Re: PHP's support to contravariance and covariance

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. >