StateT - no F.scala

import cats.{Eval, Monad}

class StateT[F[_], S, A](val run: S => F[(S, A)]) extends AnyVal

object StateT {
  implicit def monad[F[_], S](implicit F: Monad[F]): Monad[StateT[F, S, *]] = new Monad[StateT[F, S, *]] {
    def flatMap[A, B](fa: StateT[F, S, A])(f: A => StateT[F, S, B]): StateT[F, S, B] =
      new StateT(
        s =>
          F.flatMap(
            F.flatMap(F.pure(s))(fa.run)
          )(sa => f(sa._2).run(sa._1))
      )

    def tailRecM[A, B](a: A)(f: A => StateT[F, S, Either[A, B]]): StateT[F, S, B] = ???

    def pure[A](x: A): StateT[F, S, A] = new StateT(s => F.pure((s, x)))
  }
}

object Test {
  def main(args: Array[String]) = {
    val F = Monad[StateT[Eval, Unit, *]]
    val unit = F.unit
    val count = 20000
    val resultL = (0 until count).foldLeft(unit) { (acc, _) =>
      F.flatMap(acc)(_ => unit)
    }
    val resultR = (0 until count).foldLeft(unit) { (acc, _) =>
      F.flatMap(unit)(_ => acc)
    }
    println(resultL.run(()).value)
    println(resultR.run(()).value)
    println("done")
  }
}