sample code for Cats MTL FunctorListen

functorListen.sc

import cats.{FlatMap, Monad}
import cats.data.{Chain, WriterT}
import cats.effect.{IO, Sync}
import cats.syntax.flatMap._
import cats.syntax.functor._
import cats.mtl.syntax.listen._
import cats.mtl.{FunctorListen, FunctorTell}
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 send[F[_]: Sync](logs: Logs): F[Unit] =
  Sync[F].delay { println(s"Sending to log server: $logs") }

import cats.mtl.instances.listen._

def program[F[_]: Sync: λ[F[_] => FunctorListen[F, Logs]]](n: Int) =
  tellFunc[F](n).listen >>= { case (_, logs) =>
    send[F](logs)
  }
program[WriterT[IO, Logs, ?]](12345).run.unsafeRunSync()
// n = 12345
// Sending to log server: Chain(begin, end)
// res0: (cats.data.Chain[String], Unit) = (Chain(begin, end),())