PHP: Covarianza y Contravarianza

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.

Una declaración de tipo covarianza se considera más específica en el siguiente caso:

Se elimina un tipo de un tipo de unión
Se añade un tipo a un tipo de intersección
Un tipo de clase se cambia a un tipo de clase hijo
iterable es cambiado a array o Traversable

Una clase de tipo se considera menos específica si ocurre lo contrario.

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 3 notes

down

xedin dot unknown at gmail dot com ¶

4 years ago

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`.

down

Hayley Watson ¶

2 years ago

The gist of how the Liskov Substition Princple applies to class types is, basically: "If an object is an instance of something, it should be possible to use it wherever an instance of something is allowed". The Co- and Contravariance rules come from this expectation when you remember that "something" could be a parent class of the object.

For the Cat/Animal example of the text, Cats are Animals, so it should be possible for Cats to go anywhere Animals can go. The variance rules formalise this.

Covariance: A subclass can override a method in the parent class with one that has a narrower return type. (Return values can be more specific in more specific subclasses; they "vary in the same direction", hence "covariant").
If an object has a method you expect to produce Animals, you should be able to replace it with an object where that method produces only Cats. You'll only get Cats from it but Cats are Animals, which are what you expected from the object.

Contravariance: A subclass can override a method in the parent class with one that has a parameter with a wider type. (Parameters can be less specific in more specific subclasses; they "vary in the opposite direction", hence "contravariant").
If an object has a method you expect to take Cats, you should be able to replace it with an object where that method takes any sort of Animal. You'll only be giving it Cats but Cats are Animals, which are what the object expected from you.

So, if your code is working with an object of a certain class, and it's given an instance of a subclass to work with, it shouldn't cause any trouble:
It might accept any sort of Animal where you're only giving it Cats, or it might only return Cats when you're happy to receive any sort of Animal, but LSP says "so what? Cats are Animals so you should both be satisfied."

down

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>';

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