Play Json Format helper for value classes

SingleFieldFormat.scala

import play.api.libs.functional.syntax._
import play.api.libs.json.{Format, Json}

object SingleFieldFormat {
  def format[A, B](f: A => B, g: B => A)(implicit fa: Format[A]) = fa.inmap(f, g)
  // expanded:
//    syntax.toInvariantFunctorOps(fa)(Format.invariantFunctorFormat).inmap(f, g)
  // inmap's underlying:
//    Format[B](fa.map(f), Writes[B](b => fa.writes(g(b))))
}


case class AgentId(id: String) extends AnyVal

object AgentId {

  def fromIntId(i: Int) = i match {
    case 47 => apply("Hitman: Agent 47")
    case x: Int => apply(x.toString)
  }

  implicit val format = SingleFieldFormat.format(AgentId.apply, unlift(AgentId.unapply))
}

case class Profile(userId: AgentId, dob: String)
object Profile {
  implicit val format = Json.format[Profile]
}


val profile = Profile(AgentId.fromIntId(47), "09/05/1964")
val profileSerialized: String = Json.stringify(Json.toJson(profile))
val profileDeserialized: Profile = Json.fromJson[Profile](Json.parse(profileSerialized)).get
assert(profileDeserialized == profile)

// Console output:
// profile: Profile = Profile(AgentId(Hitman: Agent 47),09/05/1964)
// profileSerialized: String = {"userId":"Hitman: Agent 47","dob":"09/05/1964"}
// profileDeserialized: Profile = Profile(AgentId(Hitman: Agent 47),09/05/1964)
// res0: Unit = ()