Functions, Lambdas and Extension Functions

Function Basics

fun discount(price: Int, pct: Int = 10): Int {
    return price - price * pct / 100
}
discount(500)            // default arg
discount(price = 500, pct = 20)  // named args

Single-Expression Functions

fun square(n: Int) = n * n
fun isAdult(age: Int) = age >= 18

Lambdas and Higher-Order Functions

val nums = listOf(1, 2, 3, 4)
nums.filter { it % 2 == 0 }.map { it * 10 }  // [20, 40]
fun retry(times: Int, block: () -> Unit) {
    repeat(times) { block() }
}
retry(3) { println("trying") }  // trailing lambda

Extension Functions

fun String.slug(): String =
    lowercase().replace(Regex("[^a-z0-9]+"), "-")
"Hello World!".slug()  // hello-world

Add behaviour to any class - even JDK and library classes - without inheritance.

Scope Functions at a Glance

• let - transform nullable values.
• apply - configure an object, return it.
• also - side effects (logging) in chains.
• run/with - compute with an object in scope.

Key Points

• Defaults plus named args kill most overloads.
• Trailing lambdas make DSLs and retries beautiful.
• Extensions add power without subclassing.