Functions are preceded with the fun keyword. Function parameters require types, and are immutable. Return types should be supplied after the function name, but in some cases may also be inferred by the compiler.

Named Functions

Named function have a name assigned to them that can be used to invoke them directly (this is the expected form of a “function” in most cases, and the form that you’re probably expecting).

// no parameters required
fun main() {
    println(sum1(1, 2))

// parameters which require type annotations
fun sum1(a: Int, b: Int): Int { 
    return a + b 

// return types can be inferred based on the value you return
// it's better form to explicitly include the return type in the signature
fun sum2(a: Int, b: Int) {
    a + b // Kotlin knows that (Int + Int) -> Int

Single-Expression Functions

Simple functions in Kotlin can sometimes be reduced to a single line aka a single-expression function.

// previous example
fun sum(a: Int, b: Int) {
  a + b // Kotlin knows that (Int + Int) -> Int

// this is equivilant
fun sum(a: Int, b: Int) = a + b

// this works since we evaluate a single expression
fun minOf(a: Int, b: Int) = if (a < b) a else b

Function Parameters

Default arguments

We can use default arguments for function parameters. When called, a parameter with a default value is optional; if the value is not provided by the caller, the default will be used.

// Second parameter has a default value, so it’s optional
fun mult(a:Int, b:Int = 1): Int { 
	return a * b 

fun main() {
	mult(1) // 1 
	mult(5,2) // 10 
	// mult() will throw an error, `a` must be provided

Named parameters

You can (optionally) provide the parameter names when you call a function. If you do this, you can even change the calling order!

fun repeat(s:String="*", n:Int=1):String {
    return s.repeat(n)

fun main() {
	println(repeat()) // *
	println(repeat(s="#")) // *
	println(repeat(n=3)) // ***
	println(repeat(s="#", n=5)) // #####
	println(repeat(n=5, s="#")) // #####

Variable-length arguments

Finally, we can have a variable length list of arguments:

// Variable number of arguments can be passed!
// Arguments in the list need to have the same type 

fun sum(vararg numbers: Int): Int { 
    var sum: Int = 0 
    for(number in numbers) { 
        sum += number
  return sum 

fun main() {
    sum(1) // 1
    sum(1,2,3) // 6 
    sum(1,2,3,4,5,6,7,8,9,10) // 55