Named Parameters

One thing you may have noticed in the last section was how Swift has named parameters. At first this can be annoying if you’re not used to it. I certainly felt that way, but now, they’re one of my favorite features in Swift and I miss it when I use other languages.

When calling a function, you must specify the parameter names.

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

let sum = add(x: 3, y: 4)

You might think that this means you can specify the parameters in any order, but that is not true:

let sum2 = add(y: 4, x: 3) // Not allowed

This is often used in API design to make function calls readable as English, like the following example:

board.move(task: myTask, from: .inProgress, to: .done)

In cases like the above example, you may want to use a different name to refer to your variables inside the function body than what the caller provides. This is also possible by specifying first the argument label (what the caller types), and then the parameter name (variable name within the function).

func move(task: Task, from sourceList: ListType, to destList) {
    ...
}

In some cases, like the add(x,y) example above, an argument label doesn’t provide any additional context or value. In that case, you can make a parameter unnamed by providing _ as the argument label:

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

In addition to providing additional context when making a function call, argument labels also allow you to omit any parameters that you are leaving at their default values, while still providing values that appear after them:

func addTask(_ description: String, to board: Board, completed: Bool = false, category: TaskCategory = .misc, size: Int = 3) {
    ...
}

addTask("Take out trash", to: myBoard)
addTask("Do dishes", to: myBoard, size: 2)
addTask("Write code", to: myBoard, completed: true, size: 4)
addTask("Do homework", to: myBoard, category: .school)

Next: Optionals