sealed trait Either[+E, +A] {
  def map[B](f: A => B): Either[E, B]
  def flatMap[EE >: E, B](f: A => Either[EE, B]): Either[EE, B]
  def orElse[EE >: E,B >: A](b: => Either[EE, B]): Either[EE, B]
  def map2[EE >: E, B, C](b: Either[EE, B])(f: (A, B) => C): Either[EE, C]
}

case class Left[+E](value: E) extends Either[E, Nothing] {
  def map[B](f: (Nothing) => B): Either[E, B] = this
  def flatMap[EE >: E, B](f: Nothing => Either[EE, B]): Either[EE, B] = this
  def orElse[EE >: E,B >: Nothing](b: => Either[EE, B]): Either[EE, B] = b
  def map2[EE >: E, B, C](b: Either[EE, B])(f: (Nothing, B) => C): Either[EE, C] = this
}

case class Right[+A](value: A) extends Either[Nothing, A] {
  def map[B](f: A => B): Either[Nothing, B] = {
    Right(f(value))
  }
  def flatMap[EE >: Nothing, B](f: A => Either[EE, B]): Either[EE, B] = {
    f(value)
  }
  def orElse[EE >: Nothing,B >: A](b: => Either[EE, B]): Either[EE, B] = {
    this
  }
  def map2[EE >: Nothing, B, C](b: Either[EE, B])(f: (A, B) => C): Either[EE, C] = {
    for {
      bb<- b
    } yield f(value, bb)
  }
}