Either

Allows use of a merge method to extract values from Either instances regardless of whether they are Left or Right.

val l = Left(List(1)): Either[List[Int], Vector[Int]]
val r = Right(Vector(1)): Either[List[Int], Vector[Int]]
l.merge: Seq[Int] // List(1)
r.merge: Seq[Int] // Vector(1)

Projects an Either into a Right.

Because Either is already right-biased, this class is not normally needed. (It is retained in the library for now for easy cross-compilation between Scala 2.11 and 2.12.)

If the condition is satisfied, return the given B in Right, otherwise, return the given A in Left.

val userInput: String = readLine()
Either.cond(
  userInput.forall(_.isDigit) && userInput.size == 10,
  PhoneNumber(userInput),
  s"The input ($userInput) does not look like a phone number"