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Date: Sun, 15 Feb 2015 07:53:55 +0900
Message-ID: <CAGa2bXa1UHH61FaR47Qtvku3jfsAJGom3qxSwCrDmDOTpCGr4w@mail.gmail.com>
To: Robert Stoll <php@tutteli.ch>
Cc: francois <francois@php.net>, Dmitry Stogov <dmitry@zend.com>, 
	Joe Watkins <pthreads@pthreads.org>, Stanislav Malyshev <smalyshev@gmail.com>, 
	PHP Internals <internals@lists.php.net>
Content-Type: multipart/alternative; boundary=001a113553f2b6e1f7050f143c52
Subject: Re: [PHP-DEV] Design by Contract
From: yohgaki@ohgaki.net (Yasuo Ohgaki)

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Content-Type: text/plain; charset=UTF-8

Hi Robert,

D and PHP differs a lot with respect to types. D is very strongly typed
while PHP is very weakly typed.
Therefore, we need a little different approach.

On Sat, Feb 14, 2015 at 8:06 PM, Robert Stoll <php@tutteli.ch> wrote:

> 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. Consider the following (no longer sure which
> syntax is the one you intend to use, so I just use pre and post):
>
> class A{
>   function foo($x){
>      pre($x > 100);
>   }
> }
>
> class B extends A{
>   function foo($x){
>     pre($x > 50); // less restrictive, that's fine
>   }
> }
>
> class C{
>   function foo($x){
>     pre($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
> }
>
> foo(new A()); //that's fine
> foo(new B()); //fine as well
> foo(new C()); //nope, C violates LSP and thus will result in an error
>

Correct. D does not allow to change parents contract. D does fancy check
for "overridden" methods
so that child cannot change parent behavior. I agree this is correct for
type safety.

Since D is strongly typed, D has "overloading" and D user can get around
with the previous restriction
easily.
e.g. foo(x, y) in class B will ignore A::foo contract at all. Same applies
to type difference, since
this is what "overloading" is.

PHP is weakly typed. There is no point follow D principle for "overridden"
methods because PHP type
is weak and PHP does not have "overloading".

I think this answers to your question.

Regards,

P.S. Are we really going to consider strict type safety for PHP??

--
Yasuo Ohgaki
yohgaki@ohgaki.net

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