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


Version on this page:1.2.0
LTS Haskell 22.12:1.4.0
Stackage Nightly 2023-12-26:1.4.0
Latest on Hackage:1.4.0

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-1.2.0@sha256:aa69793111562c3c7142f8a2441c0ee7ea7b7a8b3d7638cf8e5a73367207de6c,1591

Module documentation for 1.2.0



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