trait StringWrites[-A] {
    def writes(o: A): String
  }

  object StringWrites {
    def apply[A](f: A => String): StringWrites[A] = new StringWrites[A] {
      def writes(a: A): String = f(a)
    }
  }

  implicit def mapWrites[K, V](implicit kFmt: StringWrites[K], vFmt: Writes[V]) = Writes[Map[K, V]] { map =>
    JsObject(map.map {
      case (k, v) => (kFmt.writes(k), vFmt.writes(v))
    }.toSeq)
  }

  case class UserId(value: String) extends AnyVal

  implicit val userIdFmt: StringWrites[UserId] = StringWrites[UserId] {
    userId => userId.value
  }

  implicit val userIdJsFmt = Json.format[UserId]

  val map = Map(UserId("one") -> UserId("asd")) //{"one":{"value":"asd"}}