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Callable values of user-defined nominal types

Available from Swift 5.2

Paul Hudson      @twostraws

SE-0253 introduced statically callable values to Swift, which is a fancy way of saying that you can now call a value directly if its type implements a method named callAsFunction(). You don’t need to conform to any special protocol to make this behavior work; you just need to add that method to your type.

For example, we could create a Dice struct that has properties for lowerBound and upperBound, then add callAsFunction so that every time you call a dice value you get a random roll:

struct Dice {
    var lowerBound: Int
    var upperBound: Int

    func callAsFunction() -> Int {
        (lowerBound...upperBound).randomElement()!
    }
}

let d6 = Dice(lowerBound: 1, upperBound: 6)
let roll1 = d6()
print(roll1)

That will print a random number from 1 through 6, and it’s identical to just using callAsFunction() directly. For example, we could call it like this:

let d12 = Dice(lowerBound: 1, upperBound: 12)
let roll2 = d12.callAsFunction()
print(roll2)

Swift automatically adapts your call sites based on how callAsFunction() is defined. For example, you can add as many parameters as you want, you can control the return value, and you can even mark methods as mutating if needed.

For example, this creates a StepCounter struct that tracks how far someone has walked and reports back whether they reached their target of 10,000 steps:

struct StepCounter {
    var steps = 0

    mutating func callAsFunction(count: Int) -> Bool {
        steps += count
        print(steps)
        return steps > 10_000
    }
}

var steps = StepCounter()
let targetReached = steps(count: 10)

For more advanced usage, callAsFunction() supports both throws and rethrows, and you can even define multiple callAsFunction() methods on a single type – Swift will choose the correct one depending on the call site, just like regular overloading.

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