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.

Answer 2

As of Swift 3.0 this has changed again: all methods, functions, and initializers require argument labels for all parameters, unless you have explicitly opted out using the external name _. This means methods such as addChildViewController(_:) are now written like this:

func addChildViewController(_ childController: UIViewController) 

This was proposed and approved as part of the Swift Evolution process, and was implemented in SR-961.