Argument Labels

Argument labels are the friendly words you type before a value when calling a function, so the call reads like a sentence.

In this page:

By default, calling a function requires using each parameter's name as a label, which makes the call self-documenting.

Example: Default Argument Labels

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func multiply(x: Int, y: Int) -> Int {
    return x * y
}
print(multiply(x: 6, y: 7))

You can give a parameter a different label for callers than the name used inside the function body, improving readability at the call site.

Note: Here person is what callers type, while name is used inside the function body.

Example: Custom External Labels

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func greet(person name: String) -> String {
    return "Hello, \(name)"
}
print(greet(person: "Alice"))

Using an underscore _ as the external label allows the function to be called without naming that particular argument.

Example: Omitting Labels with Underscore

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func square(_ number: Int) -> Int {
    return number * number
}
print(square(5))

Common Mistakes

Confusing the argument label (used at the call site) with the parameter name (used inside the function body) -- they can be different words.
Forgetting that using _ as the label removes the requirement to write a label at all when calling.
Assuming all Swift functions require labels; the first parameter's label is often omitted by convention when it reads naturally without one.

Chapter Summary

By default, a parameter's name serves as both its external label and internal name.
You can specify a different external label before the internal name: func f(externalLabel internalName: Type).
Using _ as the external label lets the function be called without naming that argument.
Good argument labels make call sites read like natural English.

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