Scope Functions & Extensions

Scope Functions & Extension Functions

Kotlin provides five scope functions that execute a block of code on an object: let, also, apply, run, and with. They differ in how they refer to the object and what they return.

let

let passes the object as it and returns the lambda result. Often used with safe calls:

val name: String? = "Alice"
val len = name?.let { it.length }
println(len) // 5

also

also passes the object as it and returns the object itself. Great for side effects like logging:

val numbers = mutableListOf(1, 2, 3)
numbers.also { println("Before: $it") }
       .add(4)
println(numbers) // [1, 2, 3, 4]

apply

apply gives access to the object as it and returns the object itself. Useful for configuring objects:

val result = mutableListOf(1, 2).apply { it.add(3) }
println(result) // [1, 2, 3]

run

run gives access to the object as it and returns the lambda result:

val greeting = "Hello".run { "$it, World!" }
println(greeting) // Hello, World!

with

with is not an extension — you pass the object as an argument. It returns the lambda result:

val numbers = listOf(1, 2, 3)
val summary = with(numbers) { "Count: ${it.size}" }
println(summary) // Count: 3

Extension Functions

Extension functions let you add new methods to existing types without modifying them:

fun String.exclaim(): String = this + "!"
fun String.repeat3(): String = this + this + this

println("Hello".exclaim()) // Hello!
println("ha".repeat3())    // hahaha

Your Turn

1. Write an extension function String.shout() that returns the string uppercased with "!" appended.
2. In main, use let to transform a string, also for a side effect, and with to build a summary.