Invariant to restrict function overriding in Hack PHP

Question

I have a base class in PHP Hack with a function:

// This method is used to return just one Apple Type
protected static function Apple(): AppleType {
    return AppleType;
}

Now I have two types of classes - one using a base trait, one doesn't.

The trait has the below function:

// This method is used to return more than one Apple Type
protected static function Apples(): keyset[AppleType] {
    return keyset[AppleType];
}

The child classes which don't use this trait can override the base class Apple() method. But the classes which do use the trait, must override Apples() and not Apple().

Now I want to provide an invariant exception:

Something like:

invariant(Apple() is not overridden in this class, 'Class must override Apples() and not Apple()');

i.e. invariant to provide enforcement that a class using trait must not be able to override base class's Apple() and throws exception during runtime.

Please help me in writing this invariant. I tried many things, but somehow it doesn't work right.

Traits shouldn't create requirements unless it uses them. You _could_ force subclasses to implement an interface with an `apples()` signature via [trait and interface requirements](https://docs.hhvm.com/hack/classes/trait-and-interface-requirements), but _only if_ methods in that trait use the method `apples()`. Should `apples()` be abstract in your example? Can you share more about why you want to enforce this invariant? — concat, Jul 19 '20 at 03:06

score 0 · Answer 1 · answered Jul 20 '20 at 19:54

You could define a class with a final method, and require that trait users extends that class.

enum AppleType: int {
  Fresh = 1;
  Stale = 2;
}

class Overridable {
  protected static function Apple(): AppleType {
    return AppleType::Fresh;
  }
}

class MustUseApples extends Overridable {
  <<__Override>>
  final protected static function Apple(): AppleType {
    return parent::Apple();
  }
}

trait MyTrait {
  require extends MustUseApples;

  protected static function Apples(): keyset<AppleType> {
    return keyset[AppleType::Stale];
  }
}

However, if you want to require that trait users must provide an implementation of Apples, you should probably make it an abstract method on MustUseApples.

Invariant to restrict function overriding in Hack PHP

1 Answers1