常量'结果'推断为具有type（），这可能是意外的

@IBAction func operate(sender: UIButton) {

     if let operation = sender.currentTitle {
         if let result = brain.performOperation(operation) {

            displayValue   = result
         }
         else {
            displayValue = 0.0
         }

    }
}
@IBAction func operate(sender: UIButton) {