title: how to filter list by type in haskell
body: <p>I have a list, with elements of some Node type, which may be either UNode or LNode or RNode</p>
  
  <pre><code>[
      UNode("a"), LNode("b"), RNode("c")
  ]
  </code></pre>
  
  <p>How can I filter it by the type ( UNode for example)</p>