Extension functions and default arguments

Add a function to an existing type

Extension syntax places the receiver type before the function name:

The following example shows extension syntax. Integer extension functions still need CLI support for binding this.

func int.withBonus(bonus: int): int {
    return this + bonus
}

func main() {
    var score = 20
    print(score.withBonus(5))
    print(score)
}

Use this ordinary function for the same calculation:

func withBonus(score: int, bonus: int): int {
    return score + bonus
}

func main() {
    var score = 20
    print(withBonus(score, 5))
    print(score)
}

This returns the calculated value. The original variable remains 20. Import the function's namespace to reuse it across files.

Set a default argument

func reward(base: int, bonus: int = 5): int {
    return base + bonus
}

func main() {
    print(reward(10))
    print(reward(10, 20))
}

The first call uses the default and prints 15. The second call supplies bonus and prints 30. Place parameters with defaults at the end of the parameter list. Supply arguments by position.

Read Lambdas and function types to pass behavior to another function.