title: Cannot call a value of non-function type DateCallBack
body: <p>The piece of code I am writing used to work charm before migrating to swift 3. But the, doomsday, I am migrating it to XCode 8 and I can't seem to resolve this bug in my code. This issue is out of my understanding. I think  I have missed the basics over here.
  <code>DateCallBack</code> is a typealias I defined before declaring my class.</p>
  
  <pre><code>typealias DateCallBack = ((_ year: Int?, _ month: Int?) -&gt; Void)
  </code></pre>
  
  <p>Then I have declared an instance of DateCallBack.</p>
  
  <pre><code>private var callBack: DateCallBack!
  </code></pre>
  
  <p>And then within an extension of the class, I have implemented two methods</p>
  
  <pre><code> func cancelButtonAction() {
      self.callBack(year: nil, month: nil)
      self.animateOut { (completion) in
          if completion {
              self.removeFromSuperview()
          }
      }
  }
  
  func doneButtonAction(_ yearIndex: Int!, monthIndex: Int!) {
      self.callBack(year: yearIndex, month: monthIndex)
      self.animateOut { (completion) in
          if completion {
              self.removeFromSuperview()
          }
      }
  }
  </code></pre>
  
  <p>And the error I get is <code>Cannot call a value of non-function type DateCallBack</code>on both functions self.callBack that is second line of both the functions.
  Please explain me what I have missed too.</p>