import cats.Monoid
import cats.Semigroup
import cats.instances.int._
import cats.instances.option._
import cats.syntax.semigroup._

case class Order(totalCost: Double, quantity: Double)

implicit def orderMonoid: Monoid[Order] = new Monoid[Order] {
  def empty: Order = Order(0, 0)
  def combine(x: Order, y: Order): Order =
    Order(x.totalCost + y.totalCost, x.quantity + y.quantity)
}

def addCats[A: Monoid](items: List[A]): A =
  items.foldLeft(Monoid[A].empty)(_ |+| _)

addCats(List(1, 2, 3, 4, 5))
addCats(List(Option(1), Option(2), None, Option(3), None, Option(4), Option(5)))

val order1 = Order(2.5, 1)
val order2 = Order(3.5, 2)
val order3 = Order(4.0, 3)

addCats(List[Order](order1, order2, order3))