类型'（）'不符合协议'IntegerLiteralConvertible'

Question

func makeIncrementer() -> (Int -> Int) {
    func addOne(number: Int) -> Int {
        return 1 + number
    }
    return addOne
}

above is a simple example code for Function as first-class type in Swift now, when i call the call the function in the following way:

var increment = makeIncrementer()
increment(7)

it perfectly gives the answer

But out of curiosity i tried the direct approach ie

makeIncrementer(7)  // error

and it gives an error

why is it so??? PS I am a beginner in Swift

Answer 1

The call makeIncrementer() returns the function, so to call it you pass the parameter in a second set of parentheses:

makeIncrementer()(7)

The error message is given because Swift interprets makeIncrementer(7) as 7 being passed to makeIncrementer which doesn't take any parameters. Hopefully Swift error messages are made more friendly in the future. While technically correct, the error message given leads to a lot of confusion.