title: How map only left value from scala Either?
body: <p>Consider a code:</p>
  
  <pre><code>val some: OneCaseClass Either TwoCaseClass = ???
  val r = some.left.map(_.toString)
  </code></pre>
  
  <p>Why is r <code>Serializable with Product with Either[String, TwoCaseClass]</code> type instead of <code>Either[String, TwoCaseClass]</code>?</p>
  
  <p>How to map only left value?</p>