writerreader.scala

  type R[A]       = Reader[Env, A]
  type Z[F[_], A] = WriterT[F, Output, A]
  type W[A]       = Z[R, A]

  def evalLocal(x: String, i: Int, exp: Exp): W[Int] = {
    WriterT[R,Output,Int](
      Reader[Env, (Output, Int)] { (env: Env) =>
        eval(exp).run.run(env + (x -> i))
      }
    )
  }

  def mySet(i: Int): WriterT[R, Output, Int] = {
    WriterT[R,Output,Int](
      Reader[Env, (Output, Int)] { (env: Env) =>
        (List(i.toString), i)
      }
    )
  }