title: Scala play json combinators for validating equality
body: <p>I'm using play 2.2.0 Reads for validating incoming request in my application.</p>
  
  <p>I'm trying to implement a very simple thing with json API.
  I have a json like this: </p>
  
  <pre><code>{
    "field1": "some value",
    "field2": "some another value"
  }
  </code></pre>
  
  <p>I already have <code>Reads</code> that checks for other stuff like minimal length </p>
  
  <pre><code>case class SomeObject(field1: String, field2: String)
  implicit val someObjectReads = (
    (__ \ "field1").read(minLength[String](3)) ~
    (__ \ "field2").read(minLength[String](3))
  )(SomeObject)
  </code></pre>
  
  <p>I want to create a parser combinator that will match values of two fields and return <code>JsSuccess</code> if values are equal and otherwise <code>JsError</code> and combine it with existing <code>Reads</code>. </p>
  
  <p>How can I achieve this?</p>
  
  <p>UPDATE: clarified the question and changed the code.</p>