leigh-perry / log4zio

What the world needed most – a logging library

Version Matrix


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// available for Scala 2.12, 2.13
libraryDependencies += "com.github.leigh-perry" %% "log4zio-core" % "0.3.3"


This library targets error-free, composable logger creation.

  1. The log4zio interface assumes that the user doesn't want to experience logging failures. Logging is most important under failure conditions, so it is best to log via a fallback mechanism rather than fail altogether. However, if you prefer to expose logging errors to handle them explicitly, there are error-ful (error-prone?) versions of the standard loggers available.

  2. The library lets the user create a new logging capability by composing refinements on top of a base logging implementation.

Contravariant composition

Contravariant composition allows you to, if you have a simple-text console logger, create a logger that includes a timestamp on every message. Or, it means that you can create a logger that encodes as JSON, writing the JSON data out to a string logger. Alternatively, you can create a logger that accepts only a specialised SafeString data type for its log messages. Implementing this is easy since the SafeString can be converted to String for the log-writing phase. Or perhaps all your log messages take the form of an integer code, and you want to be able to merely call log.info(101) to write whatever that means to your log.

This compositional behaviour is that of a contravariant functor for the LogMedium class. Contravariant functors are characterised by the contramap method:

  def contramap[B](f: B => A): LogMedium[B] = ...

It says, if you have a LogMedium for type A, contramap will give you a LogMedium for any B, so long as you can convert your B to an A.



LogMedium conveys the basic abstraction of logging: a function from some A to Unit, for example taking a String and (effectfully) writing it somewhere.

LogMedium[A] is a contravariant functor, so it can be reused to implement another LogMedium[B] via contramap, so long as B can be converted to an A.


TaggedLogMedium encapsulates the conventional logging pattern, with a logging level (such as INFO) and timestamp. Addition of the level and timestamp info is also achieved by contramap-ing the additional level and timestamp tags onto a raw logger. Eg:

final case class Tagged[A](level: Level, message: () => A)

object TaggedLogMedium {
  final case class TimestampedMessage[A](level: Level, message: () => A, timestamp: String)

  def console[A](prefix: Option[String]): LogMedium[Tagged[A]] =
    withTags(prefix, RawLogMedium.console)

  def silent[A]: LogMedium[Tagged[A]] =
    LogMedium(_ => ZIO.unit)

  def withTags[A](prefix: Option[String], base: LogMedium[String]): LogMedium[Tagged[A]] =
    base.contramap {
      m: TimestampedMessage[A] =>
        "%s %-5s - %s%s".format(
          prefix.fold("")(s => s"$s: "),
    }.contramapM {
      a: Tagged[A] =>
          .catchAll(_ => UIO("(timestamp error)"))
          .map(TimestampedMessage[A](a.level, a.message, _))

This sample also illustrates the philosophy that logging should not fail. It is the responsibility of LogMedium implementations to implement fallback behaviour. In the above example, catchAll handles this requirement.


The ZIO service pattern is implemented in the Log class:

trait Log[A] {
  def log: Log.Service[A]

object Log {
  def log[A]: ZIO[Log[A], Nothing, Log.Service[A]] =

  def stringLog: ZIO[Log[String], Nothing, Log.Service[String]] =

  trait Service[A] {
    def error(s: => A): UIO[Unit]
    def warn(s: => A): UIO[Unit]
    def info(s: => A): UIO[Unit]
    def debug(s: => A): UIO[Unit]

As most logging is done to String-based output media, there is a shortcut stringLog accessor for this case.


Example programs can be found here.

Logging in an application

object AppMain extends zio.App {

  final case class AppEnv(log: Log.Service[String]) extends Log[String]

  val appName = "logging-app"
  override def run(args: List[String]): ZIO[zio.ZEnv, Nothing, Int] =
    for {
      logsvc <- Log.console[String](Some(appName))
      log = logsvc.log
      pgm = Application.execute.provide(AppEnv(log))
    } yield 0

  val doSomething: ZIO[Log[String], Nothing, Unit] =
    for {
      log <- Log.stringLog
      _ <- log.info(s"Executing something")
      _ <- log.info(s"Finished executing something")
    } yield ()

  val execute: ZIO[Log[String], Nothing, Unit] =
    for {
      log <- Log.stringLog
      _ <- log.info(s"Starting app")
      _ <- doSomething
      _ <- log.info(s"Finished app")
    } yield ()

A more realistic sample application using SLF4J logging is found here.

Logging Int messages

You probably won't do this using Int logging, but custom error types can be created by contramap-ing a string-based logger:

object AppMain extends zio.App {

  def intLogger: UIO[Log[Int]] =

  def intRendered(base: LogMedium[String]): LogMedium[Tagged[Int]] =
    base.contramap {
      m: Tagged[Int] =>
        val n: Int = m.message()
        "%-5s - %d:%s".format(m.level.name, n, "x" * n)

  final case class AppEnv(log: Log.Service[Int]) extends Log[Int]

  override def run(args: List[String]): ZIO[zio.ZEnv, Nothing, Int] =
    for {
      logsvc <- intLogger
      log = logsvc.log
      pgm = execute.provide(AppEnv(log))
    } yield 0

  val doSomething: ZIO[Log[Int], Nothing, Unit] =
    for {
      log <- Log.log[Int]
      _ <- log.info(1)
      _ <- log.info(2)
    } yield ()

  val execute: ZIO[Log[Int], Nothing, Unit] =
    for {
      log <- Log.log[Int]
      _ <- log.info(3)
      _ <- doSomething
      _ <- log.info(4)
    } yield ()


git tag -a v${VERS} -m "v${VERS}"
git push origin v${VERS}