title: Correct way to unwrap an Optional Integer in Swift 3
body: <p>What is the correct way to set <code>fullLength</code> here? </p>
  
  <pre><code>var index = foo.index(foo.startIndex, offsetBy: SOME_NUM)
  let length = Int(foo.substring(to: index))
  
  if let fullLength = length? + SOME_NUM {
    return
  }
  </code></pre>
  
  <p>The problem here is <code>length?</code> is an Optional. I had this working with:</p>
  
  <p><code>fullLength = length! + SOME_NUM</code>, but I don't want to risk using <code>!</code> in the event <code>length</code> is nil. </p>