class Expr(f: Function1[Map[Symbol,Int], Int]) {
  def apply(env: Map[Symbol, Int]): Int = f(env)
}

object Lambda extends App {
  val number: Int => Expr = num => new Expr(env => num)
  val variable = (id: Symbol) => new Expr(env => env(id))
  val add = (a: Expr, b: Expr) => new Expr(env => a(env) + b(env))
  val multiply = (a: Expr, b: Expr ) => new Expr(env => a(env) * b(env))

  val environment = Map('a -> 1, 'b -> 2, 'c -> 3)
  val expr_tree = add(variable('a), multiply(number(2), variable('b)))

  println(expr_tree(environment))
}