runarorama

sealed trait Interact[A]

case class Ask(prompt: String)
  extends Interact[String]

case class Tell(msg: String)
  extends Interact[Unit]

trait Monad[M[_]] {
  def pure[A](a: A): M[A]

travisbrown

Suppose we want to lift an OptionT[List, Int] into a ReaderT[OptionT[List, _], String, Int]. The nice way (liftReaderT) currently doesn't work:

scala> List(1, 2, 3).liftM[OptionT].liftReaderT[String]
<console>:14: error: could not find implicit value for parameter F0: scalaz.UnapplyCo[scalaz.Monad,scalaz.OptionT[List,Int]]
              List(1, 2, 3).liftM[OptionT].liftReaderT[String]
                                 ^
<console>:14: error: value liftReaderT is not a member of scalaz.OptionT[List,Int]
              List(1, 2, 3).liftM[OptionT].liftReaderT[String]
 ^