defining 'reads' in companion w/ private constructor

ReadsExample.scala

scala> :paste
// Entering paste mode (ctrl-D to finish)

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

case class Person private(age: Int)

object Person {
  def build(age: Int): Option[Person] = 
    if (age >= 0 && age <= 125) Some(Person(age)) else None

  implicit val reads: Reads[Person] = 
   (__ \ 'age).read[Int].map(a => Person(a))

}

Corrected:

...
  implicit val reads: Reads[Person] = 
   (__ \ 'age).read[Int].map(a => Person.build(a).getOrElse(throw new RuntimeException("invalid age!")))
}

https://gist.github.com/dwijnand/2874b497654ed4867528d1728bcb14a7