But actually in Crystal you will have the same behaviour: class Person end class Employer < Person end class Employee < Person end x = some_co

But actually in Crystal you will have the same behaviour: class Person end class Employer < Person end class Employee < Person end x = some_condition ? Employer.new : Employee.new # x is a Person+ This is not said in the "happy birthday" article (or anywhere else, IIRC). In the beginning we typed x as Employer | Employee. But, as the hierarchy grew bigger compile times became huge. Then we decided to let x be the lowest superclass of all the types in the union (and mark it with a "+", meaning: it's this class, or any subclass). This made compile times much faster, and in most (if not all) cases this is what you want when you assign different types under the same hierarchy to a variable. What this does mean, though, is that the following won't compile: # Yes, there are abstract classes in Crystal abstract class Animal end class Dog < Animal def talk end end class Cat < Animal def talk end end class Mouse < Animal end x = foo ? Dog.new : Cat.new x.talk # undefined method 'talk' for Mouse That is, even though "x" is never assigned a Mouse, Crystal infers the type of "x" to be Animal+, so it really doesn't know which types are in and considers all cases. Again, this is most of the time something good: if you introduce a new class in your hierarchy you probably want it to respond to some same methods as the other classes in the hierarchy.