Generalized Monoids

GenMonoid.scala

trait Op[A] {
  def apply(x: A, y: A): A
}

class Multiply extends Op[Int] {
  def apply(x: Int, y: Int) = x * y
}

trait Semigroup[A, O <: Op[A]]

val intSemigroup = new Semigroup[Int, Multiply]{}

trait Monoid[A, O <: Op[A], I <: A with Singleton] extends Semigroup[A, O]

val intMonoid = new Monoid[Int, Multiply, 0]{}