This function performs arithmetic operations on all the elements of a list and returns a result. To understand the concept of the fold function, we'll use addition as the operation. Let's add all the elements of the list and return a result. The fold function takes an integer parameter and a lambda expression. The first parameter indicates the initial value, and the second parameter takes a lambda expression for adding two values:

var numbers = listOf<Int>(1,2,3,4,5)

var result = numbers.fold(0){i,j -> i + j}
println("From beginning : add all elements of the list, Initial value is 0: " + result)

The fold function takes 0 as an initializer, adds all elements, and returns a result. To understand how things works under the hood, create a function that matches the signature of the lambda expression that takes two parameters of the integer type and returns a result:

fun foldHelper(i : Int, j : Int) : Int{
    println("$i , $j")
    return i + j
}

We can print the the value of i and j, and return the value by adding them together. Pass foldHelper to the fold function, as shown here, and execute the program:

var numbers = listOf<Int>(1,2,3,4,5)
var result = numbers.fold(0, ::foldHelper)
println("Answer = " + result)

The output of this function will be as follows:

0 , 1
1 , 2
3 , 3
6 , 4
10 , 5
Answer = 15

On each iteration, the fold function takes an element from the list, assigns it to i, and stores it in j after the arithmetic operation. There are two variants of the fold function.