A mtl-style monad transformer for general purpose & compositional logging https://github.com/ocharles/logging-effect

Version on this page:1.2.1
LTS Haskell 11.1:1.2.4
Stackage Nightly 2018-03-12:1.2.4
Latest on Hackage:1.2.4

See all snapshots logging-effect appears in

BSD3 licensed by Ollie Charles
Maintained by ollie@ocharles.org.uk

Module documentation for 1.2.1



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
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