Decorator

fun main() {

    val printer = Printer.Base()
    val milk = Product.Milk.Base(60)
    val milkWithDiscount = Product.Milk.MilkWithDiscount(milk)

    val milkWithWithDiscount = Product.Milk.MilkWithDiscount(
        milkWithDiscount
    )

    milk.printPrice(printer)
    milkWithDiscount.printPrice(printer)
    milkWithWithDiscount.printPrice(printer)
}

interface Product {

    fun printPrice(printer: Printer)

    fun price() : Int


    interface Milk : Product {

        class Base(private val price: Int) : Milk {

            override fun printPrice(printer: Printer) = printer.print(price)

            override fun price() : Int = price
        }

        class MilkWithDiscount(private val milk: Milk) : Milk {
            override fun printPrice(printer: Printer) = printer.print(price())

            override fun price() : Int = milk.price() - 20
        }
    }
}

interface Printer {

    fun print(value: Int)

    class Base : Printer {
        override fun print(value: Int) = println("Price is value $value")
    }
}