jsonExample.scala

import play.api.libs.json._
import play.api.libs.json.Reads._
import play.api.libs.functional.syntax._
import scala.util.control.Exception._

object Main extends App {

  case class Person(name: String, height: Double)

  val goodJson = Json.parse("""
    {
      "foo": "bar",
      "baz": "1.4"
    }
  """)

  val badJson = Json.parse("""
    {
      "foo": "bar",
      "baz": "Unparseable"
    }
  """)

  def toNum(implicit r: Reads[String]) = Reads[JsNumber](
    r.reads(_).flatMap(s =>
      allCatch.either(s.toDouble).fold(
        _ => JsError("error.expected.jsnumber"),
        d => JsSuccess(JsNumber(d)))))

  val readPerson = (
    (__ \ 'foo).read[String] ~
    (__ \ 'baz).json.update(toNum).andThen(
      (__ \ 'baz).read[Double]
    )
  )(Person)

  println(goodJson.validate[Person](readPerson)) // JsSuccess(Person(bar,1.4),)
  println(badJson.validate[Person](readPerson)) // JsError(List((/baz,List(ValidationError(List(error.expected.jsnumber),WrappedArray())))))
}