Stacking ReaderT and OptionT.

gistfile1.scala

import scalaz._,Scalaz._,scalaz.effect._,OptionT._,Kleisli._

type OptionTIO[+X] = OptionT[IO, X]

val reader1: ReaderT[Option, Int, String] = kleisli[Option, Int, String](n => n.toString.some)
val reader2: ReaderT[IO, Int, String] = kleisli[IO, Int, String](n => IO((n * 2).toString))

val convertedReader1: ReaderT[OptionTIO, Int, String] = reader1.mapK[OptionTIO, String](opt => optionT(IO(opt)))
val convertedReader2: ReaderT[OptionTIO, Int, String] = reader2.mapK[OptionTIO, String](io => io.liftM[OptionT])

val composed = for {
  first <- convertedReader1
  second <- convertedReader2
} yield first + " - " + second

composed
  .run(4)          // ReaderT
  .run             // OptionT
  .unsafePerformIO // IO