log4cats

Project Goals

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"    % "<version>",  // Only if you want to Support Any Backend
  "org.typelevel" %% "log4cats-slf4j"   % "<version>",  // Direct Slf4j Support - Recommended
)

Examples

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

object MyThing {
  // Impure But What 90% of Folks I know do with log4s
  implicit def unsafeLogger[F[_]: Sync] = 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"))
      .attempt.flatMap{
        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.Sync
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(_ => error"We got an error!")
  } yield ()

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.