sample code for Cats MTL

catsMtlVer.sc

import cats.data.{EitherT, State, StateT}
import cats.Monad
import cats.syntax.flatMap._
import cats.syntax.functor._
import cats.instances.either._
import cats.mtl.{FunctorRaise, MonadState}
import MonadState._
import FunctorRaise._

type MSI[F[_]] = MonadState[F, Int]
type FRS[F[_]] = FunctorRaise[F, String]

def decr[F[_]: Monad: MSI: FRS]: F[Unit] = for {
  x <- get
  _ <- if (x > 0) set(x - 1) else raise("error")
} yield ()

object M {
  import cats.mtl.implicits._
  type SET[R] = StateT[Either[String, ?], Int, R]
  type EST[R] = EitherT[State[Int, ?], String, R]

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