Script from class

ExpressionProblem.scala

// Based on "Independently Extensible Solutions to the Expression Problem" by
// by Matthias Zenger and Martin Odersky, from FOOL 2005.
//
// http://lampwww.epfl.ch/~odersky/papers/ExpressionProblem.html
//
// I was using Scala 2.10.3

trait Base {

  trait Exp {
    def eval(): Int
  }

  type exp <: Exp

  trait Num extends Exp {
    val n : Int
    def eval() = n
  }
}

trait AddFeature extends Base {

  trait Add extends Exp {
    val e1 : exp
    val e2 : exp
    def eval() = e1.eval() + e2.eval()
  }

}

// @author John
trait ShowFeature extends Base {

  trait Exp extends super.Exp {
    def show(): String
  }

  type exp <: Exp

  trait Num extends super.Num with Exp {
    def show() = n.toString
  }
}

trait ShowAddFeature extends ShowFeature with AddFeature {
  
  trait Add extends super.Add
            with super[ShowFeature].Exp {
    def show() = s"${e1.show()} + ${e2.show()}"
  }

}

trait PrintFeature extends Base {

  trait Exp extends super.Exp {
    def print(): String
  }

  type exp <: Exp

  trait Num extends super.Num with Exp {
    def print() = s"new Num(${n.toString()})"
  }

}

trait PrintAddFeature extends PrintFeature with AddFeature {
  trait Add extends super.Add
            with super[PrintFeature].Exp {
    def print() = s"new Add(${e1.print()}, ${e2.print()})"
  }

}

trait BothFeatures extends ShowAddFeature with PrintAddFeature {

  trait Exp extends super[ShowAddFeature].Exp
            with super[PrintAddFeature].Exp

  type exp <: Exp

  trait Num extends super[ShowAddFeature].Num
            with super[PrintAddFeature].Num
            with Exp

  trait Add extends super[ShowAddFeature].Add
            with super[PrintAddFeature].Add
            with Exp

}

object MyExpHierarchy extends BothFeatures {
   type exp = Exp
   def add (x : Exp, y : Exp) = new Add { val e1 = x; val e2 = y }
   def num (v : Int) = new Num { val n = v }
}
import MyExpHierarchy._

val e = add(num(200), num(300))
println("Result: " + e.eval())
println("AST: " + e.print())
println("Pretty: " + e.show())