Example using WriterT and liftT

WriterTExample.scala

package examples
import cats._, data._
import cats.implicits._

object writerTExample {
  
  type Env = Int
  type Err = String
  type V[A] = Either[Err,A]
  type ReaderV[A] = Kleisli[V,Env,A]
  
  def checkPos(x: Int): ReaderV[Int] = Kleisli.lift(
    if (x > 0) Either.right(x)
     else Either.left(s"$x is not positive")
  )
  
  type WriterV[A] = WriterT[ReaderV,String,A]
  
  // The following definition is required by liftT
  type WriterV1[F[_], A] = WriterT[F, String, A]
  
  def checkPosW(x: Int) : WriterV[Int] = for {
    v <- checkPos(x).liftT[WriterV1]  
    _ <- WriterT.tell[ReaderV,String]("Writing in w1")
  } yield x

  val e: Env = 0
  
  def runW[A](c: WriterV[A]) = c.run.run(e)
  
  val rOK = runW(checkPosW(3))
  val rFail = runW(checkPosW(-3))
  
}