Named arguments

Label an argument with the corresponding parameter name:

fun area(width: Int, height: Int) Int {
    return width * height;
}

var pixels = area(width = 1920, height = 1080);

Reorder arguments

Named arguments may appear in a different order:

var pixels = area(height = 1080, width = 1920);

Each label must match a parameter, and a parameter cannot receive more than one argument.

Improve call-site meaning

fun retry(task: String, attempts: Int, delayMs: Int) {
    println(task);
}

retry(
    task = "upload",
    attempts = 3,
    delayMs = 500
);

Labels are useful when adjacent arguments have the same type or when a literal would otherwise have unclear meaning.

Overload resolution

Labels can identify parameters in an overload set:

fun mix(x: Int, y: Float) Int { return 1; }
fun mix(x: Float, y: Int) Int { return 2; }

var first = mix(y = 1.0, x = 2);
var second = mix(y = 3, x = 4.0);

The declared parameter names are therefore part of the function’s source-level calling interface.