Mutual dependencies across objects

HierarchicalModules.scala

trait Eval { outer =>
  val t: Trees
  val n: Normalize { val t: outer.t.type }
  type Val
  val eval: t.Exp => Val
  val normalizingEval: t.Exp => Val =
    exp => eval(n.normalize(exp))
}

trait Normalize { outer =>
  val t: Trees
  val e: Eval { val t: outer.t.type }
  val reify: e.Val => t.Exp
  val normalize: t.Exp => t.Exp =
    exp => reify(e.eval(exp))
}

class Interpreter { outer =>
  object t extends TreesImpl
  object e extends EvalImpl {
    val t = outer.t; val n = outer.n }
  object n extends NormalizeImpl {
    val t = outer.t; val e = outer.e }
}

object interpreter extends Interpreter

trait EvalImpl extends Eval {
  val t: TreesImpl; import t._
  type Val = Int
  val eval: Exp => Val = exp => exp match {
    case Add(l, r) => eval(l) + eval(r)
    case Lit(n)    => n
  }
}
trait NormalizeImpl extends Normalize { outer =>
  val t: TreesImpl
  val e: EvalImpl { val t: outer.t.type }
  val reify = t.Lit
}