PosFoldOverPricers.scala

type Quantity = Int
type Amount = BigDecimal
type Pricer = Quantity => (Quantity, Amount)

def total(pricings: List[(Quantity, List[Pricer])]): Amount = {
  pricings.foldLeft(BigDecimal(0)) { case (grandTotal, (quantity, pricers)) =>
    grandTotal + total(quantity, pricers)
  }
}

private def total(quantity: Quantity, pricers: List[Pricer])  = {
  val (_, total) = pricers.foldLeft((quantity, BigDecimal(0))) {
    case ((quantity, total), pricer) =>
      val (remainingQuantity, amount) = pricer(quantity)
      (remainingQuantity, total + amount)
  }
  total
}

val priceByTier: (Quantity, Amount) => Pricer = { (quantityPerTier, tierPrice) => { quantity =>
  val tiers = quantity / quantityPerTier
  (quantity % quantityPerTier, tiers * tierPrice)
} }

val priceByUnit: Amount => Pricer = { unitPrice => { quantity =>
  (0, quantity * unitPrice)
} }