How to say that a type parameter must have one supertype of alternative supertypes?

Question

One can say a type parameter T must have a specific supertype S_1:

class Test[T <: S_1] 

Is there a way to say, that a type parameter must have at least one supertype of multiple supertype alternatives ? Something like (pseudocode) :

class Test[T <: S_1 || S_2] 

Or: Is this not possible, because such a construction makes no sense and would be a hint of a design mistake in the code ?

Answer 1

Short answer: The intuitive solution is to make S_1 and S_2 share a common trait that represents the set of abilities you require for your type parameter T. Use that trait as the upper bound for T.

More possibilities:

• If S_1 and S_2 are unrelated in nature and your requirement for the type T is that it has certain members (that both S_1 and S_2 happen to implement), you can use a structural type to formulate that (the concept behind is called duck typing).

• If for some reason you really require T to be a subclass of S_1 or S_2, and you can't change those types, you can use implicits to convert both of these to a newly introduced internal type S_1_or_2, which you can then use as an upper bound for your T.

Answer 2

Let me expand on Niklas second alternative. Implicit parameters can be used to prove something about the type, so this seems like just the thing. It would go like this:

class ThingIWantToProve[T]
object ThingIWantToProve {
  // Here I define the proofs I need
  implicit def s1IsProvable: ThingIWantToProve[S_1] = new ThingIWantToProve[S_1]
  implicit def s2IsProvable: ThingIWantToProve[S_2] = new ThingIWantToProve[S_2]
}
class Test[T : ThingIWantToProve] // here I use a context bound