title: error Nil is not compatible with expected argument type '() -> Void'
body: <p><strong>This code:</strong></p>
  
  <pre><code>runAction(moveleft, completion: nil) 
  </code></pre>
  
  <p>Gives error:</p>
  
  <blockquote>
    <p>Nil is not compatible with expected argument type '() -> Void'</p>
  </blockquote>
  
  <p>Why is that?</p>