syntax - When are argument labels required in Swift?

Question

In answering this question it came about that argument labels were required for a call to init. This is normal in Swift.

class Foo {
    init(one: Int, two: String) { }
}

let foo = Foo(42, "Hello world") // Missing argument labels 'one:two:' in call

However, stranger forces are at play:

extension Foo {
    func run(one: String, two: [Int]) { }
}

foo.run(one: "Goodbye", two: []) // Extraneous argument label 'one:' in call

To use an argument label here it would have to be declared explicitly.

I haven't seen something very thorough explaining all of this in the documentation. For which varieties of class/instance/global functions are argument labels required? Are Obj-C methods always exported and imported with argument labels?

Answer 1

All init methods require parameter names:

var view = NSView(frame: NSRect(x: 10, y: 10, width: 50, height: 50))
class Foo {
    init(one: Int, two: String) { }
}

let foo = Foo(one: 42, two: "Hello world")

All methods called on an object use parameter names for everything but the first parameter:

extension Foo {
    func run(one: String, two: [Int]) { }
}
foo.run("Goodbye", two: [])

All including class functions in Swift and objective-c follow the same pattern. You also can explicitly add external names.

extension Foo{
class func baz(one: Int, two: String){}
class func other(exOne one: Int,  exTwo two: String){}
}
Foo.baz(10, two:"str")
Foo.other(exOne: 20, exTwo:"str")

Swift functions that are not a class function don't require parameter names, but you still can explicitly add them:

func bar(one: Int, two: String){}
bar(1, "hello")

As Bryan said, it's to make Swift method calls make sense when called on objective-c methods that have parameter names in the method signature. Init methods include the first parameter because Swift changes the init methods from objective-c from initWith:... to Class() so the first parameter name is no longer included in the method name.