title: Cannot assign to value: 'message' is a 'let' constant in a fucntion
body: <p>I'm trying to create a function that checks if a value is nil and if it is give it a default value.  This is what I have so far:</p>
  
  <pre><code>func appendMessage(message: String?, fixLength: Int){
          if let message = message {
          } else {
               message = DEFAULT_CHAR//this is a default value, error is here
          }
            messageBody.append(getFixedLengthString(message, fixLength))
      }
  </code></pre>