ep.scala

scala> trait Exp { def eval() : Int }
defined trait Exp

scala> trait Lit extends Exp {
     |   val x : Int 
     |   def eval() = x
     | }
defined trait Lit

scala> trait Add extends Exp {
     |   val e1, e2 : Exp
     |   def eval() = e1.eval + e2.eval
     | }
defined trait Add

scala> trait Sub extends Exp {
     |   val e1, e2 : Exp
     |   def eval() = e1.eval - e2.eval
     | }
defined trait Sub

scala> trait ExpP extends Exp { def print(): String }
defined trait ExpP

scala> trait LitP extends Lit with ExpP {
     |   def print() = "" + x
     | }
defined trait LitP

scala> trait AddP extends Add with ExpP {
     |   val e1, e2 : ExpP //type refined
     |   def print() = "(" + e1.print + "+" + e2.print + ")"
     | }
defined trait AddP