Re: How do I handle covariant parameters and not fall foul of LSP.
| From: | Stefan A. | Date: | Mon, 13 Feb 2017 16:36:03 +0000 |
| Subject: | Re: How do I handle covariant parameters and not fall foul of LSP. | ||
| References: | 1 | Groups: | php.general |
| Request: | Send a blank email to php-general+get-325959@lists.php.net to get a copy of this message | ||
I know it might look like deflecting the issue but maybe this could work
abstract class BaseRepo
{
protected function doProcessEntity(BaseEntity $entity){}
}
class PersonRepo extends BaseRepo
{
public function processEntity(Person $entity)
{
$this->doProcessEntity($entity);
}
}
On Mon, Feb 13, 2017 at 5:13 PM, Richard Quadling <rquadling@gmail.com>
wrote:
> Hi.
>
> I have an abstract base repo and an abstract base entity. Both provide
> functionality
> appropriate to all repos/entities.
>
> I extend these to allow a specific repo to handle a specific entity.
> But I can't
> declare the specific entity type hint in the specific repo as it is in
> conflict.
>
> I'm hitting my head against LSP, but wanting
> https://en.wikipedia.org/wiki/Covariance_and_contravariance_
> (computer_science)#Contravariant_method_argument_type
>
> <?php
>
> abstract class BaseEntity{}
>
> abstract class BaseRepo
> {
> public function processEntity(BaseEntity $entity){}
> }
>
> class Person extends BaseEntity{}
>
> class PersonRepo extends BaseRepo
> {
> public function processEntity(Person $entity)
> {
> parent::processEntity($entity);
> }
> }
>
> $person = new Person;
> $personRepo = new PersonRepo;
> $personRepo->processEntity($person);
>
>
> Warning: Declaration of PersonRepo::processEntity(Person $entity)
> should be compatible
> with BaseRepo::processEntity(BaseEntity $entity)
>
> I know some people 'get' this. But for me, it really seems to make next to
> no
> sense at all.
>
> I have some base behaviour for all entities. I have a base repo that can
> work
> with that base entity, regardless of how that entity is extended.
>
> If the base type were not abstract, then I can see that I wouldn't
> substitute
> anything logically, but I'm using abstract base types. And if set theory is
> to be believed, then if B extends A, B is still an A. And so, where I can
> receive an A, I can receive a B.
>
> Obviously, I'm missing / lacking an understanding of LSP as it relates to
> my
> code.
>
> I'm happy to read more, but I'd appreciate some recommendations on what to
> read.
>
> Of course, if anyone can 'correct' my code such that a concrete repo works
> on
> concrete entities, but also allows for the base behaviour, then I'd be very
> grateful.
>
> Regards,
>
> Richard.
>