My first Scala program

main.scala

object Main {

  abstract class Tree

  case class Sum(l: Tree, r: Tree) extends Tree

  case class Var(n: String) extends Tree

  case class Const(v: Int) extends Tree

  type Environment = String => Int

  def eval(t: Tree, env: Environment): Int = t match {
    case Sum(l, r) => eval(l, env) + eval(r, env)
    case Var(n) => env(n)
    case Const(v) => v
  }

  def main(args: Array[String]) {
    val exp: Tree = Sum(Sum(Var("x"), Var("x")), Sum(Const(4), Var("y")))
    val env: Environment = {
      case "x" => 4
      case "y" => 7
    }
    println(eval(exp, env))
  }
}