title: Swift 4 - Private method inside extension
body: <p>I'm looking for a way to write a private function inside an extension. </p>
  
  <p>For example: </p>
  
  <pre><code>class A: UIViewController {
      override viewDidLoad() {
          privateFoo()
      }
  }
  
  private extension A {
      func foo() { 
          privateFoo()
      }
  
      private func privateFoo() { //Helper function for foo(), expected to be called inside the scope of this extension only
  
      }
  }
  </code></pre>
  
  <p>However, even if I declare privateFoo() as <code>private</code>, I can still call it from outside of the extension, which is not what I expected it to be. </p>
  
  <p>Can you someone please help me how to achieve my goal? </p>