Swift 2: guard

In Swift 2, there is a new keyword for handling the control-flow of a block of code. It guarantees, that a specific condition holds true for the further code. Otherwise, it ensures that the following code will not be executed.

Let us start with a simple example. Imagine you have a function, that has an optional as an argument:

func testItOld(a: Int?) {
    if let a = a {
        print("a: (a)")
    } else {
        print("not ok")
    }
}

Because a  is an optional, you need to check whether a  is nil or not.

In this situation, guard gives you a simpler possibility:

func testItWithGuard(a: Int?) {
    guard let a = a else {
        print("not ok")
        return
    }
    
    print("a: (a)")
}

In the guard condition, there is conditional binding of a . If it fails, the else part of the guard statement must ensure that the following code will not be executed. In this case, the function is returned.

After the guard statement, a is not an optional anymore, which makes the handling of optionals easier then before.

Generally, the guard statement can be used everywhere, where one of the following statements can be called:

return, break, continue, throw

If none of these keywords are called, there will be a compiler error!

So you can for example ensure, that a condition holds for a loop iteration:

var numbers: [Int?] = [1,2,3,nil,5]

for a in numbers {
    guard let a = a else {
        continue
    }
    print("value (a)")
}

Another good application case is throwing an error. Our previous example can be rewritten to

enum anErrorType: ErrorType {
    case numberNotValid
}

func testItWithGuard(a: Int?) throws {
    guard let a = a else {
        print("not ok")
        throw anErrorType.numberNotValid
    }
    
    print("a: (a)")
}