gistfile1.scala

sealed trait Validation[+E, +A] {
  def fold[X](failure: E => X = identity[E] _, success: A => X = identity[A] _): X
  def map[B](f: A => B): Validation[E, B];
}

final case class Success[E, A](a: A) extends Validation[E, A] {
  def fold[X](failure: E => X = identity[E] _, success: A => X = identity[A] _): X = success(a)
  def map[B](f: A => B): Validation[E, B] = Success(f(a))
}

final case class Failure[E, A](e: E) extends Validation[E, A] {
  def fold[X](failure: E => X = identity[E] _, success: A => X = identity[A] _): X = failure(e)
  def map[B](f: A => B): Validation[E, B] = Failure(e)
}