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Date: Tue, 31 Jan 2017 20:30:28 +0100
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To: "guilhermeblanco@gmail.com" <guilhermeblanco@gmail.com>
Cc: PHP internals <internals@lists.php.net>
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Subject: Re: [PHP-DEV] PHP's support to contravariance and covariance
From: rasmus@mindplay.dk (Rasmus Schultz)

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We were talking about this exact issue at work today.

Supporting this for interfaces would be particularly useful - if an
implementation better than satisfies the requirements defined by an
interface, it should be able to implement that interface.

I'd be very happy to see an RFC for this :-)


On Tue, Jan 31, 2017 at 7:30 PM, guilhermeblanco@gmail.com <
guilhermeblanco@gmail.com> wrote:

> Hi internals,
>
> During my regular open source work, I realized that PHP yet do not support
> contravariance and covariance for user classes.
>
> Some examples that I found that could be really easy to implement and
> useful to end user are highlighted here:
>
> - contravariance.php - https://3v4l.org/I3v0u
> - covariance.php - https://3v4l.org/i79O5
>
> For all those lazy people that don't want to open 2 new tabs to understand
> what I mean, here's a more elaborate example covering both scenarios:
>
> <?php
>
> class A {}
>
> class B extends A {}
>
> class Foo {
>     public function bar(B $b) : A {}
> }
>
> class Woo extends Foo {
>     public function bar(A $b) : B {}
> }
>
> $w = new Woo();
>
> Basically, here are the highlights:
> - method arguments are ok to reference wider parameter types (and we just
> approved the PR for Parameter Type Widening) - this is what we call
> contravariant, as contra inheritance direction
> - return types are ok to enforce strictness for subtypes - this is what we
> call covariant, as Woo is a subtype of Foo
>
> I've walked through PHP source and it looks like the only needed place to
> modify is
> https://github.com/php/php-src/blob/master/Zend/zend_inheritance.c#L184,
> which would require to be decoupled in 2 functions to check which direction
> is should support (covariance or contravariance).
>
> Is there anything else that I am missing? I'm happy to write an RFC for
> that... =)
>
> Cheers,
>
> --
> Guilherme Blanco
> Senior Technical Architect at Huge Inc.
>

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