Basic Kotlin

Pizza.Kt

sealed class Topping {
	class Pepperoni: Topping() 
	class Cheese(val type: CheeseType, val extra: Boolean): Topping() 
	class Sausage(val thickness: Double): Topping() 
} 

fun Topping.toString(): String {
	return when (this) {
		is Cheese -> "${extra ? "extra " : ""}$type cheese"
		is Pepperoni -> "pepperoni"
		is Sausage -> "$thickness-inch thick sausage"
	} 
} 

enum class CheeseType {
	mozzarella, feta, four
} 

data class Pizza(val toppings: List<Topping>) {
	override fun toString(): String {
		return "A pizza with " + toppings.reduce { description, topping ->
			(description ?: "") + topping.toString() 
		} 
	} 
}