Validation.scala

sealed trait Validation[+E, +A] {
  def map[B](f: A => B): Validation[E, B]
  def flatMap[EE >: E, B](f: A => Validation[EE, B]): Validation[EE, B]
  def <*>[EE >: E, AA, B](aa: Validation[EE, AA])(implicit valueToFunction: A <:< (AA => B)): Validation[EE, B]
}

object Validation {

  case class Success[+A](value: A) extends Validation[Nothing, A] {
    def map[B](f: A => B): Validation[Nothing, B] =
      Success(f(value))
    def flatMap[EE, B](f: A => Validation[EE, B]): Validation[EE, B] =
      f(value)
    def <*>[EE, AA, B](aa: Validation[EE, AA])(implicit valueToFunction: A <:< (AA => B)): Validation[EE, B] =
      aa map valueToFunction(value)
  }

  case class Failure[+E](errors: Seq[E]) extends Validation[E, Nothing] {
    def map[B](f: Nothing => B): Validation[E, B] =
      this
    def flatMap[EE >: E, B](f: Nothing => Validation[EE, B]): Validation[EE, B] =
      this
    def <*>[EE >: E, AA, B](aa: Validation[EE, AA])(implicit valueToFunction: Nothing <:< (AA => B)): Validation[EE, B] =
      aa match {
        case Success(_) => this
        case Failure(otherErrors) => Failure(errors ++ otherErrors)
      }
  }

}