// From https://kotlinlang.org/docs/reference/whatsnew11.html, 
//   also https://kotlinlang.org/docs/reference/sealed-classes.html

// This works when compiled with kotlinc and run as a class file:
///    $ kotlinc expr.kt
//     $ kotlin ExprKt
//     e is 5.0 , e1 is 6.0 , e2 is NaN, e3 is 7.0
// 
//  but fails when :load-ed from kotlinc-jvm (probably because 
//    there compiling goes by line by line:
//    "error: this type is sealed, so it can be inherited by only its own nested classes or objects"

// For ADT rationale, see
//   https://medium.com/car2godevs/kotlin-adt-74472319962a
//   http://engineering.pivotal.io/post/algebraic-data-types-in-kotlin/

sealed class Expr    // base class, absract parent of all others

data class Const(val number: Double) : Expr()    // pretty-printing, equality, ... 

data class Sum(val e1: Expr, val e2: Expr) : Expr()  // 

data class Prod(val e1: Expr, val e2: Expr) : Expr() 

object NotANumber : Expr()

// recursive arithmeic eval 
fun eval(expr: Expr): Double = 
  when (expr) {
     is Const -> expr.number
     is Sum -> eval(expr.e1) + eval(expr.e2)
     is Prod -> eval(expr.e1) * eval(expr.e2)
     NotANumber -> Double.NaN
  }


val e  = eval( Sum( Const(2.0), Const(3.0) ))
val e1 = eval( Sum( Const(1.0), Sum( Const(2.0), Const(3.0) )))
val e2 = eval( Sum( Const(1.0), Sum( Const(2.0), NotANumber )))
val e3 = eval( Sum( Const(1.0), Prod( Const(2.0), Const(3.0) )))

fun main(args: Array<String>) {
    println("e is $e , e1 is ${e1} , e2 is ${e2}, e3 is ${e3}") 
}  // e is 5.0 , e1 is 6.0 , e2 is NaN, e3 is 7.0