/*
 * This example is modified from expr.kt to include a package
 *   and to make eval(..) a static function of the Expr class
 *   in that package.
 *
 *   Since Kotlin does not support static class functions,
 *   Expr.eval is wrapped in a compaion object, a gimmick:
 *    https://kotlinlang.org/docs/reference/object-declarations.html#companion-objects
 */

// This works when compiled with kotlinc and run as a class file:
//    $ kotlinc expr1.kt
//    $ kotlin com.netfluke.calc.Expr1Kt
//       e is 5.0 , e1 is 6.0 , e2 is NaN, e3 is 7.0
//
// For ADT rationale, see
//   https://medium.com/car2godevs/kotlin-adt-74472319962a
//   http://engineering.pivotal.io/post/algebraic-data-types-in-kotlin/

package com.netfluke.calc

sealed class Expr { // abstract class, parent for all actual node types below
   companion object {
     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
           }
   }
}

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()

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