Union Types

A union type A | B has as values all values of type A and also all values of type B.

case class UserName(name: String)
case class Password(hash: Hash)

def help(id: UserName | Password) =
  val user = id match
    case UserName(name) => lookupName(name)
    case Password(hash) => lookupPassword(hash)
  ...

Union types are duals of intersection types. | is commutative: A | B is the same type as B | A.

The compiler will assign a union type to an expression only if such a type is explicitly given. This can be seen in the following REPL transcript:

scala> val password = Password(123)
val password: Password = Password(123)

scala> val name = UserName("Eve")
val name: UserName = UserName(Eve)

scala> if true then name else password
val res2: Object = UserName(Eve)

scala> val either: Password | UserName = if true then name else password
val either: Password | UserName = UserName(Eve)

The type of res2 is Object & Product, which is a supertype of UserName and Password, but not the least supertype Password | UserName. If we want the least supertype, we have to give it explicitly, as is done for the type of either.

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