Does Scala support an OR composition?

Question

That question may be a dumb one where any search engine should give me a quick answer. However I did not find anything, so I may use the wrong terms.

If F# one can do an exclusive (OR) composition with the | symbol:

type LeftOrRightOrNone = 
   | Left of string
   | Right of string
   | None

Which mean that a LeftOrRightOrNone can only be of type Left OR Right OR None .

We can have the same constraint with a sealed trait :

sealed trait LeftOrRightOrNone
case class Left(..) extends LeftOrRightOrNone
case class Right(..) extends LeftOrRightOrNone
case object None extends LeftOrRightOrNone

But I wonder if there is a simpler way to declare such types in Scala?

Thanks

Answer 1

In Scala 2 you need to write it with sealed traits and case classes like you described. Scala 3 has much nicer syntax for this:

enum Either[+A, +B]:
   case Left(a: A)
   case Right (b: B)

https://dotty.epfl.ch/docs/reference/enums/adts.html

Answer 2

There is no such thing in Scala. You have to approximate it via sealed traits as shown.

However, there is a way to encode union types leveraging the Curry-Howard Isomorphism.