Key points

I hope you can recognize that Swift takes code safety seriously. This takes a number of forms, of which the most important ones are:

1. All your variables and constants must have a specific type, like Int or String. There is a special Any type, used when you want to mix data together, but even then you’ll need to typecast values back to Int or whatever in order to use them.
2. If you create a function that says it will return a value, Swift will ensure it always does, no matter what route is taken through its code.
3. Optionals ensure you always work with real values. Swift won’t let you touch optionals if they are nil, unless you specifically force override it – there’s a reason ! is sometimes called “the crash operator”.
4. When you use a switch/case block, it must be exhaustive – it must cover all possible cases.

Let’s take another look at some optional code. Consider this:

var a: Int? = 5
var b: Int? = 10

That creates two variables, a and b, both of which store optional integers. That means they might contain a number like 0, 10, 2309832, or they also be nil – they might have no value at all. How would you add those two numbers together?

Remember: Swift doesn’t let you use optionals directly; you need to unwrap them safely first. This is because they can be nil, and “5 + nil” is undefined behavior that used to crash your app or produce weird behavior before Swift came along with its optionals.

In this case, we can unwrap a and b using if let. This creates new constants that are regular integers rather than optional, so we can work with them directly:

if let unwrappedA = a {
    if let unwrappedB = b {
        let c = unwrappedA + unwrappedB
    }
}

That let c line will only be reached if both a and b had a value, so Swift knows it’s safe to run.

Swift lets you merge those two if let statements into one, like this:

if let unwrappedA = a, let unwrappedB = b {
    let c = unwrappedA + unwrappedB
}

While that’s common practice, I prefer to avoid it in this book because it’s a bit harder to read.

Another important take-away is that Swift is a modern programming language. It used many of the latest development techniques when it launched, as has since evolved further. This means we benefit from great features like optionals, closures, switch/case ranges, and more.

Finally, I hope you feel like you’re able to go back to the playground now and start writing a little of your own code. Sure, there’s no iOS code yet, but you can still have a go at attempting the challenge below.

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