title: Swift `in` keyword meaning?
body: <p>I am trying to implement some code from parse.com and I notice a keyword <code>in</code> after the void.</p>
  
  <p>I am stumped what is this ? The second line you see the <code>Void in</code></p>
  
  <pre><code>PFUser.logInWithUsernameInBackground("myname", password:"mypass") {
    (user: PFUser?, error: NSError?) -&gt; Void in
    if user != nil {
      // Do stuff after successful login.
    } else {
      // The login failed. Check error to see why.
    }
  }
  </code></pre>
  
  <p>The docs don't document this. I know the <code>in</code> keyword is used in <code>for</code> loops.</p>
  
  <p>Anyone confirm?</p>