cannot assign to value 'colorTouched' is a 'let' constant : <p>beginner of swift, follow the book and get the error ""</p>

<p>here is the code:</p>

<pre><code>@IBAction func buttonTouched(sender : UIButton) {
    var buttonTag : Int = sender.tag

    if let colorTouched = ButtonColor(rawValue: buttonTag) {
        if currentPlayer == .Computer {
            return
        }

   //error here:     
   if colorTouched = inputs[indexOfNextButtonToTouch] {
            indexOfNextButtonToTouch += 1

            if indexOfNextButtonToTouch == inputs.count {
                // 玩家成功地完成了这一轮
                if advanceGame() == false {
                    playerWins()
                }
                indexOfNextButtonToTouch = 0
            }
            else {
            }
        }
        else {
            playerLoses()
            indexOfNextButtonToTouch = 0
        }
    }
}
</code></pre>

<p>so if I cannot use "if let colorTouched", what should I do with it?</p>
