Trying to get a "scala" version of https://github.com/lkuper/aw/ and have to do this to get it to :load in the REPL. The REPL only recognizes companion objects defined on the same line as the original class so here we have everything defined on the same "line". If I'm wrong someone please tell me how to make this less awkward.

gistfile1.scala

abstract class Expr {
}; case class Number(a: Int) extends Expr {
}; case class Plus(a: Expr, b: Expr) extends Expr {
}; case class Times(a: Expr, b: Expr) extends Expr {
}; case class Minus(a: Expr, b: Expr) extends Expr {
}; object Expr {
  def interp(expr: Expr): Int = 
  expr match {
    case Number(a) => a

    case Plus(a,b) => interp(a) + interp(b)

    case Times(a,b) => interp(a) * interp(b)

    case Minus(a,b) => interp(a) - interp(b)
  }
}