Log Effect

Start

Log Effect is available for Scala 2.12, 2.13 and 3. Helper constructors are provided for Cats Effect's Sync F[_], for Fs2's Stream and for ZIO's Task. Add

libraryDependencies += "io.laserdisc" %% "log-effect-fs2" % <latest-fs2-version>

for Fs2 or Cats Effect. Add instead

libraryDependencies += "io.laserdisc" %% "log-effect-zio" % <latest-zio-version>

for ZIO. If the intention is, instead, to create your own implementation of the typeclass, adding this

libraryDependencies += "io.laserdisc" %% "log-effect-core" % <latest-core-version>

will be enough. For the latest versions available please refer to the badges below the title.

Backends

Currently Log Effect supports the following backends

• Log4s
• Java Logging (Jul)
• Scribe
• Console
• No log sink

Dependencies

	Cats	Fs2	Cats Effect	Log Effect Core
	2.9.0	3.4.0	3.4.1

	Zio	Log Effect Core
	2.0.4

	Log4cats	Log Effect Core
	2.5.0

	Log4s	Scribe
	1.10.0	3.10.5

Examples

Get Logs

Cats Effect Sync

To get an instance of LogWriter for Cats Effect's Sync the options below are available (see here)

full compiling example here

val log4s1: F[LogWriter[F]] =
  F.delay(l4s.getLogger("test")) map log4sLog[F]

val log4s2: F[LogWriter[F]] = log4sLog("a logger")

val log4s3: F[LogWriter[F]] = {
  case class LoggerClass()
  log4sLog(classOf[LoggerClass])
}

val jul1: F[LogWriter[F]] =
  F.delay(jul.Logger.getLogger("a logger")) map julLog[F]

val jul2: F[LogWriter[F]] = julLog

val scribe1: F[LogWriter[F]] =
  F.delay(scribe.Logger("a logger")) map scribeLog[F]

val scribe2: F[LogWriter[F]] = scribeLog("a logger")

val scribe3: F[LogWriter[F]] = {
  case class LoggerClass()
  scribeLog(classOf[LoggerClass])
}

val console1: LogWriter[F] = consoleLog

val console2: LogWriter[F] = consoleLogUpToLevel(LogLevels.Warn)

val noOp: LogWriter[F] = noOpLog[F]

Fs2 Stream

Similarly, to get instances of LogWriter for Fs2's Stream the constructors below are available here

full compiling example here

val log4s1: fs2.Stream[F, LogWriter[F]] =
  Stream.eval(F.delay(l4s.getLogger("test"))) >>= log4sLogStream[F]

val log4s2: fs2.Stream[F, LogWriter[F]] = log4sLogStream("a logger")

val log4s3: fs2.Stream[F, LogWriter[F]] = {
  case class LoggerClass()
  log4sLogStream(classOf[LoggerClass])
}

val jul1: fs2.Stream[F, LogWriter[F]] =
  Stream.eval(F.delay(jul.Logger.getLogger("a logger"))) >>= julLogStream[F]

val jul2: fs2.Stream[F, LogWriter[F]] = julLogStream

val scribe1: fs2.Stream[F, LogWriter[F]] =
  Stream.eval(F.delay(scribe.Logger("a logger"))) >>= scribeLogStream[F]

val scribe2: fs2.Stream[F, LogWriter[F]] = scribeLogStream("a logger")

val scribe3: fs2.Stream[F, LogWriter[F]] = {
  case class LoggerClass()
  scribeLogStream(classOf[LoggerClass])
}

val console1: fs2.Stream[F, LogWriter[F]] = consoleLogStream

val console2: fs2.Stream[F, LogWriter[F]] = consoleLogStreamUpToLevel(LogLevels.Warn)

val noOp: fs2.Stream[F, LogWriter[F]] = noOpLogStream

See here for an example with Laserdisc

Zio Task

To create instances for ZIO some useful constructors can be found here. Note as they exploit the power and expressiveness of RLayer an the RIO pattern as shown below.

Create LogWriter as a Layer

full compiling example here

// Case 1: from a possible config
val logNameLiveFromConfig: ULayer[ZLogName] =
  aConfigLive >>> ZLayer(ZIO.service[AConfig].map(c => LogName(c.logName)))

val log4sCase1: TaskLayer[ZLogWriter] =
  logNameLiveFromConfig >>> log4sLayerFromName

val scribeCase1: TaskLayer[ZLogWriter] =
  logNameLiveFromConfig >>> scribeLayerFromName

// Case 2: from a name
val log4sCase2: TaskLayer[ZLogWriter] =
  logNameLive >>> log4sLayerFromName

val scribeCase2: TaskLayer[ZLogWriter] =
  logNameLive >>> scribeLayerFromName

// Case 3: from a logger
val log4sCase3: TaskLayer[ZLogWriter] =
  log4sLoggerLive >>> log4sLayerFromLogger

val julCase3: TaskLayer[ZLogWriter] =
  julLoggerLive >>> julLayerFromLogger

val scribeCase3: TaskLayer[ZLogWriter] =
  scribeLoggerLive >>> scribeLayerFromLogger

Create LogWriter as RIO

full compiling example here

// Case 1: from a possible config in a Layer (gives a Layer)
val log4sCase1: RLayer[AConfig, ZLogWriter] =
  ZLayer(ZIO.serviceWithZIO { c =>
    log4sFromName.provideEnvironment(ZEnvironment(c.logName))
  })
val scribe4sCase1: RLayer[AConfig, ZLogWriter] =
  ZLayer(ZIO.serviceWithZIO { c =>
    scribeFromName.provideEnvironment(ZEnvironment(c.logName))
  })

// Case 2: from a name
val log4sCase2: Task[Unit] =
  log4sFromName.provideEnvironment(ZEnvironment(aLogName)).flatMap { logger =>
    someZioProgramUsingLogs.provideEnvironment(ZEnvironment(logger))
  }
val scribeCase2: Task[Unit] =
  scribeFromName.provideEnvironment(ZEnvironment(aLogName)).flatMap { logger =>
    someZioProgramUsingLogs.provideEnvironment(ZEnvironment(logger))
  }

// Case 3: from a logger
val log4sCase3: Task[Unit] =
  for {
    logger    <- ZIO.attempt(l4s.getLogger(aLogName))
    logWriter <- log4sFromLogger.provideEnvironment(ZEnvironment(logger))
    _         <- someZioProgramUsingLogs.provideEnvironment(ZEnvironment(logWriter))
  } yield ()
val julCase3: Task[Unit] =
  for {
    logger    <- ZIO.attempt(jul.Logger.getLogger(aLogName))
    logWriter <- julFromLogger.provideEnvironment(ZEnvironment(logger))
    _         <- someZioProgramUsingLogs.provideEnvironment(ZEnvironment(logWriter))
  } yield ()
val scribeCase3: Task[Unit] =
  for {
    logger    <- ZIO.attempt(scribe.Logger(aLogName))
    logWriter <- scribeFromLogger.provideEnvironment(ZEnvironment(logger))
    _         <- someZioProgramUsingLogs.provideEnvironment(ZEnvironment(logWriter))
  } yield ()

// Case 4: from a class
val log4sCase4: Task[Unit] = {
  case class LoggerClass();
  log4sFromClass.provideEnvironment(ZEnvironment(classOf[LoggerClass])).flatMap { logger =>
    someZioProgramUsingLogs.provideEnvironment(ZEnvironment(logger))
  }
}
val scribeCase4: Task[Unit] = {
  case class LoggerClass();
  scribeFromClass.provideEnvironment(ZEnvironment(classOf[LoggerClass])).flatMap { logger =>
    someZioProgramUsingLogs.provideEnvironment(ZEnvironment(logger))
  }
}

// Case 5 (Jul): from global logger object
val julCase5: Task[Unit] =
  julGlobal.flatMap(logger =>
    someZioProgramUsingLogs.provideEnvironment(ZEnvironment(logger))
  )

// Case 6: console logger
val console1: Task[Unit] =
  someZioProgramUsingLogs.provideEnvironment(ZEnvironment(consoleLog))

val console2: Task[Unit] =
  someZioProgramUsingLogs.provideEnvironment(
    ZEnvironment(consoleLogUpToLevel(LogLevels.Warn))
  )

// Case 7: No-op logger
val noOp: Task[Unit] =
  someZioProgramUsingLogs.provideEnvironment(ZEnvironment(noOpLog))

Submit Logs

The following ways of submitting logs are supported:

• in a monadic sequence of effects

import cats.effect.Sync
import cats.syntax.flatMap._
import cats.syntax.functor._
import log.effect.LogWriter

def process[F[_]](implicit F: Sync[F], log: LogWriter[F]): F[(Int, Int)] =
  for {
    _ <- log.trace("We start")
    a <- F.delay(10)
    _ <- log.trace("Keep going")
    b <- F.delay(20)
    _ <- log.trace("We reached this point")
    _ <- log.info(s"Process complete: ${(a, b)}")
  } yield (a, b)

• in a streaming environment using LogWriter's syntax

import java.nio.channels.AsynchronousChannelGroup

import cats.effect.{ ConcurrentEffect, ContextShift, Timer }
import cats.syntax.flatMap._
import log.effect.LogWriter

import scala.concurrent.ExecutionContext

sealed trait RedisClient[F[_]] {
  def address: String
}
object RedisClient {
  def apply[F[_]](addr: String): fs2.Stream[F, RedisClient[F]] =
    fs2.Stream.emit(new RedisClient[F] { val address = addr })
}

implicit def EC: ExecutionContext         = ???
implicit def CG: AsynchronousChannelGroup = ???

def redisClient[F[_]: ConcurrentEffect: ContextShift: Timer](
  address: String
)(implicit log: LogWriter[F]): fs2.Stream[F, RedisClient[F]] =
  RedisClient[F](address) evalMap { client =>
    log.info(s"Laserdisc Redis client for $address") >> ConcurrentEffect[F].pure(client)
  }

• in a streaming environment using fs2 streams' syntax

import java.nio.channels.AsynchronousChannelGroup

import cats.effect.{ ConcurrentEffect, ContextShift, Timer }
import log.effect.LogWriter
import log.effect.fs2.syntax._

import scala.concurrent.ExecutionContext

sealed trait RedisClient[F[_]] {
  def address: String
}
object RedisClient {
  def apply[F[_]](addr: String): fs2.Stream[F, RedisClient[F]] =
    fs2.Stream.emit(new RedisClient[F] { val address = addr })
}

implicit def EC: ExecutionContext         = ???
implicit def CG: AsynchronousChannelGroup = ???

def redisCache[F[_]: ConcurrentEffect: ContextShift: Timer](
  address: String
)(implicit log: LogWriter[F]): fs2.Stream[F, RedisClient[F]] =
  for {
    _      <- log.infoS(s"About to connect a Laserdisc Redis client for $address")
    client <- RedisClient[F](address)
    _      <- log.infoS("The connection went fine. Talking to the server")
  } yield client

• through the companion's syntax

import cats.effect.Sync
import cats.syntax.apply._
import cats.syntax.flatMap._
import log.effect.LogWriter

def double[F[_]: Sync: LogWriter](source: fs2.Stream[F, Int]): fs2.Stream[F, Int] =
  source evalMap { n =>
    LogWriter.debug("Processing a number") >>
      LogWriter.debug(n.toString) >>
      Sync[F].pure(n * 2) <*
      LogWriter.debug("Processed")
  } handleErrorWith { th =>
    fs2.Stream eval (
      LogWriter.error("Ops, something didn't work", th) >> Sync[F].pure(0)
    )
  }

• through the companion's syntax for fs2 streams

import cats.effect.Sync
import cats.syntax.apply._
import cats.syntax.flatMap._
import log.effect.LogWriter
import log.effect.fs2.syntax._

def double[F[_]: Sync: LogWriter](source: fs2.Stream[F, Int]): fs2.Stream[F, Int] =
  (source >>= { n =>
    LogWriter.debugS("Processing a number") >>
      LogWriter.debugS(n.toString) >>
      fs2.Stream.eval(Sync[F].pure(n * 2)) <*
      LogWriter.debugS("Processed")
  }) handleErrorWith { th =>
    LogWriter.errorS("Ops, something didn't work", th) >> fs2.Stream.eval(Sync[F].pure(0))
  }

• through the companion's accessor for the write method

import java.nio.channels.AsynchronousChannelGroup

import cats.Show
import cats.effect.{ ConcurrentEffect, ContextShift, Timer }
import cats.instances.string._
import cats.syntax.either._
import cats.syntax.flatMap._
import log.effect.LogLevels.{ Debug, Error }
import log.effect.fs2.interop.show._
import log.effect.{ Failure, LogWriter }

import scala.concurrent.ExecutionContext

sealed trait RedisClient[F[_]] {
  def address: String
}
object RedisClient {
  def apply[F[_]](addr: String): fs2.Stream[F, RedisClient[F]] =
    fs2.Stream.emit(new RedisClient[F] { val address = addr })
}

type |[A, B] = Either[A, B]

implicit def EC: ExecutionContext         = ???
implicit def CG: AsynchronousChannelGroup = ???

def redisClient[F[_]: ConcurrentEffect: ContextShift: Timer: LogWriter](
  address: String
): fs2.Stream[F, Throwable | RedisClient[F]] = {
  // Cats Show instances are needed for every logged type
  implicit val clientShow: Show[RedisClient[F]] = ???

  RedisClient[F](address) evalMap { client =>
    LogWriter.write(Debug, "Connected client details:") >> // Or
      LogWriter.debug(address) >>                          // And
      LogWriter.debug(client) >>
      ConcurrentEffect[F].pure(client.asRight)
  } handleErrorWith { th =>
    fs2.Stream eval (
      LogWriter.write(
        Error,
        Failure("Ops, something didn't work", th)
      ) >> ConcurrentEffect[F].pure(th.asLeft)
    )
  }
}

or using the fs2 Stream specific syntax like writeS or the level alternatives for types that provide a cats.Show instance

import cats.Show
import cats.effect.Sync
import log.effect.LogLevels.Error
import log.effect.{ Failure, LogWriter }
import log.effect.fs2.syntax._

trait A
object A {
  def empty: A = ???
  implicit val aShow: Show[A] = new Show[A] {
    override def show(t: A): String = ???
  }
}

def double[F[_]: Sync: LogWriter](source: fs2.Stream[F, Int]): fs2.Stream[F, A] = {
  // Cats Show instances are needed for every logged type
  implicit def intShow: Show[Int] = ???

  def processAnInt: Int => A = ???

  (for {
    n <- source
    _ <- LogWriter.debugS("Processing a number")
    _ <- LogWriter.debugS(n) // N.B. the syntax requires a `cats.Show` for `Int`
    r <- (processAnInt andThen fs2.Stream.emit)(n)
    _ <- LogWriter.debugS("Processed")
    _ <- LogWriter.debugS(r) // Same here, a `cats.Show` for `A` is needed
  } yield r) handleErrorWith { th =>
    LogWriter.writeS(
      Error,
      Failure("Ops, something didn't work", th)
    ) >> fs2.Stream.emit(A.empty) // and `write again`
  }
}

NB: note above the LogWriter's implicit evidence given as context bound and the import log.effect.fs2.interop.show._. The latter is needed to summon an internal.Show given a cats.Show.

Interop

In some cases log-effect can be used in projects that have a different logging system already in place. A basic interoperability with other logging libraries is provided in log-effect-interop. The supported ones at the moment are

• log4cats

See an example below.

libraryDependencies += "io.laserdisc" %% "log-effect-interop" % <latest-interop-version>

import cats.effect.{Resource, Sync}
import cats.syntax.flatMap._
import io.chrisdavenport.log4cats.Logger
import log.effect.LogWriter

import log.effect.interop.log4cats._

sealed trait RedisClient[F[_]] {
  def address: String
  def write: F[Unit]
}
object RedisClient {
  def apply[F[_]: LogWriter](addr: String)(implicit F: Sync[F]): Resource[F, RedisClient[F]] =
    Resource.make(
      F.pure(
        new RedisClient[F] {
          val address        = addr
          def write: F[Unit] = LogWriter.info(address)
        }
      )
    )(_ => F.unit)
}

def buildLog4catsLogger[F[_]]: F[Logger[F]] = ???

def storeOwnAddress[F[_]: Sync](address: String): F[Unit] =
  buildLog4catsLogger[F] >>= { implicit l =>
    RedisClient[F](address).use { cl =>
      cl.write
    }
  }

NB: note the import log.effect.interop.log4cats._ that enables the derivation of a LogWriter[F] when a log4cats' Logger[F] is in scope.

License

Log-effect is licensed under the MIT License (the "License"); you may not use this software except in compliance with the License.
Unless required by applicable law or agreed to in writing, software distributed under the License is distributed on an "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the License for the specific language governing permissions and limitations under the License.