Why does Swift use parameter labels?

Paul Hudson    @twostraws    May 28th 2020

Updated for Xcode 14.2

Many programming languages either do not use parameter labels or make them optional. Swift is unusual in that it uses them extensively, and it even lets us differentiate between external parameter names and internal parameter names – it’s doubly unusual!

To look at why labels are useful, consider code like this:

setReactorStatus(true, true, false)

That’s perfectly normal code in many languages, but you will rarely if ever see that in Swift because we prefer to give our parameters names. So, we’d write our function like this:

func setReactorStatus(primaryActive: Bool, backupActive: Bool, isEmergency: Bool) {
    // code here
}

As a result, Swift will require those Booleans to be used when calling the function, like this:

setReactorStatus(primaryActive: true, backupActive: true, isEmergency: false)

And now our code becomes much clearer – we know exactly what each value does when we call the function, rather than having to remember whether isEmergency comes first or last.

Of course, Swift takes this even further by letting us write our labels twice, like this:

func setAge(for person: String, to value: Int) {
    print("\(person) is now \(value)")
}

This solves two problems at once: it makes those labels more meaningful inside the function, and also makes those labels more meaningful outside the function.

To demonstrate this, here’s how the function is called right now:

setAge(for: "Paul", to: 40)

If we had only used the person and value labels, we would have to write this instead:

setAge(person: "Paul", value: 40)

I hope you can see that the original code can actually be read aloud as a standard English statement: “set age for Paul to 40”. On the other hand, “set age person Paul value 40” isn’t natural at all.

Going the other way, if we had used only the for and to labels, then calling the function would look natural but using those values inside the function would be weird:

func setAge(for: String, to: Int) {
    print("\(for) is now \(to)")
}

In fact, Swift wouldn’t even allow that because it will think for is the start of a loop!

By having both the internal and external labels, our functions read more naturally both where we call them and inside the function itself. They aren’t required, and often you’ll have only one label for your parameters, but it’s still nice to have them around.