Scope functions

Learn let, run, apply, also and with scope functions.

Scope functions

Scope functions work on an object in a temporary scope. Kotlin has five: let, run, apply, also and with.

Example: val list = mutableListOf<Int>().apply { add(1); add(2) }. apply configures the object and returns it. Calling val s = "hi".let { it.uppercase() } returns "HI". let produces a return value; apply returns the object. Each scope function gives a different context; the right choice makes code short and readable.

apply and let

apply and let are the most common scope functions. apply suits configuration, let suits transformation.

Example: val list = mutableListOf<Int>().apply { add(1); add(2) } returns [1, 2]. val upper = "hi".let { it.uppercase() } returns "HI". Inside apply, this refers to the object; inside let, it refers to the object. Which scope function to choose depends on the goal.

Exercise Using apply and let

Declare val list = mutableListOf<Int>().apply { add(1); add(2) }. Declare val upper = "hi".let { it.uppercase() }. In main call println(list) and println(upper) on separate lines.

Show example solution

val list = mutableListOf<Int>().apply { add(1); add(2) }
val upper = "hi".let { it.uppercase() }

fun main() {
    println(list)
    println(upper)
}

Test yourself

Match the scope functions

let

Produces a return value
run

Runs code on the object
apply

Returns the object, this
also

Returns the object, it
Which scope function configures an object and returns it?
- apply
- run
- let
- with
Show answer

apply — apply configures the object and returns it.

Complete the object with apply

val list = mutableListOf<Int>().____ { add(1); add(2) }

Show answer

val list = mutableListOf<Int>().apply { add(1); add(2) }

Next lesson: Summary - Higher-order functions

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