Abstract Union Types

[fcard]

Since I already implemented julia-style abstract types, I may as well implement union types. The easy way would be to just create a variant of TypeMatch that matches against multiple types and have the matched argument inferred as Value, which will result in the following semantics:

multifunction! {
  fn F(x: Union![i32, i64]) -> Value {
    x + 1 // x is of type Value
  }
}

fn main() {
  println!("{}", F(1i32)) // 2
  println!("{}", F(1i64)) // 2
}

But I wonder if it wouldn't be better if two methods were created, one where x is a i32 and another where x is a i64... That might be too tricky to handle even from the user's perspective, actually; in the above case, you would need to choose a return type that will work for both. One could convert x to i64 in this case because it's sort of a supertype, but if one had say a Union![f32, i64] this wouldn't work. One could convert x to Value but then that's just the first implementation, but more verbose. (Well, it's also opt-in...)

HMMMM...