Recursion

Recursion is a function invoking itself with new arguments:

fun sumRec(i: Int, numbers: List<Int>): Long {
    return if (i == numbers.size) {
        0
    } else {
        numbers[i] + sumRec(i + 1, numbers)
    }
}

We usually avoid recursion, due to Stack Overflow error that we may receive if our call stack is too deep. You can call this function with a list that contains a million numbers to experience it:

val numbers = List(1_000_000) {it}
println(sumRec(0,  numbers)) // Crashed pretty soon, around 7K 

One of the great benefits of tail recursion is that it avoids the dreaded stack overflow exception.

Let's rewrite our recursive function using a new keyword, tailrec, to avoid that problem:

tailrec fun sumRec(i: Int, sum: Long, numbers: List<Int>): Long {
    return if (i == numbers.size) {
        return sum
    } else {
        sumRec(i+1, numbers[i] + sum...