just another day in Scala...

Persistence.scala

private type H[A <: BSONValue, B] = BSONHandler[A, B]
private type DH[A] = BSONDocumentHandler[A]

object Implicits extends BSONConverter with DaoToDocDerivation {
  private implicit def `(A, B) <-> BSON`[A <: BSONValue, B, C <: BSONValue, D](implicit hab: H[A, B], hcd: H[C, D]): DH[(B, D)] = handler[Tuple2[B, D]]
  private implicit def `Coll[A] <-> BSON`[A[_], B](implicit h: H[_ <: BSONValue, B], cbf: CanBuildFrom[A[_], B, A[B]], ev: A[B] <:< Traversable[B]): H[BSONArray, A[B]] =
    BSONHandler(bsonArrayToCollectionReader[A, B].read, collectionToBSONArrayCollectionWriter[B, A[B]].write)
  private implicit def `Map[A, B] <-> BSON`[A, B](implicit htab: DH[(A, B)]): H[BSONArray, Map[A, B]] =
    `Coll[A] <-> BSON`[Vector, (A, B)].as(_.toMap, _.toVector)
  private implicit def `Set[A] <-> BSON`[A](implicit htab: H[_ <: BSONValue, A]): H[BSONArray, Set[A]] =
    `Coll[A] <-> BSON`[Set, A]

  //...
}