object Evaluation {
  type Env = Map[String, Value]
  def mtEnv(): Env = Map()
  val num: Value => Double = { case NumValue(num) => num }
  def plus(x: Value, y: Value) = NumValue(num(x) + num(y))
  def mult(x: Value, y: Value) = NumValue(num(x) * num(y))
  def subtract(x: Value, y: Value) = NumValue(num(x) - num(y))
  def divide(x: Value, y: Value) = NumValue(num(x) / num(y))
  def eval(expr: Expr): Value = eval(expr, mtEnv())
  def eval(expr: Expr, env: Env): Value = {
    expr match {
      case Number(d) => NumValue(d)
      case Var(name) => env(name)
      case BinOp("+", left, right) => plus(eval(left, env), eval(right, env))
      case BinOp("*", left, right) => mult(eval(left, env), eval(right, env))
      case BinOp("-", left, right) => subtract(eval(left, env), eval(right, env))
      case BinOp("/", left, right) => { require( eval(right,env) != NumValue(0) )
                                        divide(eval(left, env), eval(right, env)) }
      case Let(name, expr, body) => {
        eval(body, env + (name -> eval(expr, env)))
      }
      case Fun(param, body) => ClosureValue(param, body, env)
      case App(fun, arg) => {
        val ClosureValue(param, body, env2) = eval(fun, env)

        val eval_args   = arg.map((a: Expr) => eval(a, env))
        eval(body, env2 ++ param.zip(eval_args))
      }
    }
  }
}