co-log-core: Composable Contravariant Comonadic Logging Library

[ comonad, contravariant, library, logging, mpl ] [ Propose Tags ]

This package provides core types and functions to work with the LogAction data type which is both simple and powerful.

newtype LogAction m msg = LogAction
    { unLogAction :: msg -> m ()

The ideas behind this package are described in the following blog post:

See the following packages for different implementations based on co-log-core:

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Versions [faq] 0.0.0, 0.1.0, 0.1.1,,,
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Dependencies base (>= && <4.15) [details]
License MPL-2.0
Copyright 2018-2020 Kowainik
Author Dmitrii Kovanikov
Maintainer Kowainik <>
Revised Revision 1 made by shersh at 2020-06-03T10:30:16Z
Category Logging, Contravariant, Comonad
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Source repo head: git clone
Uploaded by shersh at 2020-04-18T20:12:25Z
Distributions LTSHaskell:0.1.1, NixOS:, Stackage:
Downloads 2239 total (58 in the last 30 days)
Rating 2.25 (votes: 2) [estimated by Bayesian average]
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GitHub CI 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 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 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 a 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 purpose we have the concurrent-playground executable where we experiment with different multithreading scenarios to test the library's 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 a benchmark's name doesn't contain Message then this benchmark simply dumps the string "message" to output, otherwise it works with the 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 |