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log4cats attempts to make referentially transparent logging a reality. These F algebras allow you to write code for logging knowing you won't be doing side-effects as it offers no way to do so. We provide our own slf4j layer, or you can use any of the supported backends, or create your own.

Quick Start

To use log4cats in an existing SBT project with Scala 2.12 or a later version, add the following dependency to your build.sbt:

libraryDependencies ++= Seq(
  "org.typelevel" %% "log4cats-core"    % "2.7.0",  // Only if you want to Support Any Backend
  "org.typelevel" %% "log4cats-slf4j"   % "2.7.0",  // Direct Slf4j Support - Recommended


import org.typelevel.log4cats.Logger
import org.typelevel.log4cats.slf4j.Slf4jLogger
import cats.effect._
import cats.implicits._

object MyThing {
  // Impure But What 90% of Folks I know do with log4s
  implicit def logger[F[_]: Sync]: Logger[F] = Slf4jLogger.getLogger[F]

  // Arbitrary Local Function Declaration
  def doSomething[F[_]: Sync]: F[Unit] =
    Logger[F].info("Logging Start Something") *>
    Sync[F].delay(println("I could be doing anything"))
        case Left(e) => Logger[F].error(e)("Something Went Wrong")
        case Right(_) => Sync[F].pure(())

def safelyDoThings[F[_]: Sync]: F[Unit] = for {
    logger <- Slf4jLogger.create[F]
    _ <- logger.info("Logging at start of safelyDoThings")
    something <- Sync[F].delay(println("I could do anything"))
      .onError{case e => logger.error(e)("Something Went Wrong in safelyDoThings")}
    _ <- logger.info("Logging at end of safelyDoThings")
  } yield something

def passForEasierUse[F[_]: Sync: Logger] = for {
    _ <- Logger[F].info("Logging at start of passForEasierUse")
    something <- Sync[F].delay(println("I could do anything"))
      .onError{case e => Logger[F].error(e)("Something Went Wrong in passForEasierUse")}
    _ <- Logger[F].info("Logging at end of passForEasierUse")
  } yield something

Laconic syntax

It's possible to use interpolated syntax for logging. Currently, supported ops are: trace, debug, info, warn, error. You can use it for your custom Logger as well as for Slf4j Logger.

import cats.Applicative
import cats.effect._
import org.typelevel.log4cats.Logger
import org.typelevel.log4cats.syntax._

def successComputation[F[_]: Applicative]: F[Int] = Applicative[F].pure(1)
def errorComputation[F[_]: Sync]: F[Unit] = Sync[F].raiseError[Unit](new Throwable("Sorry!"))

def log[F[_]: Sync: Logger] = 
  for {
    result1 <- successComputation[F]
    _ <- info"First result is $result1"
    _ <- errorComputation[F].onError {
      case _ => error"We got an error!"
  } yield ()

Logging using capabilities

You can work with logging using capabilities. It's implemented via the LoggerFactory trait. You instantiate it once (dependent on the specific logging backend you use) and pass this around in your application.

This brings several advantages:

If you are unsure how to create a new LoggerFactory[F] instance, then you can look at the log4cats-slf4j, or log4cats-noop modules for concrete implementations.

The quickest fix might be to create an instance of a Slf4jFactory and pass it around implicitly:

import cats.effect.IO
import cats.Monad
import cats.syntax.all._
import org.typelevel.log4cats._
// assumes dependency on log4cats-slf4j module
import org.typelevel.log4cats.slf4j.Slf4jFactory

// create our LoggerFactory
implicit val logging: LoggerFactory[IO] = Slf4jFactory.create[IO]

// we summon LoggerFactory instance, and create logger
val logger: SelfAwareStructuredLogger[IO] = LoggerFactory[IO].getLogger
logger.info("logging in IO!"): IO[Unit]

// basic example of a service using LoggerFactory
class LoggerUsingService[F[_]: LoggerFactory: Monad] {
  val logger = LoggerFactory[F].getLogger
  def use(args: String): F[Unit] = 
    for {
      _ <- logger.info("yay! effect polymorphic code")
      _ <- logger.debug(s"and $args")
    } yield ()
new LoggerUsingService[IO].use("foo")

CVE-2021-44228 ("log4shell")

log4cats is not directly susceptible to CVS-2021-44228. The log4cats-slf4j implementation delegates all logging operations to slf4j. if you use log4cats-slf4j, your configured slf4j provider may put you at risk. See slf4j's comments on CVE-2021-44228 for more.