mess.scala

trait Reference
trait Database extends Reference
trait REST extends Reference
trait NoRef extends Reference

trait RefType[A, R <: Reference] {
  type Repr
}

trait Customer

trait Product

object RefType {
  implicit val customerDatabaseRefType = instance[Customer, Database, Int]
  implicit val customerRestRefType = instance[Customer, REST, URI]
  implicit val productDatabaseRefType = instance[Product, Database, Int]
  implicit val productRestRefType = instance[Product, REST, URI]
  implicit def aNoRefType[A] = instance[A, NoRef, A]

  def instance[A, R <: Reference, _Repr]: RefType[A, R] = new RefType[A, R] {
    type Repr = _Repr
  }
}

trait Order {
  type R <: Reference
  val customerRef: RefType[Customer, R]
  val productRef: RefType[Product, R]

  def id: Int
  def customer: customerRef.Repr
  def product: Seq[productRef.Repr]
}

object Order {
  def apply[_R <: Reference](_id: Int)(implicit _cR: RefType[Customer, _R], _pR: RefType[Product, _R]) = new {
    def apply(_customer: _cR.Repr, _product: Seq[_pR.Repr]) = {
      new Order {
        type R = _R
        val customerRef = _cR
        val productRef = _pR
        def id = _id
        def customer = _customer.asInstanceOf[customerRef.Repr]
        def product = _product.asInstanceOf[Seq[productRef.Repr]]
      }
    }
  }
}