PosCombineAmountsWithMonoid.scala

import cats.kernel.Monoid
import cats.syntax.monoid._

type Quantity = Int
type Pricer[A] = Quantity => (Quantity, A)

def total[A: Monoid](pricings: List[(Quantity, List[Pricer[A]])]): A = {
  pricings.foldLeft(Monoid[A].empty) { case (grandTotal, (quantity, pricers)) =>
    grandTotal |+| total(quantity, pricers)
  }
}

private def total[A: Monoid](quantity: Quantity, pricers: List[Pricer[A]]): A = {
  val (_, total) = pricers.foldLeft((quantity, Monoid[A].empty)) {
    case ((quantity, total), pricer) =>
      val (remainingQuantity, amount) = pricer(quantity)
      (remainingQuantity, total |+| amount)
  }
  total
}

def priceByTier[A: Monoid](quantityPerTier: Quantity, tierPrice: A): Pricer[A] = { quantity =>
  val tiers = quantity / quantityPerTier
  (quantity % quantityPerTier, tierPrice.combineN(tiers))
}

def priceByUnit[A: Monoid](unitPrice: A): Pricer[A] = { quantity =>
  (0, unitPrice.combineN(quantity))
}