While I was learning about "Functions", I met an issue showing "Extraneous argument label 'number:' in call" error msg for my code. I wonder why I shouldn't place "number" in argument?

func makeIncrementer() -> ((Int) -> Int) {
    func addOne(number: Int) -> Int {
        return 1   number
    }
    return addOne
}
var increment = makeIncrementer()
increment(number: 7)

enter image description here

CodePudding user response：

The problem is that you're returning a (Int) -> Int, which doesn't specify a name for its argument. Ideally, you'd want to write

func makeIncrementer() -> ((number: Int) -> Int)

but that isn't allowed in Swift:

function types cannot have argument labels; use '_' before 'number'

The best you can do is

func makeIncrementer() -> ((_ number: Int) -> Int)

This strategy might make your code a little clearer because giving the argument a name makes its purpose more obvious. Unfortunately, you still have to omit the number: label when you call the returned function:

let increment = makeIncrementer()
increment(7)  // <- no `number:` label

What's the rationale?

Specifying argument labels for function types was possible before Swift 3. The problem was that you could write

func add(numToAdd: Int) -> Int { ... }
func subtract(numToSubtract: Int) -> Int { ... }

let f: ((numToAdd: Int) -> Int) = subtract

and then calling f(numToAdd: 3) would actually call subtract(numToSubtract: 3), which is quite surprising and can cause confusion.

Check out this proposal for the full rationale behind removing this feature.