Narrowing-down the return type of an implemented method

Question

I\'m trying to specify two interfaces for class A that returns instances of class B, and for class B itself.

I\'m declaring return types on the interface.

Sa

Answer 1

Happy to answer my own question after a year or so:

This will be possible in PHP 7.4

This will be possible:

interface X {
  function m(Y $z): X;
}
interface Y extends X {
  // not permitted but type-safe
  function m(X $z): Y;
}

Check out this RFC.

Answer 2

This is not possible with any version of PHP lower than 7.4.

If your interface contains:

public function get( $key ) : ElementInterface;

Then your class needs to be:

class MyRepository implements RepositoryInterface {

    public function get( $key ) : ElementInterface {

        returns new MyElement();
        // which in turn implements ElementInterface
    }
}

The declaration of a class implementing an interface has to match exactly the contract laid out by the interface.

By declaring that it has to return a particular interface instead of an specific implementation, you are giving you leeway on how to implement it (now you could return MyElement or AnotherElement as long as both implemented ElementInterface); but the method declaration has to be the same anyway.

See it working here.

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Starting with PHP 7.4, due to be released in November 2019, covariance will be supported for return types. By then, this would work.