Example code using Trampolines [Doesn't Work]

build.sc

import mill._, scalalib._;

object cub extends ScalaModule {
    def scalaVersion = "0.23.0-RC1";
    def finalMainClass = "Factorial";
    // def scalaVersion = "2.13.1";
}

Factorial.scala

import scala.annotation.tailrec

sealed trait Trampoline[+A]

case class Done[A](x : A) extends Trampoline[A]
case class More[A](x : () => Trampoline[A]) extends Trampoline[A]
case class Act[A, B](x : Trampoline[B], f : (B => Trampoline[A])) extends Trampoline[A]

object Runner {
    @tailrec
    def runT[A](c : Trampoline[A]) : A = c match {
        case Done(x) => x
        case More(x) => runT(x())
        case Act(x, f) => x match {
            case Done(y) => runT(f(y))
            case More(y) => runT(Act(y(), f))
            case Act(y, g) => runT(Act(y, q => Act(g(q), f)))
        }
    }
}

object Factorial {

    import Runner._;

    def tFact(n : BigInt) : Trampoline[BigInt] = {
        if (n <= 1)
            Done(1)
        else Act(More(() => tFact(n - 1)), (p : BigInt) => Done(n * p))
    }

    def main(args : Array[String]) = {
        println(args(1))
        if (args.length < 2) {
            println("Usage: .. <number>")
        } else {
            val n = BigInt(args(1))
            println(runT(tFact(n)).toString)
        }
    }
}