Multiple_Failures_Support.scala

  sealed trait BuildStep[+E, +A]{
    def toEither: Either[E,A]
  }

  final case class Continue[A](v: A) extends BuildStep[Nothing,A] {
    def toEither = Right(v)
  }
  final case class Failure[E](e: List[E]) extends BuildStep[List[E], Nothing] {
    def toEither = Left(e)
  }

  def applicativeBuilder[E] = new Applicative[({type f[x] = BuildStep[List[E], x]})#f] {
    def unit[A](a: A) = Continue(a)

    override def map2[A, B, C](fa: BuildStep[List[E], A], fb: BuildStep[List[E], B])(f: (A, B) => C) = {

      (fa, fb) match{
        case (Continue(a), Continue(b)) => Continue(f(a,b))
        case (Failure(l1), Failure(l2)) => Failure(l1 ++ l2)
        case (f @ Failure(_), _) => f
        case (_, f @ Failure(_)) => f
      }

    }
  }