Using Kotlin run scope function

Posted on Jan 26, 2022

Learn how Kotlin run scope function works with code examples


The run keyword is used to create a scoped function in Kotlin.

A scoped function is a function that’s called under the context of an object.

While a scoped function doesn’t add any new capabilities to your source code, it helps in writing a more concise code.

Let’s learn how to use the run function with code examples.

Suppose you have a class named Car with the following definitions:

class Car(var name: String, var year: Int) {
    fun hello() {
        println("Hello! I'm a $name")
    }
}

Next, you create an instance of the Car class:

val car = Car("Tesla S", 2021)

When you want to change the properties of the car instance above, you might do it like this:

car.name = "Ford Mustang Mach-E"
car.year = 2022

Using the run method, you can do the same changes with the following syntax:

car.run {
    name = "Ford Mustang Mach-E"
    year = 2022
}

println(car.name) // Ford Mustang Mach-E
println(car.year) // 2022

Instead of the usual function call with parentheses () before the opening bracket, the run method above is called with a lambda expression as its body block.

Inside the body of the function, the car object is available for you to access using the this keyword.

But you can also omit the this keyword as you can see in the example above.

The run keyword also returns the result of the lambda expression that you can save in a variable if you need it.

For example, the following run method returns a string Hello World:

val result = car.run {
    car.name = "Ford Mustang Mach-E"
    car.year = 2022
    "Hello World"
}

println(result) // Hello World

When there’s no value returned from the function body, then an empty Unit will be returned:

val result = car.run {
    car.name = "Ford Mustang Mach-E"
    car.year = 2022
}

println(result) // kotlin.Unit

And that’s how the run scope function works in Kotlin.

It can be called from any valid object in Kotlin, so you can also call it from an instance of Kotlin built-in object.

The example below calls the run function from a String instance:

val str = "Hello World"

val char = str.run {
    println(length) // 11
    get(1)
}

println(char) // e

Great work learning about the run scope function in Kotlin. 👍

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