JsonWriter type class demo

json.scala

// Scala script: run with "scala json.scala"

// JSON Library ---------------------------------

sealed trait JsValue {
  def stringify: String = this match {
    case JsNull => "null"
    case JsString(value) => "\"" + value + "\""
    case JsNumber(value) => value.toString
    case JsBoolean(value) => value.toString
    case JsArray(values) => "[" + values.map(_.stringify).mkString(",") + "]"
    case JsObject(fields) => "{" + fields.map { case (k, v) => "\"" +k + "\":" + v.stringify + "\"" }.mkString(",") + "}"
  }
}
case object JsNull extends JsValue
final case class JsString(value: String) extends JsValue
final case class JsNumber(value: Double) extends JsValue
final case class JsBoolean(value: Boolean) extends JsValue
final case class JsArray(values: List[JsValue]) extends JsValue
final case class JsObject(fields: List[(String, JsValue)]) extends JsValue

trait JsonWriter[A] {
  def write(value: A): JsValue
}

object DefaultJsonWriters {
  implicit val stringWriter: JsonWriter[String] =
    new JsonWriter[String] {
      def write(value: String): JsValue =
        JsString(value)
    }

  implicit def listWriter[A](implicit valueWriter: JsonWriter[A]): JsonWriter[List[A]] =
    new JsonWriter[List[A]] {
      def write(values: List[A]): JsValue =
        JsArray(values.map(valueWriter.write))
    }
}

object JsonSyntax {
  implicit class JsonOps[A](value: A) {
    def toJson(implicit writer: JsonWriter[A]): JsValue =
      writer.write(value)
  }
}

// Application ----------------------------------

case class Email(address: String) {
  def toJson: JsValue =
    JsString(address)
}

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

object AppJsonWriters {
  import JsonSyntax._
  import DefaultJsonWriters._

  implicit val emailWriter: JsonWriter[Email] =
    new JsonWriter[Email] {
      def write(email: Email): JsValue =
        email.address.toJson
    }

  implicit val personWriter: JsonWriter[Person] =
    new JsonWriter[Person] {
      def write(person: Person): JsValue =
        JsObject(List(
          "name" -> person.name.toJson,
          "email" -> person.email.toJson
        ))
    }
}

// Main -----------------------------------------

import JsonSyntax._
import DefaultJsonWriters._
import AppJsonWriters._

val dave  = Person("Dave", Email("[email protected]"))
val dave2 = Person("Dave2", Email("[email protected]"))
val daves = List(dave, dave2)

println(dave.toJson.stringify)
println(dave.email.toJson.stringify)
println(daves.toJson.stringify)
println(List(daves, daves).toJson.stringify)