Store.kt

inline fun <T> Iterable<T>.sumByPrice(selector: (T) -> Price): Price {
    return this.map(selector).reduce({acc: Price, price: Price -> acc + price })
}

inline fun <T> Iterable<T>.sumByWeight(selector: (T) -> Weight): Weight {
    return this.map(selector).reduce({acc: Weight, weight: Weight -> acc + weight })
}

data class Price(private val value: BigDecimal) {
    operator fun plus(price: Price) = Price(this.value + price.value)

    operator fun times(num: Int) = Price(this.value * BigDecimal(num))
}
data class Weight(private val value: BigDecimal) {
    operator fun plus(weight: Weight) = Weight(this.value + weight.value)

    operator fun times(num: Int) = Weight(this.value * BigDecimal(num))
}

data class Product(val name: String, val price: Price, val weight: Weight)

data class OrderLine(private val product: Product, private val count: Int) {
    fun weight() = product.weight * count

    fun amount() = product.price * count
}

object Store {
    @JvmStatic
    fun main(args: Array<String>) {
        val toothPaste = Product(
                "Tooth paste",
                Price(BigDecimal(1.5)),
                Weight(BigDecimal(0.5)))
        val toothBrush = Product(
                "Tooth brush",
                Price(BigDecimal(3.5)),
                Weight(BigDecimal(0.3)))

        val orderLines = listOf(
                OrderLine(toothPaste, 2),
                OrderLine(toothBrush, 3)
        )

        val weight = orderLines.sumByWeight { it.weight() }
        val price = orderLines.sumByPrice { it.amount() }

        println("Total price: $price")
        println("Total weight: $weight")
    }
}