Type class example 2

type_class_example2.scala

sealed trait Json

final case class JsObject(get: Map[String, Json]) extends Json

final case class JsString(get: String) extends Json

final case class JsNumber(get: Double) extends Json

case object JsNull extends Json


// 1 the type class
trait JsonWriter[A] {

  def write(value: A): Json
}

final case class Person(name: String, email: String)

// 2 type class instances
object JsonWriter {

  implicit val personWriter: JsonWriter[Person] =
    new JsonWriter[Person] {
      override def write(value: Person): Json = JsObject(Map(
        "name" -> JsString(value.name),
        "email" -> JsString(value.email)
      ))
    }

  implicit def optionWriter[A](implicit writer: JsonWriter[A]): JsonWriter[Option[A]] =
    new JsonWriter[Option[A]] {
      def write(value: Option[A]): Json = value match {
        case Some(a) => writer.write(a)
        case None => JsNull
      }
    }
}


object Json {
  // 3 type class interface
  def toJson[A](value: A)(implicit w: JsonWriter[A]) =
    w.write(value)
}

// 4 Interface syntax - extend existing types with interface methods
object JsonSyntax {

  implicit class JsonWriterOps[A](value: A) {
    def toJson(implicit w: JsonWriter[A]): Json =
      w.write(value)
  }

}


object Main extends App {

  import Json._
  val p = Person("julian", "foo@bar.com")
  println(toJson(p)) // JsObject(Map(name -> JsString(julian), email -> JsString(foo@bar.com)))

  import JsonSyntax._
  println(p.toJson) // same result as above but much nicer syntax as it extends the existing type

  val pOpt: Option[Person] = Some(p)
  println(pOpt.toJson) //same as above, uses OptionWriter instance

}