foldRightのfoldLeftを用いた実装。　ついでにstackless

foldRight.scala

def foldRightSimple[A, B](l: List[A])(z: B)(f: (A, B) => B): B = l match {
  case Nil => z
  case x :: xs => f(x, foldRightSimple(xs)(z)(f))
}

trait Trampoline[T]
case class Done[T](v: T) extends Trampoline[T]
case class Suspend[T](f: () => Trampoline[T]) extends Trampoline[T]

@annotation.tailrec
def runTrampoline[T](t: Trampoline[T]): T = t match {
  case Done(t) => t
  case Suspend(f) => runTrampoline(f())
}


def foldRightUsingFoldLeft[A, B](l: List[A])(z: B)(f: (A, B) => B): B = {
  val tTrampoline = l.foldLeft((b: B) => Done(b): Trampoline[B]) { (accF, e) =>
    acc => Suspend(() => accF(f(e, acc)))
  }
  runTrampoline(tTrampoline(z))
}