title: How to make a closure as function parameter optional
body: <p>I have a helper to display an AlertView:</p>
  
  <pre><code>func showAlertBoxOK(headline:String, message:String, OkButtonText:String, viewController:UIViewController, completionOK:() -&gt; Void){
  
      let alertController = UIAlertController(title: headline, message: message, preferredStyle:UIAlertControllerStyle.Alert)
  
      alertController.addAction(UIAlertAction(title: OkButtonText, style: UIAlertActionStyle.Default)
          { action -&gt; Void in
              if completionOK() != nil {
                  completionOK()
              }
          })
      viewController.presentViewController(alertController, animated: true, completion: nil)
  }
  </code></pre>
  
  <p>Now I would like to make the parameter <code>completionOK</code>optional. I´ve tried
  completionOK:() -> Void? = nil` but his gives me a compiler error. My calls should be like this without parameter:</p>
  
  <pre><code>showAlertBoxOK("Could not retrieve position", "Edit iOS Settings - GeoLocation denied. Sorry. Please fix that und restart App.", "OK", self)
  </code></pre>
  
  <p>and with parameter as this:</p>
  
  <pre><code>showAlertBoxOK("Could not retrieve position", "Edit iOS Settings - GeoLocation denied. Sorry. Please fix that und restart App.", "OK", self, { println("hello world") })
  </code></pre>
  
  <p>Any help?</p>