sample code for Cats MTL FunctorTell

functorTell.sc

import cats.FlatMap
import cats.data.{Chain, WriterT}
import cats.mtl.FunctorTell
import cats.syntax.flatMap._
import cats.syntax.functor._
import cats.data.Chain._
import cats.effect.{IO, Sync}
import cats.mtl.instances.listen._
import Chain.one

type Logs = Chain[String]

def tellFunc[F[_]: FlatMap: Sync](n: Int)(implicit F: FunctorTell[F, Logs]): F[Unit] =
  for {
    _ <- F.tell(one("begin"))
    _ <- Sync[F].delay { println(s"n = $n") }
    _ <- F.tell(one("end"))
  } yield ()

def program: IO[Logs] = tellFunc[WriterT[IO, Logs, ?]](42).written

program.unsafeRunSync()
// n = 42
// res0: Logs = Chain(begin, end)