resource_tailrecM.scala

// something like this, I didn't actually compile it.
def tailRecM[A, B](a: A)(fn: A => Resource[F, Either[A, B]]): Resource[F, B] = {
  def step(adis: (A, F[Unit])): F[Either[(A, F[Unit]), (B, F[Unit])]] = {
    val (a, dispose) = dis
    val next: F[(Either[A, B], F[Unit])] = fn(a).allocate
    // todo: this might not be stack safe, we might
    // need a tailRecM type thing on bracket.
    def compDisp(d: F[Unit]): F[Unit] =
      F.bracket(d)(F.pure)(_ => dispose)
    next.map {
      case (Left(a), nextDispose) => Left((a, compDisp(nextDispose)))
      case (Right(b), nextDispose) => Right((b, compDisp(nextDispose)))
    }
  }
  
  Resource(Monad[F].tailRecM((a, Monad[F].pure(())))(step))
}