Scala Worksheet: How to inject an implicit value into a Play json serializer...?

gistfile1.txt

import play.api.libs.json._
import play.api.libs.json.Reads._
import play.api.libs.functional.syntax._

case class GlobalConfig(defaultAge: Int)
val config = GlobalConfig(34)

case class Person(name: String, age: Int)

object Person {
  implicit val reads: Reads[Person] = (
    (JsPath \ "name").read[String] and
      (JsPath \ "age").readNullableWithDefault(????)[Int] // default age with some sort of di...
    ) (Person.apply _)
}

val json: JsValue = Json.parse("""
  {
    "name" : "John",
    "age": 23
  }
  """)

val result: JsResult[Person] = (json).validate[Person]

result.get