Learn

Functions use fun. Parameter types are required; return type goes after :. Type can be inferred for single-expression functions.

fun add(a: Int, b: Int): Int {
    return a + b
}

// Single-expression form:
fun multiply(a: Int, b: Int) = a * b   // return type inferred as Int

Default arguments and named arguments make signatures very flexible:

fun greet(name: String, greeting: String = "Hello") = "$greeting, $name!"

greet("Alice")                         // Hello, Alice!
greet("Bob", greeting = "Howdy")        // Howdy, Bob!
greet(name = "Carol")                  // Hello, Carol!

Vararg with vararg:

fun sum(vararg nums: Int) = nums.sum()
sum(1, 2, 3, 4)   // 10