OneOf3.scala

/** A boxed union of 3 types; same principle applies for > 3. */
sealed trait OneOf3[A, B, C] {
  /** There are plenty of useful methods we could define, but folds provides
      an alternative to explicit pattern matching and a basis for other
      useful things. */
  def fold[X](fax: A => X, fbx: B => X, fcx: C => X): X
}

/* Implementations are straight forward: */

final case class FirstOf3[A, B, C](a: A) extends OneOf3[A, B, C] {
  def fold[X](fax: A => X, fbx: B => X, fcx: C => X): X = fax(a)
}
final case class SecondOf3[A, B, C](b: B) extends OneOf3[A, B, C] {
  def fold[X](fax: A => X, fbx: B => X, fcx: C => X): X = fbx(b)
}
final case class ThirdOf3[A, B, C](c: C) extends OneOf3[A, B, C] {
  def fold[X](fax: A => X, fbx: B => X, fcx: C => X): X = fcx(c)
}