A mtl-style monad transformer for general purpose & compositional logging


Version on this page:1.3.9
LTS Haskell 22.26:1.4.0@rev:1
Stackage Nightly 2024-06-20:1.4.0@rev:1
Latest on Hackage:1.4.0@rev:1

See all snapshots logging-effect appears in

BSD-3-Clause licensed by Ollie Charles
Maintained by [email protected]
This version can be pinned in stack with:logging-effect-1.3.9@sha256:4fd36d7c19f22569d510907c55d6d49cb5d80fb30767c6b3f85736b98f57d7c0,1678

Module documentation for 1.3.9


1.3.9 – 2020-01-25

Other Changes

  • Support prettyprinter-1.6

1.3.8 – 2019-11-10

Other Changes

  • Support prettyprinter-1.5

1.3.7 – 2019-10-22

Other Changes

  • Support prettyprinter-1.4

1.3.6 – 2019-09-19


  • Added MonadFail instances.

1.3.5 – 2019-09-17

Other Changes

  • Support base-4.13
  • Support GHC 8.8.1
  • Support prettyprinter-1.3

1.3.4 – 2019-05-15

Other Changes

  • Support semigroups-0.19.

1.3.3 – 2018-09-30

Other Changes

  • Support stm-2.5.
  • Support base-4.12.

1.3.2 – 2018-07-06

Other Changes

  • Support free-5.1.


  • Add MonadUnliftIO instance for LoggingT.


Major Changes

  • Switch from wl-pprint-text to prettyprinter.

Other changes

  • Change the type of the ribbonFrac parameter of withFDHandler from Float to Double to reflect the underlying prettyprinter API.


Other Changes

  • Increased upper bound of base and support GHC 8.4.


Other Changes

  • Increased upper bound of exceptions.


Other Changes

  • Increased upper bound of exceptions.


Other Changes

  • Increased upper of async.


Other Changes

  • Increased upper bound of free and time.


Other Changes

  • Increased upper bound of base to allow < 4.11.


Major Changes

  • withFDHandler now explicitly flushes the file handle whenever log entries are rendered out. Thanks to @filterfish for identifying this omission that could lead to log messages being dropped.

    Upgrade steps: no changes other than updating logging-effect.


Other Changes

  • Increased upper bound of time to allow < 1.9.


Other changes

  • Increased upper bound of time


  • withBatchedHandler no longer prints empty log messages. Previously, if you ran a program that didn’t log but used withBatchedHandler (or anything that used that), an empty log message would be output. Thanks to @codedmart for fixing this.


Breaking changes:

  • MonadLog no longer has logMessage as a function. It now has logMessageFree which takes a free monoid of log messages. If you were just using logging-effect then this won’t affect you, as logMessage still exists with the same signature outside the type class.

  • MonadLog now comes with a law that states that logging is a monoid homomorphism. This essentially means that you have to treat all log messages uniformly.

  • Pass-through instances for all “stock” monad transformers have been added (all of transformers, CatchT from exceptions and FreeT/FT from free).

  • WithSeverity now has instances of Traversable and Foldable

  • WithTimestamp now has instances of Eq, Ord, Read and Show.


  • A set of convenience functions have been added for quickly logging with severity. The combinators are: logDebug, logInfo, logNotice, logWarning, logError, logCritical, logAlert and logEmergency.

  • mapLogMessage got a companion function mapLogMessageM that works with monadic tranformations.


  • Many documentation bug fixes.

  • INLINEABLE pragmas added.


  • Initial release