Swift 2.2上的可选绑定错误？

Question

if let mathematicalSymbol = sender.currentTitle {
    brain.performOperation(mathematicalSymbol)
}

The code above introduces the error below;

Value of optional type 'String?' not unwrapped; did you mean to use '!' or '?'?

As can be seen in this screen shot;

sender.currentTitle is an optional.

Here is an excerpt from Apple's " The Swift Programming Language (Swift 2.2) " with its example code just below it;

If the optional value is nil , the conditional is false and the code in braces is skipped. Otherwise, the optional value is unwrapped and assigned to the constant after let , which makes the unwrapped value available inside the block of code.

Here is the sample code for that excerpt;

var optionalName: String? = "John Appleseed"
var greeting = "Hello!"
if let name = optionalName {
    greeting = "Hello, \(name)"
}

So for these reasons, I'm thinking that either I'm missing something or that I'm hitting a bug .

I've also tried something similar on a Playground, and didn't get a similar error;

Here is my Swift version;

Apple Swift version 2.2 (swiftlang-703.0.18.8 clang-703.0.31)
Target: x86_64-apple-macosx10.9

Answer 1

If you look at currentTitle , you'll see it is likely inferred to be String?? . For example, go to currentTitle in Xcode and hit the esc key to see code completion options, and you'll see what type it thinks it is:

I suspect you have this in a method defining sender as AnyObject , such as:

@IBAction func didTapButton(sender: AnyObject) {
    if let mathematicalSymbol = sender.currentTitle {
        brain.performOperation(mathematicalSymbol)
    }
}

But if you explicitly tell it what type sender is, you can avoid this error, namely either:

@IBAction func didTapButton(sender: UIButton) {
    if let mathematicalSymbol = sender.currentTitle {
        brain.performOperation(mathematicalSymbol)
    }
}

Or

@IBAction func didTapButton(sender: AnyObject) {
    if let button = sender as? UIButton, let mathematicalSymbol = button.currentTitle {
        brain.performOperation(mathematicalSymbol)
    }
}