title: How to use substringToIndex in Swift?
body: <p>I get compiler error at this line:</p>
  
  <pre><code>UIDevice.currentDevice().identifierForVendor.UUIDString.substringToIndex(8)
  </code></pre>
  
  <p><em>Type 'String.Index' does not conform to protocol 'IntegerLiteralConvertible'</em></p>
  
  <p>My intention is to get the substring, but how?</p>