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To: <francois@php.net>,
	"'Yasuo Ohgaki'" <yohgaki@ohgaki.net>
Cc: "'Dmitry Stogov'" <dmitry@zend.com>,
	"'Joe Watkins'" <pthreads@pthreads.org>,
	"'Stanislav Malyshev'" <smalyshev@gmail.com>,
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Date: Sun, 15 Feb 2015 12:07:30 +0100
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Subject: AW: [PHP-DEV] Design by Contract
From: php@tutteli.ch ("Robert Stoll")

Hi Fran=C3=A7ois

> -----Urspr=C3=BCngliche Nachricht-----
> Von: Fran=C3=A7ois Laupretre [mailto:francois@php.net]
> Gesendet: Sonntag, 15. Februar 2015 11:43
> An: 'Robert Stoll'; 'Yasuo Ohgaki'
> Cc: 'Dmitry Stogov'; 'Joe Watkins'; 'Stanislav Malyshev'; 'PHP =
Internals'
> Betreff: RE: [PHP-DEV] Design by Contract
>=20
> > De : Robert Stoll [mailto:php@tutteli.ch]
> >
> > The theory is actually quite simple. Roughly it says that if you use =
a
> > type hint of a certain class then you can rely on all
> > pre/post-conditions of this class even if a sub-class is passed. =
Hence
> > the sub-class cannot have more restrict conditions.
>=20
> > I guess from the example above it should be clear why D has
> > implemented it this way
>=20
> OK. Thanks for your explanations.
>=20
> What I meant is that, while I understand the theory, I don't =
understand the reason *why* it must be done this way.
>=20
> An additional point is that, IMO, It is impossible for a software to =
enforce the theory and check that a set of conditions is
> more or less restrictive than another one, except analyzing them, =
which is far beyond our possibilities. So, IMO, we can just
> give it as a convention, which is quite poor. That's why I preferred =
to use another logic. But I would like to know if it won't
> bring other problems later.
>=20
> Regards
>=20
> Fran=C3=A7ois
>=20
>=20
> --
> PHP Internals - PHP Runtime Development Mailing List To unsubscribe, =
visit: http://www.php.net/unsub.php

Unfortunately, I am not familiar with Eiffel or D, so I am just assuming =
and cannot really give you the specific reason behind the design =
decision.=20
Having LSP-safe contracts allow that a type checker has do perform a =
conformity check only once and not for each case which is most probably =
way to slow and further cannot guarantee contract-safety at compile-time =
anyway (Eiffel and D are both statically typed).

Considering my example again, this time with some kind of static typing:

class A{
  function foo(int $x){
     pre($x > 100);
  }
}

class B extends A{
  function foo(int $x){
    pre($x > 50); // less restrictive, that's fine
  }
}

class C{
  function foo(int $x){
    pre($x > 150); //more restrictive that's not allowed
  }
}

function foo(A $a){
  $a->foo(101); // that is ok as long as LSP is not violated=20
}

foo(new A()); //that's fine
foo(new B()); //fine as well
A $a =3D new C(); //the type checker thinks $a is of type A, does not =
know that it has actually type C
foo($a); //not possible for a type checker to see the violation


With the LSP rule in place the type checker does not need to verify the =
function call foo since everything of type A will conform anyway.
Makes sense no? But as I said, I am just assuming. Someone familiar with =
Eiffel or D can probably give some insights.

I totally agree with you that such a check is not feasible for PHP, a =
dynamically typed language in general respectively.

Cheers,
Robert