Scala Tail Recursion From a Flatmap - scala

Question

I have a recursive call as defined below:
def getElems[A](a: A)(f: A => List[A]): List[A] = {
f(a)
}
def parse[A](depth: Int, elems: List[A], f: A => List[A]): List[A] = {
elems.flatMap(elem => {
if (depth > 0) {
parse(depth - 1, getElems(elem)(f), f)
} else elems
})
}
As it can be seen that for every elem in the elems, I run a function that in turn gives me back another List. I do this until I reach the depth 0. So for example., I start with a certain elems and a certain depth like:
parse(depth = 2, elems = List("1", "2"), someFnThatGivesBackAListOfString)
What I'm doing with my code above is that for each element in elems, I check the depth value and if the depth is > 0, I run the function for that elem and go over the same process until I hit a depth of 0. This works as expected, but as it can be seen that it is not stack safe, I'm thiking of getting a tail recursive implementation. To my understanding tail recursion is about reduction, but here it is not the case. So how do I make it stack safe or how can I do a tail recursive logic here?
I started with something like this, but this is not quite right:
def firstAttempt[A](ls: List[A], depthOrig: Int)(f: (A => List[A])): List[A] = {
#annotation.tailrec
def helper(acc: List[A], ls: List[A], depth: Int): List[A] =
ls match {
case Nil => acc
case sublist # (head :: tail) =>
// Check if the entry is available in the bloom filter
if (depth > 0)
helper(acc ::: f(head), tail, depth - 1)
else
helper(acc.appended(head), tail, depthOrig)
}
helper(Nil, ls, depthOrig)
}

Answer 1

I got this to work by attaching the current depth to each element.
def parse[A](depth:Int, elems:List[A], f:A => List[A]): List[A] = {
#annotation.tailrec
def loop(todo:List[(A,Int)], acc:List[A]): List[A] = todo match {
case Nil => acc
case (_,dpth)::_ if dpth < 1 =>
val (zs, td) = todo.span(_._2 < 1)
loop(td, acc ++ zs.flatMap(_ => zs.map(_._1)))
case (elm,dpth)::tl =>
loop(f(elm).map(_ -> (dpth-1)) ++ tl, acc)
}
loop(elems.map(_ -> depth), Nil)
}