gistfile1.scala

trait LeftFolder[C <: Coproduct, In, F] extends Serializable {
  def apply(c: C, in: In): In
}

object LeftFolder {
  def apply[C <: Coproduct, In, F](implicit folder: LeftFolder[C, In, F]) = folder

  implicit def hdLeftFolder[H, T <: Coproduct, In, F]
  (implicit f: Case2.Aux[F, In, H, In]): LeftFolder[H :+: T, In, F] = new LeftFolder[H :+: T, In, F] {
    def apply(c: H :+: T, in: In): In = c match {
      case Inl(h) => f(in, h)
      case Inr(t) => in
    }
  }

  implicit def tlLeftFolder[H, T <: Coproduct, In, F]
  (implicit st: LeftFolder[T, In, F]): LeftFolder[H :+: T, In, F] = new LeftFolder[H :+: T, In, F] {
    def apply(c: H :+: T, in: In) =
      c match {
        case Inl(h) => in
        case Inr(t) => st(t,in)
    }
  }
}