eff.scala

scala> trait Eff[+R[_], A]
// defined trait Eff

scala> enum Reader[I, A] { case Ask[I]() extends Reader[I, I] }
// defined class Reader

scala> enum Writer[W, A] { case Tell[W](value: W) extends Writer[W, Unit] }
// defined class Writer

scala> def run[A](eff: Eff[Nothing, A]): A = ???
def run[A](eff: Eff[Nothing, A]): A

scala> def runWriter[W, R[_], A](eff: Eff[[A] =>> Writer[W, A] | R[A], A]): Eff[R, (W, A)] = ???
def runWriter
  [W, R[_$2], A](eff: Eff[[A] => Writer[W, A] | R[A], A]): Eff[R, (W, A)]

scala> def runReader[I, R[_], A](i: I)(eff: Eff[[A] =>> Reader[I, A] | R[A], A]): Eff[R, A] = ???
def runReader
  [I, R[_$2], A](i: I)(eff: Eff[[A] => Reader[I, A] | R[A], A]): Eff[R, A]

scala> def e: Eff[[A] =>> Writer[String, A] | Reader[Int, A], Int] = ???
def e: Eff[[A] => Writer[String, A] | Reader[Int, A], Int]

scala> def r = run(runReader(0)(runWriter(e)))
def r: (String, Int)