Playing with typesafe builder patterns

RefOrNull.scala

object RefOrNull {
  type Id[A >: Null <: AnyRef] = A
  type No[A >: Null <: AnyRef] = Null
  def nullAs[A] = null.asInstanceOf[A]
  def sideEffect[A](result: A, exprs: Any*): A = result
  def set[A, B](result: A, exprs: Any*): B = result.asInstanceOf[B]
}
import RefOrNull._

final class Data[
  S[SP >: Null <: AnyRef] <: SP,
  W[WP >: Null <: AnyRef] <: WP,
  D[DP >: Null <: AnyRef] <: DP,
] private (
  val id: String,
  private[Data] var _schedule: Int,
  private[Data] var _warehouse: String,
  private[Data] var _delivered: Int,
) {
  def schedule: Int = _schedule
  def warehouse[A >: Null <: AnyRef](implicit ev: W[A] =:= Id[A]): String = _warehouse
}

object Data {
  type InitT = Data[No, No, No]
  type ScheduledT = Data[Id, No, No]
  type WarehouseAssignedT = Data[Id, Id, No]
  type DeliveredT = Data[Id, Id, Id]

  def create(id: String): InitT = new InitT(id, nullAs, nullAs, nullAs)

  implicit class Schedulable(private val data: InitT) extends AnyVal {
    def schedule_=(schedule: Int): ScheduledT = new ScheduledT(data.id, schedule, nullAs, nullAs)
  }

  implicit class WarehouseAssignable(private val data: ScheduledT) extends AnyVal {
    def warehouse_=(warehouse: String): WarehouseAssignedT =
      set[WarehouseAssignedT](data, data._warehouse = warehouse)
  }

  implicit class Deliverable(private val data: WarehouseAssignedT) extends AnyVal {
    def delivered_=(deliveryNum: Int): DeliveredT =
      new DeliveredT(data.id, data.schedule, data.warehouse, deliveryNum)
  }
}