Converting case classes in an HList to reactivemongo based BSONDocuments using Shapeless

converter.scala

import reactivemongo.bson.Macros
import shapeless._

object ToBson extends Poly1 {

  import reactivemongo.bson._

  implicit def caseClass[E](
    implicit
    ev: E <:< Product,
    gen: Generic.Aux[E, _ <: HList],
    writer: BSONWriter[E, _ <: BSONValue]
  ) =
    at[E](e => BSON.write(e))
}

case class Foo(a: String, b: Int)
object Foo {
  implicit val fooHandler = Macros.handler[Foo]
}

case class Bar(c: Boolean, d: Double)
object Bar {
  implicit val barHandler = Macros.handler[Bar]
}

val h = Foo("bla", 123) :: Bar(false, 12.0) :: HNil

h map ToBson // res0: shapeless.::[reactivemongo.bson.BSONValue,shapeless.::[reactivemongo.bson.BSONValue,shapeless.HNil]] = BSONDocument(<non-empty>) :: BSONDocument(<non-empty>) :: HNil

/*
Then, by converting the HList to a traversable and initialising a BSONArray, the pretty print will yield this result:

{
  0: {
    a: "bla",
    b: BSONInteger(123)
  },
  1: {
    c: BSONBoolean(false),
    d: BSONDouble(12.0)
  }
}
*/