xmappy.scala

// JsonFormat is from spray-json, reproduced here
trait JsonFormat[A] {
  def write(a: A): JsValue
  def read(js: JsValue): A
}

implicit class FormatInvariantFunctor[A](format: JsonFormat[A]) {
  def xmap[B](f: A => B, g: B => A): JsonFormat[B] =
    new JsonFormat[B] {
      def write(b: B): JsValue = format.write(g(b))
      def read(js: JsValue): B = f(format.read(js))
    }

  // is there a name for this?
  def using[B](f: A => B, g: B => A)(J: JsonFormat[B]): JsonFormat[A] =
    new JsonFormat[A] {
      def write(a: A): JsValue = J.write(f(a))
      def read(js: JsValue): A = g(J.read(js))
    }
}