Named Arguments
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fun functionName(param1: Type, param2: Type = defaultValue) { }
functionName(param1 = value1, param2 = value2)
functionName(param2 = value2, param1 = value1)Basic Named Arguments
Instead of relying on position, you can specify parameterName = value explicitly when calling a function, which makes the call self-documenting.
Example: Basic Named Arguments
// Define the function `createRectangle` taking `width`, `height` and returning a String
fun createRectangle(width: Int, height: Int): String {
// Return `"Rectangle ${width}x$height"` from this function
return "Rectangle ${width}x$height"
}
// Entry point: execution of the program starts here
fun main() {
// Print `createRectangle(width = 10, height = 5)` to the console, with a trailing newline
println(createRectangle(width = 10, height = 5))
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Reordering with Named Arguments
Because named arguments are matched by name rather than position, they can be passed in a different order than they were declared.
Example: Reordering with Named Arguments
// Define the function `createRectangle` taking `width`, `height` and returning a String
fun createRectangle(width: Int, height: Int): String {
// Return `"Rectangle ${width}x$height"` from this function
return "Rectangle ${width}x$height"
}
// Entry point: execution of the program starts here
fun main() {
// Print `createRectangle(height = 5, width = 10)` to the console, with a trailing newline
println(createRectangle(height = 5, width = 10))
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Skipping Defaults with Named Arguments
Named arguments let you override just one later default parameter without having to also repeat the earlier defaults you're happy to keep.
Example: Skipping Defaults with Named Arguments
// Define the function `createUser` taking `name`, `age`, `active` and returning a String
fun createUser(name: String, age: Int = 18, active: Boolean = true): String {
// Return `"$name (age $age, active=$active)"` from this function
return "$name (age $age, active=$active)"
}
// Entry point: execution of the program starts here
fun main() {
// Print `createUser("Carol", active = false)` to the console, with a trailing newline
println(createUser("Carol", active = false))
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Improving Readability
For functions with several parameters of the same type, such as multiple booleans, named arguments make the call site far easier to understand at a glance.
Example: Improving Readability
// Define the function `configure` taking `darkMode`, `notifications`, `autoSave`
fun configure(darkMode: Boolean, notifications: Boolean, autoSave: Boolean) {
// Print "dark=$darkMode, notify=$notifications, autoSave=$autoSave" to the console, with a trailing newline
println("dark=$darkMode, notify=$notifications, autoSave=$autoSave")
}
// Entry point: execution of the program starts here
fun main() {
configure(darkMode = true, notifications = false, autoSave = true)
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- Mixing named and positional arguments incorrectly; once you use a named argument, arguments after it should generally also be named.
- Assuming named arguments change performance or behavior; they only affect readability and call-site flexibility.
- Not using named arguments for functions with several
Booleanparameters, making call sites hard to read (e.g.f(true, false, true)).
- Named arguments are passed as
parameterName = value, independent of declaration order. - They greatly improve readability for functions with many parameters or several parameters of the same type.
- Named arguments can be combined with default parameters to skip earlier defaults while setting a later one.
- Once a named argument is used, any following arguments in that call should also be named for clarity (and sometimes are required to be).
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