with-ambiguous-implicits.sc

import cats._, cats.data._, cats.syntax._, cats.implicits._

implicit def readerTMonadError[F[_], I, E](implicit AE: ApplicativeError[ReaderT[F, I, ?], E], M: FlatMap[ReaderT[F, I, ?]]) =
  new MonadError[ReaderT[F, I, ?], E] {
    def pure[A](x: A) = AE.pure(x)

    def handleErrorWith[A](fa: ReaderT[F, I, A])(f: E => ReaderT[F, I, A]) =
      AE.handleErrorWith(fa)(f)

    def raiseError[A](e: E) =
      AE.raiseError[A](e)

    def flatMap[A, B](fa: ReaderT[F, I, A])(f: A => ReaderT[F, I, B]) =
      M.flatMap(fa)(f)

    def tailRecM[A, B](a: A)(f: A => ReaderT[F, I, Either[A, B]]) =
      M.tailRecM(a)(f)
  }

implicit def readerMonadState[F[_], I, O, S](implicit M: Monad[ReaderT[F, I, ?]], ST: MonadState[F, S]) =
  new MonadState[ReaderT[F, I, ?], S] {
    def flatMap[A, B](fa: ReaderT[F, I, A])(f: A => ReaderT[F, I, B]) = M.flatMap(fa)(f)

    def tailRecM[A, B](a: A)(f: A => ReaderT[F, I, Either[A, B]]) = M.tailRecM(a)(f)

    def pure[A](a: A) = M.pure(a)

    def get = ReaderT[F, I, S] { env => ST.get }

    def set(s: S) = ReaderT[F, I, Unit] { env => ST.set(s) }
  }

def a: ReaderT[StateT[util.Try, String, ?], String, Int] = ???
def b: ReaderT[StateT[util.Try, String, ?], String, Int] = ???

a >> b
// output:
// error: value >> is not a member of cats.data.ReaderT[[γ$5$]cats.data.StateT[scala.util.Try,String,γ$5$],String,Int]
//     a >> b
//       ^
// 

without-ambigous-implicits.sc

import cats._, cats.data._, cats.syntax._, cats.implicits._

implicit def readerTMonadError[F[_], I, E](implicit AE: ApplicativeError[ReaderT[F, I, ?], E], M: FlatMap[ReaderT[F, I, ?]]) =
  new MonadError[ReaderT[F, I, ?], E] {
    def pure[A](x: A) = AE.pure(x)

    def handleErrorWith[A](fa: ReaderT[F, I, A])(f: E => ReaderT[F, I, A]) =
      AE.handleErrorWith(fa)(f)

    def raiseError[A](e: E) =
      AE.raiseError[A](e)

    def flatMap[A, B](fa: ReaderT[F, I, A])(f: A => ReaderT[F, I, B]) =
      M.flatMap(fa)(f)

    def tailRecM[A, B](a: A)(f: A => ReaderT[F, I, Either[A, B]]) =
      M.tailRecM(a)(f)
  }

// implicit def readerMonadState[F[_], I, O, S](implicit M: Monad[ReaderT[F, I, ?]], ST: MonadState[F, S]) =
//   new MonadState[ReaderT[F, I, ?], S] {
//     def flatMap[A, B](fa: ReaderT[F, I, A])(f: A => ReaderT[F, I, B]) = M.flatMap(fa)(f)

//     def tailRecM[A, B](a: A)(f: A => ReaderT[F, I, Either[A, B]]) = M.tailRecM(a)(f)

//     def pure[A](a: A) = M.pure(a)

//     def get = ReaderT[F, I, S] { env => ST.get }

//     def set(s: S) = ReaderT[F, I, Unit] { env => ST.set(s) }
//   }

def a: ReaderT[StateT[util.Try, String, ?], String, Int] = ???
def b: ReaderT[StateT[util.Try, String, ?], String, Int] = ???

a >> b
// output:
// scala.NotImplementedError: an implementation is missing
//  at scala.Predef$.$qmark$qmark$qmark(Predef.scala:230)
//  at .a(<pastie>:43)
//  ... 50 elided
//
// which is ok since I didn't implement 'a' and 'b'