Listen

Listen extends Tell and expands upon its ability to write to a log, by also providing a way to access the log when given a value F[A].

This means, given an F[A] we can use listen to see the accumulated value inside, by turning it into an F[(A, L)]. That comes in handy whenever you want to do something with your accumulated log. For example, we might want to send our logs to some external server. Let's have a look at an example of how one could do this:

import cats._
import cats.data._
import cats.syntax.all._
import cats.mtl._
import cats.mtl.implicits._

def sendToServer[F[_]: Monad](logs: Chain[String]): F[Unit] =
  // impure implementation for demonstrative purposes, please don't do this at home
  Monad[F].pure(println(show"Sending to server: $logs"))

def sendLogsToServer[F[_]: Monad, A](logProgram: F[A])(implicit F: Listen[F, Chain[String]]): F[A] =
  logProgram.listen.flatMap {
    case (a, logs) => sendToServer[F](logs).as(a)
  }

Now we have a function sendLogsToServer that takes any program with logging and will send those logs to the server. To see if it works, let's write some logging program and run it all with Writer.

def logging[F[_]: Monad](implicit F: Tell[F, Chain[String]]): F[Unit] =
  // Example of some logging activity in your application
  for {
    _ <- F.tell(Chain.one("First log"))
    _ <- F.tell(Chain.one("Second log"))
  } yield ()

val result = sendLogsToServer(logging[Writer[Chain[String], *]]).value
// Sending to server: Chain(First log, Second log)

And there, we can see it prints out the log. In summary Listen can be used to access or "listen to" the logged values, which is pretty useful whenever you want to actually do something with the logs.