How to use TailCalls?

Question

If I understand correctly, scala.util.control.TailCalls can be used to avoid stack overflows for non-tail-recursive functions by using a trampoline. The example given in the

Answer 1

This question is more than 6 years old, but the accepted answer doesn't seem to answer the question:

So my question is how to write a factorial or fibonacci function correctly using TailCalls (yes, I know how to use accumulators to get them tail-recursive)?

So, here it is:

object Factorial {

  /**
    * Returns the nth factorial
    */
  def apply(n: Long): BigInt = {
    if (n < 0)
      throw new IllegalArgumentException("Can't factorial to an index less than 0")
    else {
      import scala.util.control.TailCalls._
      def step(current: Long): TailRec[BigInt] = {
        if (current <= 0)
          done(1)
        else
          tailcall {
            for {
              next <- step(current - 1)
            } yield current * next
          }
      }
      step(n).result
    }
  }

}

assert(Factorial(0) == 1)
assert(Factorial(7) == 5040)
assert(Factorial(10) == 3628800)

One of the big use-cases for using TailCalls is to do something that is right-fold-like.