How I Finally Understood Swift Optionals

I had a bit of trouble understanding Swift Optionals. When should I use a question mark, and when should I use an exclamation point? The iBook was a bit confusing on this point for me. Until I was sitting at IndyCocoaHeads last night and @AndyDuff was giving a talk about Swift. When he got to the part about Optionals, something finally clicked for me and I was able to finally understand them.

Optionals are like Schrödinger’s Cat!

Think of it like this, you can have an integer, and we all know that integers are whole numbers (-1, 0, 1 , 2, 3…) within a certain range depending on what type of system you are on. In Swift you would declare it as such:

var n: Int = 23

The compiler does not need the type here, but I included it for clarity. This is telling the compiler that while, n “could” be any integer, here, it’s 23. Now lets look at an Optional:

var n: Int? = 23

In this line we are telling the compiler something else. Currently n is 23, but n “could” be any integer OR nil. If you are familiar with Objective-C, this is quite familiar with NS objects. NSString can equal @”Hello, World” but you can also set it to nil, which is not a string, or any type for that matter. This is why I like to say it’s like Schrödinger’s Cat If you are not familiar with the concept, I believe this link is a fun place to start. Essentially the variable can contain a value, or nil (the cat is either alive or dead), but we don’t know until we check.

if (n != nil) {
    println("n = \(n!)." // Note the ! after n.
}

Once you are 100% sure that n does have a value, then you can force it to behave like a normal integer by using “!”, if you use ! and n is a nil value, then your app will crash.

var n: Int? // Optionals default their initial value to nil.
println("n = \(n)." // This will crash because it's trying to print nil.

Once you get the hang of it, it becomes natural and feels like things we have done in Objective-C for quite some time now just we now have access to this feature on “Primitive” types.