do you think the factorial can benefit from a

tailrect

function?

var result = 1
for (i in 1..a)
    result *= i
result

Yess! For factorial No! Because we don't need to implement it as recursive. using tail-rec reduces memory usage of stack(comparing traditional recursive). To implement it in tail-rec, the method call should be return statement

yes or no, then?

Not for factorial. I was wrong at first

Well, you can implement factorial as a tail recursive function too, but it will overflow the

Long

so quickly that it doesn't make that much sense to do it. You cannot get a

StackOverflowException

with it though.

fun factorial(n: Long): Long {
    tailrec fun fact(acc: Long, curr: Long): Long = 
        if (curr == 1L) acc
        else fact(acc * curr, curr - 1L)
    
    return fact(1, n)
}

could always just use

reduce

instead, like

(1..a).reduce { product, next -> product * next }