I would like to explain why covariance and contravariance are important, and why they apply to return types and parameter types respectively, and not the other way around.
Covariance is probably easiest to understand, and is directly related to the Liskov Substitution Principle. Using the above example, let's say that we receive an `AnimalShelter` object, and then we want to use it by invoking its `adopt()` method. We know that it returns an `Animal` object, and no matter what exactly that object is, i.e. whether it is a `Cat` or a `Dog`, we can treat them the same. Therefore, it is OK to specialize the return type: we know at least the common interface of any thing that can be returned, and we can treat all of those values in the same way.
Contravariance is slightly more complicated. It is related very much to the practicality of increasing the flexibility of a method. Using the above example again, perhaps the "base" method `eat()` accepts a specific type of food; however, a _particular_ animal may want to support a _wider range_ of food types. Maybe it, like in the above example, adds functionality to the original method that allows it to consume _any_ kind of food, not just that meant for animals. The "base" method in `Animal` already implements the functionality allowing it to consume food specialized for animals. The overriding method in the `Dog` class can check if the parameter is of type `AnimalFood`, and simply invoke `parent::eat($food)`. If the parameter is _not_ of the specialized type, it can perform additional or even completely different processing of that parameter - without breaking the original signature, because it _still_ handles the specialized type, but also more. That's why it is also related closely to the Liskov Substitution: consumers may still pass a specialized food type to the `Animal` without knowing exactly whether it is a `Cat` or `Dog`.
Covarianza y Contravarianza
En PHP 7.2.0, se introdujo la contravarianza parcial mediante la eliminación de las restricciones de tipo en los parámetros de un método hijo. A partir de PHP 7.4.0, se añadió soporte completo de covarianza y contravarianza.
La covarianza permite que el método hijo devuelva un tipo más específico que el tipo de devolución del método de su padre. Mientras que la contravarianza permite que un tipo de parámetro sea menos específico en un método hijo, que el de su padre.
Covarianza
Para ilustrar cómo funciona la covarianza, una simple clase de padre abstracta, Animal es creada. Animal se extenderá a las clases hijo, Cat, y Dog.
<?php
abstract class Animal
{
protected string $name;
public function __construct(string $name)
{
$this->name = $name;
}
abstract public function speak();
}
class Dog extends Animal
{
public function speak()
{
echo $this->name . " barks";
}
}
class Cat extends Animal
{
public function speak()
{
echo $this->name . " meows";
}
}
Observe que no hay ningún método que devuelva valores en este ejemplo. Unas pocas fábricas que devuelven un nuevo objeto de tipo clase Animal, Cat, o Dog.
<?php
interface AnimalShelter
{
public function adopt(string $name): Animal;
}
class CatShelter implements AnimalShelter
{
public function adopt(string $name): Cat // instead of returning class type Animal, it can return class type Cat
{
return new Cat($name);
}
}
class DogShelter implements AnimalShelter
{
public function adopt(string $name): Dog // instead of returning class type Animal, it can return class type Dog
{
return new Dog($name);
}
}
$kitty = (new CatShelter)->adopt("Ricky");
$kitty->speak();
echo "\n";
$doggy = (new DogShelter)->adopt("Mavrick");
$doggy->speak();
El resultado del ejemplo sería:
Ricky meows Mavrick barks
Contravarianza
Continuando con el ejemplo anterior con las clases Animal, Cat, y Dog, una clase llamada Food y AnimalFood serán incluidas, y un método eat(AnimalFood $food) es añadido a la clase abstracta Animal.
<?php
class Food {}
class AnimalFood extends Food {}
abstract class Animal
{
protected string $name;
public function __construct(string $name)
{
$this->name = $name;
}
public function eat(AnimalFood $food)
{
echo $this->name . " eats " . get_class($food);
}
}
Para ver el comportamiento de la contravención, el método eat se sobrescribe en la clase Dog para permitir cualquier tipo de objeto Food. La clase Cat permanece sin cambios.
<?php
class Dog extends Animal
{
public function eat(Food $food) {
echo $this->name . " eats " . get_class($food);
}
}
El siguiente ejemplo mostrará el comportamiento de la contravarianza.
<?php
$kitty = (new CatShelter)->adopt("Ricky");
$catFood = new AnimalFood();
$kitty->eat($catFood);
echo "\n";
$doggy = (new DogShelter)->adopt("Mavrick");
$banana = new Food();
$doggy->eat($banana);
El resultado del ejemplo sería:
Ricky eats AnimalFood Mavrick eats Food
¿Pero qué pasa si el (gatito) $kitty trata de (comer) eat la $banana?
$kitty->eat($banana);
El resultado del ejemplo sería:
Fatal error: Uncaught TypeError: Argument 1 passed to Animal::eat() must be an instance of AnimalFood, instance of Food given
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User Contributed Notes 4 notes
xedin dot unknown at gmail dot com ¶
4 years ago
Anonymous ¶
4 years ago
Covariance also works with general type-hinting, note also the interface:
interface xInterface
{
public function y() : object;
}
abstract class x implements xInterface
{
abstract public function y() : object;
}
class a extends x
{
public function y() : \DateTime
{
return new \DateTime("now");
}
}
$a = new a;
echo '<pre>';
var_dump($a->y());
echo '</pre>';
maxim dot kainov at gmail dot com ¶
3 years ago
This example will not work:
<?php
class CatFood extends AnimalFood { }
class Cat extends Animal
{
public function eat(CatFood $food) {
echo $this->name . " eats " . get_class($food);
}
}
?>
The reason is:
<?php
class DogFood extends AnimalFood { }
function feedAnimal(Animal $animal, AnimalFood $food) {
$animal->eat($food);
}
$cat = new Cat();
$dogFood = new DogFood();
feedAnimal($cat, $dogFood);
?>
But you can do it with traits, like this:
<?php
trait AnimalTrait
{
public function eat(AnimalFood $food)
{
echo $this->name . " ест " . get_class($food);
}
}
class Cat
{
use AnimalTrait;
public function eat(CatFood $food) {
echo $this->name . " eats " . get_class($food);
}
}
?>
phpnet-at-kennel17-dotco-dotuk ¶
3 years ago
Following the examples above, you might assume the following would be possible.
<?php
class CatFood extends AnimalFood { ... }
class Cat extends Animal
{
public function eat(CatFood $food) {
echo $this->name . " eats " . get_class($food);
}
}
?>
However, the Liskov Substitution Prinicpal, and therefore PHP, forbids this. There's no way for cats to eat cat food, if animals are defined as eating animal food.
There are a large number of legitimate abstractions that are forbidden by PHP, due to this restriction.
