Scala Isomorphic Types

Question

Reading this blog post from Chuusai, it says:

def size(x : Either[Int, String]) = x match {
  case Left(i) => i
  case Right(s) => s.length
}

size(Left(23)) == 23    // OK
size(Right("foo")) == 3 // OK

Either[Int, String] can model the union type Int ∨ String because there is an isomorphism between the two types and their values

What's "there is an isomorphism between the two types and their values" mean?

Answer 1

First you have to define the term "values" in the context of Scala. I'm assuming that means canonical expressions with that type, expressions of the form Left(n) or Right(s) for constant integer n or string s. Then the isomorphism is a straightforward mapping onto the mathematical type Int ∪ String. But what about, e.g., null, or Right( null )? Do these have images under this mapping? Doesn't seem totally isomorphic to me.