title: Scala's either with tuple as Right
body: <p>Suppose I have following code:</p>
  
  <pre><code>val either: Either[String, (Int, Int)] = Right((1,2))
  for {
    (a, b) &lt;- either.right
  } yield a + b
  </code></pre>
  
  <p>When I evaluate it in REPL I get</p>
  
  <blockquote>
    <p>:13: error: constructor cannot be instantiated to expected
    type;  found   : (T1, T2)  required:
    scala.util.Either[Nothing,(Double, Double)]
                  (a, b) &lt;- a.right
                  ^ :14: error: not found: value a
                  } yield a + b
                          ^</p>
  </blockquote>
  
  <p>Why do I have such error? Can't I pattern match on tuple from Either's Right?</p>