sample code for monad transformer

monadTransformerVer.sc

import cats.data.{EitherT, State, StateT}
import cats.instances.either._

object A {
  type SET[E, S, R] = StateT[Either[E, ?], S, R]
  type EST[E, S, R] = EitherT[State[S, ?], E, R]

  def decrementSE: SET[String, Int, Unit] = for {
    x <-            StateT.get[Either[String, ?], Int]
    _ <- if (x > 0) StateT.set[Either[String, ?], Int](x - 1)
         else       StateT.liftF[Either[String, ?], Int, Unit](Left("error"))
  } yield ()

  def decrementES: EST[String, Int, Unit] = for {
    x <-            EitherT.liftF[State[Int, ?], String, Int](State.get[Int])
    _ <- if (x > 0) EitherT.liftF[State[Int, ?], String, Unit](State.set(x - 1))
         else       EitherT.leftT[State[Int, ?], Unit]("error")
  } yield ()

  def runSE(n: Int): Either[String, (Int, Unit)] =
    decrementSE.run(n)

  def runES(n: Int): (Int, Either[String, Unit]) =
    decrementES.value.run(n).value
}
A.runSE(0) // Left(error)
A.runSE(1) // Right((0,()))
A.runES(0) // (0,Left(error))
A.runES(1) // (0,Right(()))