title: Can I use default values in a JSON decoder?
body: <p>I make a JSON request to a web API about books. My responses <em>usually</em> look like</p>
  
  <pre><code>{
      "status": "successful",
      "author": "Roald Dahl",
      "title": "Charlie and the Chocolate Factory"
  }
  </code></pre>
  
  <p>So I decode it to a <code>Book</code> type using the author and title fields.</p>
  
  <p>But sometimes, the requested book won't be in the database, so my response is just</p>
  
  <pre><code>{
      "status": "failed"
  }
  </code></pre>
  
  <p>In this case I'd still look to return a <code>Book</code> type, but with the author and title set to "NOT FOUND".</p>
  
  <p>I'm reading the JSON documentation, but I'm not sure if anything there is helpful to me or if I can even do this in Elm in a simple way. Would appreciate some advice.</p>