ApplicativeBuilderWithSingleError.scala

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

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

      (fa, fb) match{
        case (Continue(a), Continue(b)) => Continue(f(a,b))
        case (Failure(e1), Failure(e2)) => Failure(e1)
        case (f @ Failure(_), _) => f
        case (_, f @ Failure(_)) => f
      }
    }
  }