为什么代码产生错误类型'（int，int）'不符合协议'IntegerLiteralConvertible'

Question

With the code below, I get "Type '(int, int)' does not conform to protocol 'IntegerLiteralConvertible' instead of missing argument as one would expect. What's IntegerLiteralConvertible and why do you think the compiler produces this error instead for the code below?

I have looked at other SO posts regarding this error but have not gotten any insight from them.

func add(x:Int, y:Int) {

}

add(3)

Answer 1

My best guess is that it tries to convert the (3) tuple into a (Int, Int) tuple.

In fact, this is accepted by the compiler and works as expected:

func add(x: Int, y: Int) -> Int {
  return x + y
}

let tuple = (4, 7)
add(tuple)

In playground that outputs 11 , which is the expected sum result.

Note : the code above works if the func is global, with no named parameters. If it's an instance or class/static method, then the tuple must include parameter names:

class MyClass {
    class func add(# x: Int, y: Int) -> Int {
        return x + y
    }
}

let tuple = (x: 3, y: 7)

MyClass.add(tuple) // returns 10

As for IntegerLiteralConvertible , it's used to make a class or struct adopting it to be initializable from a literal integer. Let's say you have a struct and you want to be able to instantiate by assigning an literal integer, you achieve it this way:

struct MyDataType : IntegerLiteralConvertible {
    var value: Int

    static func convertFromIntegerLiteral(value: IntegerLiteralType) -> MyDataType {
        return MyDataType(value: value)
    }

    init(value: Int) {
        self.value = value
    }
}

and then you can create an instance like this:

let x: MyDataType = 5

Answer 2

It looks like you are trying to use currying — Swift has built-in support for this, but it is not automatic, so you have to be explicit about it when declaring your function:

func add(x:Int)(y:Int) -> Int {
    return x + y
}

println(add(3)) // (Function)