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Kotlin let, run, also, apply, with

Published on August 3, 2022
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By Anupam Chugh
Developer and author at DigitalOcean.
Kotlin let, run, also, apply, with

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In this tutorial, we’ll be implementing some of the important standard library functions available in Kotlin. The kotlin-stdlib provides us with useful higher order functions implementing idiomatic patterns. We’ll see how they make programming in Kotlin so easier and faster. The functions that we’re going to discuss below are:

  • let
  • run
  • also
  • apply
  • with

Kotlin let

let takes the object it is invoked upon as the parameter and returns the result of the lambda expression. Kotlin let is a scoping function wherein the variables declared inside the expression cannot be used outside. An example demonstrating kotlin let function is given below.

fun main(args: Array<String>) {
    var str = "Hello World"
    str.let { println("$it!!") }

//Hello World!!
//Hello World

it keyword contains the copy of the property inside let. The last value from the let is returned as an argument as shown below.

var strLength = str.let { "$it function".length }
println("strLength is $strLength") //prints strLength is 25

Kotlin let example

Chaining let functions

var a = 1
var b= 2

a = a.let { it + 2 }.let { val i = it + b
println(a) //5

As you can see we’ve declared a local variable “i” inside the second let function. Setting the last statement of the let function to i returns the property to the outer property a. kotlin let chaining

Nesting let

We can set a let expression inside another let expression as shown below.

var x = "Anupam"
x.let { outer -> outer.let { inner -> print("Inner is $inner and outer is $outer") } }

//Inner is Anupam and outer is Anupam

For nested let, we can’t use it keyword. We need to assign explicit names to it in both the let functions. Only the outermost let returns the value as shown below.

var x = "Anupam"
    x = x.let { outer ->
        outer.let { inner ->
            println("Inner is $inner and outer is $outer")
            "Kotlin Tutorials Inner let"
        "Kotlin Tutorials Outer let" 
    println(x) //prints Kotlin Tutorials Outer let

kotlin nested let

let for null checks

Additionally, let is useful for checking Nullable properties as shown below.

var name : String? = "Kotlin let null check"
name?.let { println(it) } //prints Kotlin let null check
name = null
name?.let { println(it) } //nothing happens

The code inside the let expression is executed only when the property is not null. Thus let saves us from the if else null checker too!

Kotlin run

Kotlin run is another interesting function. The following example demonstrates its use cases.

var tutorial = "This is Kotlin Tutorial"
    println(tutorial) //This is Kotlin Tutorial
    tutorial = run {
        val tutorial = "This is run function"
    println(tutorial) //This is run function

Kotlin run function Kotlin run expression can change the outer property. Hence in the above code, we’ve redefined it for the local scope.

  • Similar to the let function, the run function also returns the last statement.
  • Unlike let, the run function doesn’t support the it keyword.

let and run

Let’s combine the let and run functions together.

var p : String? = null
    p?.let { println("p is $p") } ?: run { println("p was null. Setting default value to: ")
        p = "Kotlin"}

//p was null. Setting default value to: 

Kotlin also

As the name says, also expressions does some additional processing on the object it was invoked. Unlike let, it returns the original object instead of any new return data. Hence the return data has always the same type. Like let, also uses it too.

var m = 1
m = m.also { it + 1 }.also { it + 1 }
println(m) //prints 1 

Kotlin also function

Kotlin let vs also

Following code snippet shows a great example to differentiate between let and also.

data class Person(var name: String, var tutorial : String)
var person = Person("Anupam", "Kotlin")

var l = person.let { it.tutorial = "Android" }
var al = person.also { it.tutorial = "Android" }

Kotlin let vs also In the above code, we’ve used Data classes. The also expression returns the data class object whereas the let expression returns nothing (Unit) as we didn’t specify anything explicitly.

Kotlin apply

Kotlin apply is an extension function on a type. It runs on the object reference (also known as receiver) into the expression and returns the object reference on completion.

data class Person(var name: String, var tutorial : String)
var person = Person("Anupam", "Kotlin")

person.apply { this.tutorial = "Swift" }

Kotlin apply

apply vs also

data class Person(var n: String, var t : String)
var person = Person("Anupam", "Kotlin")

person.apply { t = "Swift" }

person.also { it.t = "Kotlin" }

Kotlin apply vs also Note: In apply it isn’t allowed. If the property name of the data class is unique in the function, you can omit this. We should use also only when we don’t want to shadow this.

Kotlin with

Like apply, with is used to change instance properties without the need to call dot operator over the reference every time.

data class Person(var name: String, var tutorial : String)
var person = Person("Anupam", "Kotlin")

        name = "No Name"
        tutorial = "Kotlin tutorials"

Kotlin with function Again with is similar to apply except for a few differences.

Kotlin apply vs with

  • with runs without an object(receiver) whereas apply needs one.
  • apply runs on the object reference, whereas with just passes it as an argument.
  • The last expression of with function returns a result.
var xyz = with(person)
        name = "No Name"
        tutorial = "Kotlin tutorials"
        val xyz = "End of tutorial"
    println(xyz) //End of tutorial

That’s all for Kotlin standard functions to alter variables or modify objects within the function.

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About the authors
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Developer and author at DigitalOcean.

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Was this helpful?

let null check code with give you a compile time error if anyone copies and paste it from here… !! Otherwise its very informative. Thanks.

- Soumita

    I think it’s the best article I have read on kotlin scoping function. Crystal clear information.

    - Astha

      Thanks, great examples and good job explaining the differences.

      - Gary

        Very nice easy to understand each one

        - Abhishek Srivastava

          Thanks! Very informative and understandable article.

          - Anchith

            Very clear and specific

            - Alif Hasnain

              give me some internal mechanism about these functions.

              - Nareshkumar