Easy Logging for Haskell

Easy-logger can be used to easily create logs, without handling any Monad. Just log as you go. The package provides logging for code that lives in the IO Monad, implements MonadIO m, as well as for pure code.

Usage

Initialise the logger for your package and start logging:

import qualified Data.Text                          as T

main :: IO ()
main = do
  $(initLogger) (LogFile "package.log") LogDebug
  $(logInfo) ("Starting App" :: T.Text)
  ...
  # At the end of your program, flush the buffers:
  finalizeAllLoggers

You can also include the logs of the libraries that you use and which use the easy-logger package for logging. If the library maintainer is nice, (s)he allows you to turn on/off logging for that library only. See the Library section below how to do it. To turn logging for all packages (that use easy-logger) on, do this:

import qualified Data.Text                          as T

main :: IO ()
main = do
  $(initLoggerAllPackages) (LogFile "package.log") LogAll True
  $(logInfo) ("Starting App" :: T.Text)
  ...
  # At the end of your program, flush the buffers:
  finalizeAllLoggers

You want to log in pure code without wrapping anything in a Monad? Here you go:

fromEither :: Either String Int -> Int
fromEither (Right v)  = v
fromEither (Left str) = $(pureLogPrintError) ("Parse error: " <> T.pack str) defaultValue
defaultValue = 0

Under the hood pureLogPrintError uses unsafePerformIO to log. The return value, i.e. defaultValue in the example, ensures that the log is actually executed when the error occurs.

Log Levels and Destinations

The available log levels are:

-- | Log Level. Levels are sorted. `All` < `Debug` < `Info` < `Warning` < `Error`. None disables all logging. Default: All
data LogLevel
  = LogNone
  | LogAll
  | LogDebug
  | LogInfo
  | LogWarning
  | LogError
  deriving (Show, Read, Bounded, Enum, Eq, Ord)

The logger can be used to log to stderr, stdout or a file:

-- | Logging destination. See also `setLoggingDestination`.
data LogDestination
  = LogStdErr
  | LogStdOut
  | LogFile FilePath

Library

If you are exposing a library, let your user turn on/off the logging for your library. The Template-Haskell code ensures that your package name is provided to the logger, and thus logging for this module only is turned on/off.

{-# LANGUAGE TemplateHaskell #-}
module My.Great.Package.Logging
    ( enableMyGreatPackageLogging
    , disableMyGreatPackageLogging
    ) where

import           EasyLogger

enableMyGreatPackageLogging :: LogDestination -> LogLevel -> IO ()
enableMyGreatPackageLogging = $(initLogger)

disableMyGreatPackageLogging :: IO ()
disableMyGreatPackageLogging = $(finalizeLogger)