title: Default optional parameter in Swift function
body: <p>When I set <code>firstThing</code> to default <code>nil</code> this will work, without the default value of <code>nil</code> I get a error that there is a missing parameter when calling the function.</p>
  
  <p>By typing <code>Int?</code> I thought it made it optional with a default value of <code>nil</code>, am I right? And if so, why doesn't it work without the <code>= nil</code>?</p>
  
  <pre><code>func test(firstThing: Int? = nil) {
      if firstThing != nil {
          print(firstThing!)
      }
      print("done")
  }
  test()
  </code></pre>