title: what do "_" and "in" mean in Swift programming language?
body: <p>I found some code written in Swift on <a href="https://gist.github.com/frozzare/cfb79749e56b5abb00c7" rel="noreferrer">github</a>, </p>
  
  <p>And get a little confused about this line</p>
  
  <pre><code>var done: (NSError?, NSData, NSString?) -&gt; () = { (_, _, _) -&gt; () in }
  </code></pre>
  
  <p>could you please explain the real meaning of this line? Thank you very much!</p>