Composable Contravariant Comonadic Logging Library

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MPL-2.0 licensed by Dmitrii Kovanikov
Maintained by Kowainik
This version can be pinned in stack with:co-log-,6395

Module documentation for


GitHub CI MPL-2.0 license

co-log is a composable and configurable logging framework. It combines all the benefits of Haskell idioms to provide a reasonable and convenient interface. Although the library design uses some advanced concepts in its core, we are striving to provide beginner-friendly API. The library also provides the complete documentation with a lot of beginner-friendly examples, explanations and tutorials to guide users. The combination of a pragmatic approach to logging and fundamental Haskell abstractions allows us to create a highly composable and configurable logging framework.

If you’re interested in how different Haskell typeclasses are used to implement core functions of co-log, you can read the following blog post which goes into detail about the internal implementation specifics:

Co-Log Family

Co-Log is a family of repositories for a composable and configurable logging framework co-log.

Here is the list of currently available repositories and libraries that you can check out:

co-log-core lightweight package with basic data types and general idea which depends only on base Hackage
co-log taggless final implementation of logging library based on co-log-core Hackage
co-log-polysemy implementation of logging library based on co-log-core and the polysemy extensible effects library. Hackage
co-log-benchmarks benchmarks of the co-log library -

co-log library

Logging library based on co-log-core package. Provides ready-to-go implementation of logging. This README contains How to tutorial on using this library. This tutorial explains step by step how to integrate co-log into small basic project, specifically how to replace putStrLn used for logging with library provided logging.

All code below can be compiled and run with the following commands:

$ cabal build
$ cabal exec readme

Preamble: imports and language extensions

Since this is a literate haskell file, we need to specify all our language extensions and imports up front.

{-# LANGUAGE FlexibleContexts  #-}
{-# LANGUAGE OverloadedStrings #-}

import Control.Monad.IO.Class (MonadIO, liftIO)

import Colog (Message, WithLog, cmap, fmtMessage, logDebug, logInfo, logTextStdout, logWarning,

import qualified Data.Text as Text
import qualified Data.Text.IO as TextIO

Simple IO function example

Consider the following function that reads lines from stdin and outputs different feedback depending on the line size.

processLinesBasic :: IO ()
processLinesBasic = do
    line <- TextIO.getLine
    case Text.length line of
        0 -> do
            -- here goes logging
            TextIO.putStrLn ">>>> Empty input"
        n -> do
            TextIO.putStrLn ">>>> Correct input"
            TextIO.putStrLn $ "Line length: " <> Text.pack (show n)

This code mixes application logic with logging of the steps. It’s convenient to have logging to observe behavior of the application. But putStrLn is very simple and primitive way to log things.

Using co-log library

In order to use co-log library, we need to refactor processLinesBasic function in the following way:

processLinesLog :: (WithLog env Message m, MonadIO m) => m ()
processLinesLog = do
    line <- liftIO TextIO.getLine
    case Text.length line of
        0 -> do
            -- here goes logging
            logWarning "Empty input"
        n -> do
            logDebug "Correct line"
            logInfo $ "Line length: " <> Text.pack (show n)

Let’s summarize required changes:

  1. Make type more polymorphic: (WithLog env Message m, MonadIO m) => m ()
  2. Add liftIO to all IO functions.
  3. Replace putStrLn with proper log* function.

Running actions

Let’s run both functions:

main :: IO ()
main = do

    let action = cmap fmtMessage logTextStdout
    usingLoggerT action processLinesLog

And here is how output looks like:

screenshot from 2018-09-17 20-52-01

More Tutorials

To provide a more user-friendly introduction to the library, we’ve created the tutorial series which introduces the main concepts behind co-log smoothly, please check more details here.



co-log uses PVP Versioning. The changelog is available on GitHub. - Sep 18, 2023

What’s Changed

  • Support GHC-9.6 - replace typerep-map with dependent-map. by @alaendle in #264
  • Support GHC 9.4. by @alaendle in #252
  • Add MonadUnliftIO instance by @newhoggy in #240
  • Update CI tested GHC versions & workaround for GHC < 9.4.5 (run-st, primitive-unlifted) by @alaendle in #257
  • Derive MonadFail for LoggerT by @alaendle in #260
  • docs: use relative path to benefit locally reading by @xieyuschen in #253
  • docs: refine readme by @xieyuschen in #254
  • tutorials: add demo for LoggerT and SimpleMsg, #84 by @xieyuschen in #256
  • docs: add a link to tutorial pages and aggregate all tutorial pages by @xieyuschen in #259
  • tutorials: add a tutorial for loggert and message by @xieyuschen in #261
  • Create tags and upload package candidates on version bumps. by @alaendle in #265
  • Added @alaendle as code owner. by @alaendle in #258
  • GA(deps): Bump actions/checkout from 3 to 4 by @dependabot in #263

New Contributors

  • @xieyuschen made their first contribution in #253
  • @newhoggy made their first contribution in #240

Full Changelog: - Nov 2, 2022

  • #230: Support GHC-9.2.
  • Allow mtl-2.3.
  • Allow vector-0.13.
  • Allow hedgehog-1.2.
  • #187: Remove CoLog.Concurrent module and executable.
  • #243: Improve printing in multiple threads.
  • Drop support for GHC-8.2, GHC-8.4, GHC-8.6, GHC-8.8 — , 2021

  • #223: Support GHC-9.0.1. Require typerep-map ^>= 0.4 — Apr 18, 2020

  • #186: Support GHC-8.10.1. — Jan 19, 2020

  • #120: Improve time formatting.

    Old: 29-12-2019 22:00:00.000

    New: 29 Dec 2019 22:00:00.000 +00:00

    (by @vrom911)

  • #144: Add Windows CI check. (by @vrom911)

  • #148: Support GHC-8.8.2. (by @chshersh)

  • #119: Add new message type that allows printing messages without Severity. (by @sphaso)

  • #150: Introduce formatWith — beginner-friendly alias for formatting combinator. (by @chshersh)

  • Use chronos-1.1 as 1.0.9 is not Windows-compatible. (by @vrom911)

  • #156: Improve documentation for the Colog.Concurrent module. (by @chshersh)

  • #146: Allow ansi-terminal-0.10. (by @mpilgrem)

  • #124: Implement executable playground for concurrent logging. (by @chshersh) — May 5, 2019

  • #77: Important: Use chronos time formatter. This is a breaking change because default field map in RichMessage now contains different type representing time. If you use your custom formatter for time, you should change it. Othwerwise no observable differences in the library API usage will be noticed.

  • #103: Breaking change: make Message data type polymorhic over the type of severity.

    Migration guide: this change is done in backwards-compatible way. If you use any fields of the Message data type, you should rename them according to the following scheme:

    messageSeverity -> msgSeverity
    messageStack    -> msgStack
    messageText     -> msgText
  • Export more formatting functions to make implementation of custom formatters easier.

  • #96: Add simpleMessageAction and richMessageAction to work with Messages.

  • Use co-log-core of version

0.2.0 — Nov 15, 2018

  • #45: Introduce approach for concurrent log writing.
  • #46: Moves logStringStdout, logStringStderr, logStringHandle, withLogStringFile from Colog.Actions to Colog.Core.IO
  • #77: Remove relude from dependencies. Add HLint check to Travis CI.
  • #64: Introduce basic benchmarks.
  • #20: Add experimental support for logger rotation (see Colog.Rotation module).
  • #39: Support GHC-8.2.2 and GHC-8.6.2.


  • #37: Add bounds to all dependencies. Move Prelude to the other-modules section.


  • Initially created.