Hello, brilliant Kotlin developers. Could someone explain how this closure function works?:pray: ``...

# getting-started

[Mohammadreza Khahani]

Hello, brilliant Kotlin developers. Could someone explain how this closure function works?🙏

fun createIncrementFunction(incrementBy: Int): () -> Int {
    var count = 0

    return {
        count += incrementBy
        count
    }
}

fun main() {
    val incrementByTwo = createIncrementFunction(2)
    println(incrementByTwo()) // Output: 2
    println(incrementByTwo()) // Output: 4
}

[Youssef Shoaib [MOD]]

It's equivalent to this class:

class Incrementer(private val incrementBy: Int) {
    private var count = 0

    operator fun invoke(): Int {
        count += incrementBy
        return count
    }
}

fun main() {
    val incrementByTwo = Incrementer(2)
    println(incrementByTwo()) // Output: 2
    println(incrementByTwo()) // Output: 4
}

[Mohammadreza Khahani]

It clarifies the example, now the only question left is how the count becomes part of the returned lambda function or where the count state is preserved?

In the class example, the state of count is preserved in the incrementByTwo variable but in the original example, the returned value is a lambda function of type () -> Int.

[Landry Norris]

You'd have to check the bytecode to know for sure, but it may create an actual class under the hood like shown above that holds the captured value.

[Youssef Shoaib [MOD]]

That is exactly what it does! Basically, the lambda that gets return is what's called a closure. What happens under the hood is that count is converted to an IntRef, which is a class that holds a single var.