The Problems in OO Typing(Not Finished)

 

Typing of

? Encapsulation (Solved: “existential types” [Girard 71])

?  Inheritance (Partially solved)

?

  Polymorphism (Open issue)

 

 

 

 

Typing Inheritance

1.

Covariant Typing

Animal {

   Shit eats(Food)

}

Cow extends Animal {

  CowShit eats(Grass)

}

Problem: Not type sound

Animal a = new Cow();

Food f = new Hamburger();

a.eat(f); //Food poisoning

 

2.

 

Contravariant Typing

Animal {

   Shit eats(Grass)

}

Cow extends Animal {

  CowShit eats(Food)

}

Problem:

Sound but not flexible

3.

 

Novariant Typing

Animal {

   Shit eats(Food)

}

Cow extends Animal {

  Shit eats(Food)

}

Problem:

Worst of both worlds.