HowDoICallEval.scala

//libraryDependencies += "org.scalaz" %% "scalaz-core" % "7.2.0"

import scala.language.higherKinds
import scalaz.{MonadState, MonadListen, MonadReader, MonadError, \/}
import scalaz.std.list._

// All I want to do in this code is invoke the eval function. That's it.
// This code compiles, so it's close, just needs the finishing touch.
object HowDoICallEval {

  type Mem    = Map[Int, Int]
  type Env    = Map[String, Int]
  type Output = List[String]

  type T[F[_]] =
    MonadError [F,String] with MonadListen[F,List[String]] with
    MonadReader[F,Env]    with MonadState [F,Mem]

  // it doesn't matter what eval does. i just want to call it.
  def eval[F[_]](i: Int)(implicit m: T[F]): F[Int] =
    m.raiseError("don't care")

  // TODO: how do I call eval?
  def callEval(e:Int): (Output, String \/ Int, Mem) = {
    import scalaz.{Id, EitherT, ReaderWriterState}
    import Id.Id, EitherT._, ReaderWriterState._

    type S[F[_]] = MonadListen[F,Output] with MonadReader[F,Env] with MonadState [F,Mem]
    type R[A]    = ReaderWriterState[Env, Output, Mem, A]
    type E[A]    = EitherT[R, String, A]

    val rm: S[R] = ReaderWriterState.rwstMonad[Id, Env, Output, Mem]
    //val em: T[E] = EitherT.eitherTMonad[R, String](rm)

    val x: E[Int] = eval(e)(???)
    ???
  }
}



// Contrast that code with this code, where I can call eval
// Because I have no MonadError.
object HereICanCallEval {

  type Mem    = Map[Int, Int]
  type Env    = Map[String, Int]
  type Output = List[String]

  type T[F[_]] =
    MonadListen[F,List[String]] with
    MonadReader[F,Env] with MonadState [F,Mem]

  def eval[F[_]](i: Int)(implicit m: T[F]): F[Int] =
    m.pure(i)

  def callEval(e:Int): (Output, Int, Mem) = {
    import scalaz.{Id, ReaderWriterState}
    import Id.Id

    type R[A]    = ReaderWriterState[Env, Output, Mem, A]
    val rm: T[R] = ReaderWriterState.rwstMonad[Id, Env, Output, Mem]

    eval(e)(rm).run(Map(), Map())
  }
}