title: How to write a Json Reads Combinator for a Map[Int, Long]
body: <p>I am trying to write a json reads combinator for type Map[Int, Long]</p>
  
  <p>I have this so far:</p>
  
  <pre><code>implicit val mapWrites = Json.writes[Map[Int, Long]]
  
  implicit val mapReads: Reads[Map[Int, Long]] = (
    // ???
  ) // ?
  </code></pre>
  
  <p>I'm not sure how this will work, I tried doing (Map[Int, Long].apply but that apply method was not available.  </p>
  
  <p>Looking for some help to write this Reads combinator.</p>