title: Optional values in Elixir/Phoenix EEx shared templates
body: <p>I'm trying to pass values inside Phoenix EEx shared templates (templates that I reuse). So far, so good, but now I would like to make some of them mandatory and others optional. For example:</p>
  
  <pre><code>&lt;a href="&lt;=% @url %&gt;" class="core Item &lt;%= className %&gt;"&gt;
  </code></pre>
  
  <ul>
  <li><code>url</code> should be mandatory - this is the default (OK);</li>
  <li><code>className</code> should be optional - I can't make it because if I don't include it when calling the template 
  (<code>&lt;%= render MyProject.SharedView, "myTemplate.html", url:"logo2.png" %&gt;</code>)
  there's an error.
  How can I do this?</li>
  </ul>