Minimal Interpreter

Tree.scala

case class Add(l: Tree, r: Tree) extends Tree
case class Sub(l: Tree, r: Tree) extends Tree
case class Mul(l: Tree, r: Tree) extends Tree
case class Div(l: Tree, r: Tree) extends Tree
case class Num(v: Int) extends Tree

// switch ≒ match
def eval(tree: Tree): Int = tree match {
  case Add(l, r) =>  eval(l) + eval(r)
  case Sub(l, r) =>  eval(l) - eval(r)
  case Mul(l, r) =>  eval(l) * eval(r)
  case Div(l, r) =>  eval(l) / eval(r)
  case Num(v) =>  v
}


// 1 + (2 - 4)
val r = eval(
  Add(Num(1), Sub(Num(2), Num(4)))
)
println(r)