title: Swift: AnyObject cast to Float failed
body: <pre><code>let json = [
      "left" : 18,
      "deadline" : "May 10",
      "progress" : 0.6
  ] as [String: AnyObject]
  
  let ss = json["progress"] as? Float
  let sss = json["progress"] as? Double
  print("ss = \(ss)\n  sss = \(sss)")
  </code></pre>
  
  <p>I have no idea why the <code>ss</code> shows nil while <code>sss</code> shows <code>0.599999998</code>. Why does casting to <code>Float</code> get nil? Do you guys have some methods so that I can get the correct result?  </p>