Runs a StateT while some state condition is met

StateTRunner.scala

object stateTRunner {

  def runWhileUsing[F[+_], S, A, B](st: StateT[F, S, A])(init: S, p: S => Boolean, f: (S, A) => B)(implicit F: Monad[F]): F[List[B]] = {
    def runWhile0(st: StateT[F, S, A], init: S, accum: List[B]): F[List[B]] = {
      if (!p(init)) F.point(accum.reverse)
      else F.bind(st.run(init)) {
        case (s, a) => runWhile0(st, s, f(s, a) :: accum)
      }
    }
    runWhile0(st, init, Nil)
  }

  def evalWhile[F[+_], S, A](st: StateT[F, S, A])(init: S, p: S => Boolean)(implicit F: Monad[F]): F[List[A]] =
    runWhileUsing(st)(init, p, ((s, a) => a))
  }

  // Can add runWhile, execWhile, etc.

}