See http://lampsvn.epfl.ch/trac/scala/ticket/967

Encoding type members as parameter types in a Scala structural type

// See http://lampsvn.epfl.ch/trac/scala/ticket/967.
//
// Gilles' fix causes the definition of Nat to be rejected with the error 
// "Parameter type in structural refinement may not refer to a type member
// of that refinement". However we can work around the problem by 
// quantifying out the problematic parameter type and reinstating it via
// a generalized type constraint.

type Num = {
  type Rep
  val zero : Rep
  def succ[R](r : R)(implicit ev : R =:= Rep) : Rep
  def show[R](r : R)(implicit ev : R =:= Rep) : String
}

val IntNum : Num = new {
  type Rep = Int
  val zero = 0
  def succ[R](r : R)(implicit ev : R =:= Rep) : Rep = r+1
  def show[R](r : R)(implicit ev : R =:= Rep) = r.toString
}

val DoubleNum : Num = new {
  type Rep = Double
  val zero = 0.0
  def succ[R](r : R)(implicit ev : R =:= Rep) : Rep = r+1.0
  def show[R](r : R)(implicit ev : R =:= Rep) = r.toString
}

def two(n : Num) = n.show(n.succ(n.succ(n.zero)))

scala> two(IntNum)
res0: String = 2                                                                                                                                                                                       
                                                                                                                                                                                                       
scala> two(DoubleNum)
res1: String = 2.0