Dotty Documentation


Union Types

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Used on types, the | operator creates a union type.

case class UserName(name: String) {
  def lookup(admin: Admin): UserData
case class Password(hash: Hash) {
  def lookup(admin: Admin): UserData

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

Union types are dual of intersection types. Values of type A | B are all values of type A and all values of type B. | is commutative: A | B is the same type as B | A.

The compiler will assign an expression a union type only if such a type is explicitly given. This can be seen in the folling 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) name else password
val res2: Object & Product = UserName(Eve)
scala> val either: Password | UserName = if (true) name else password
val either: Password | UserName = UserName(Eve)

The type of res2 is Object & Product, which is a supertype of UserName and Product, but not the least supertype Password | UserName. If we want the least supertype, we have to give it explicitely, as is done for the type of either. More precisely, the typechecker will widen a union type to a non-union type when inferring the type of val or var, or the result type of a def, or the argument to pass for a type parameter. The widened type of A | B is usually the intersection of all class or trait types that are supertypes of both A and B; it does not include any refinements. Union types are in that sense analogous to singleton types x.type which are also widened to their underlying type unless explicitly specified.