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Extending property wrappers to function and closure parameters

Available from Swift 5.5

Paul Hudson      @twostraws

SE-0293 extends property wrappers so they can be applied to parameters for functions and closures. Parameters passed this way are still immutable unless you take a copy of them, and you are still able to access the underlying property wrapper type using a leading underscore if you want.

As an example, we could write a function that accepts an integer and prints it out:

func setScore1(to score: Int) {
    print("Setting score to \(score)")
}

When that’s called we can pass it any range of values, like this:

setScore1(to: 50)
setScore1(to: -50)
setScore1(to: 500)

If we wanted our scores to lie only within the range 0...100 we could write a simple property wrapper that clamps numbers as they are created:

@propertyWrapper
struct Clamped<T: Comparable> {
    let wrappedValue: T

    init(wrappedValue: T, range: ClosedRange<T>) {
        self.wrappedValue = min(max(wrappedValue, range.lowerBound), range.upperBound)
    }
}

Now we can write and call a new function using that wrapper:

func setScore2(@Clamped(range: 0...100) to score: Int) {
    print("Setting score to \(score)")
}

setScore2(to: 50)
setScore2(to: -50)
setScore2(to: 500)

Calling setScore2() with the same input values as before will print different output, because the numbers will get clamped to 50, 0, 100.

Tip: Our property wrapper is trivial because parameters passed into a function are immutable – we don’t need to handle re-clamping the wrapped value when it changes because it won’t change. However, you can make your property wrappers as complex as you need; they work just as they would with properties or local variables.

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