Optional便利貼

CatEatSwift

```swift=
func perform(operation:String, a:Double, b:Double) -> Double! {
    var result:Double?
    switch operation {
    case "+":
        result = a + b
    case "-":
        result = a - b
    case "/":
        result = a / b
    case "*":
        result = a * b
    default:
        print("Error operation")
    }
//    print("Performing", operation, "on", a, "and", b)
    return result
}

if let result = perform(operation: "like", a: 3.0, b: 4.0) {
    print(result)
} else {
    print("error")
}
```