Composable Contravariant Comonadic Logging Library

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LTS Haskell 16.31:
Stackage Nightly 2020-11-06:
Latest on Hackage:

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MPL-2.0 licensed by Dmitrii Kovanikov
Maintained by Kowainik
This version can be pinned in stack with:co-log-polysemy-,3591

Module documentation for


Build status Windows build MPL-2.0 license

co-log-core Hackage Stackage LTS Stackage Nightly
co-log Hackage Stackage LTS Stackage Nightly
co-log-polysemy Hackage Stackage LTS Stackage Nightly

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

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

co-log is also modular on the level of packages. We care a lot about a low dependency footprint so you can build your logging only on top of the minimal required interface for your use-case. This repository contains the following packages:

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

To provide more user-friendly introduction to the library, we’ve created the tutorial series which introduces the main concepts behind co-log smoothly:

co-log also cares about concurrent logging. For this purposes we have the concurrent-playground executable where we experiment with different multithreading scenarios to test the library behavior. You can find it here:


co-log is compared with basic functions like putStrLn. Since IO overhead is big enough, every benchmark dumps 10K messages to output. If benchmark name doesn’t contain Message then this benchmark simply dumps string "message" to output, otherwise it works with Message data type from the co-log library.

To run benchmarks, use the following command:

cabal v2-run co-log-bench
Benchmarks Time for 10K messages
Prelude.putStrLn 5.117ms
Text.putStrLn 9.220ms
ByteString.putStrLn 2.971ms
mempty 1.181ms
logStringStdout 5.107ms
logPrint 5.248ms
logTextStdout 5.351ms
logByteStringStdout 2.933ms
logByteStringStderr 17.482ms
ByteString > (stdout <> stderr) 17.715ms
Message > format > stdout 9.188ms
Message > format > ByteString > stdout 3.524ms
Message{callstack} > format > stdout 9.139ms
Message{callstack:5} > format > stdout 9.464ms
Message{callstack:50} > format > stdout 9.439ms
Message{Time,ThreadId} > format > stdout 54.160ms
Message{Time,ThreadId} > format > ByteString > stdout 54.137ms



co-log-polysemy uses PVP Versioning. The changelog is available on GitHub. — Feb 17, 2020 — Jan 19, 2020

  • #121: Add LogAction and interpreter that works directly with the Sem monad. (by @chshersh)
  • #148: Support GHC-8.8.2. (by @chshersh)
  • Bump up polysemy to the latest version. (by @Avi-D-coder) — May 5, 2019

  • Initially created.