring $kind { get =3D&gt; "fruit" }</div><div>}</div><div><br></div><div=
>class Vegetable extends Fruit {</div><div>&nbsp; public string $kind { =
get =3D&gt; "vegetable" }</div><div>}</div><div><br></div><div>function =
foo(Fruit $fruit) {}</div><div><br></div><div>foo(new Vegetable); // hmm=
m</div><div><br></div><div>This is a "soft" violation that only makes se=
nse to us humans, but PHP allows it. It requires us humans to realize we=
 are performing an LSP violation and refactor the code so that we don't =
pass a Carrot to someone expecting a Mango.</div><div><br></div><div>Thi=
s can be done through protected means as well (simply replace the proper=
ties above as protected properties used internally), and it won't be as =
obvious to consumers outside the class, but still there, nonetheless.<br=
></div><div><br></div><blockquote type=3D"cite" id=3D"qt" style=3D""><di=
v><br></div><div>Btw, I found the fatal error as well (and then useful, =
because moving a&nbsp;</div><div>protected property upwards, PHP tells w=
here duplicates are downwards in the&nbsp;</div><div>hierarchy IIRC), ho=
wever I'm also looking for something more internal for&nbsp;</div><div>u=
ser land class and objects trees and the subject caught my attention.</d=
iv><div><br></div><div>-- hakre</div><div><br></div></blockquote><div><b=
r></div><div id=3D"sig121229152">=E2=80=94 Rob</div></body></html>
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