Flexible Logging with Monad-Effect

monad-effect-logging is a pure structured logging library for the monad-effect ecosystem.

The current API is centered on one unified message payload LogDoc.

Highlights

• You build one LogDoc value and decide at the boundary how to render it:

  • plain text
  • ANSI colored text
  • different Show strategies
  • custom logger pipelines

• Color stays semantic until rendering time. A file logger can ignore color while a console logger can emit ANSI codes from the exact same log event.

• Excellent TraceId support

• Open log categories, extensible

• Easy JSON logging by tagging loggerJson

This enables you to easily add JSON logging to your files while keeping the console output colorful and readable: you can write no-color json logging to a file while use pSho and color constructors to display stuff on the screen at the same time.

Core types

• LogEvent for the event envelope
• LogWithSourceMeta for source-location metadata
• LogDoc for the structured log message
• Logger for the sink
• LogEffect for the installed effect

data LogEvent a = LogEvent
  { logEventCats :: [LogCat]
  , logEventPayload :: a
  }
data LogWithSourceMeta a = LogWithSourceMeta
  { logMetaLoc :: Maybe Loc
  , logMetaSource :: Maybe LogSource
  , logMetaDoc :: a
  }
newtype Logger m a = Logger
  { runLogger :: LogEvent a -> m ()
  }
data LogEffect m a
data LogDoc

The default installed logging effect is:

type Logging = LogEffect IO LogDoc

Building messages

String literals work through IsString, and deferred values use logShow.

import Module.Logging
example :: (Monad m, In (LogEffect m LogDoc) mods) => EffT mods es m ()
example = do
  $(logTH Info) $ "starting request " <> logShow (42 :: Int)
  $(logTH Warn) $ logFg Yellow "slow query: " <> toLog ("SELECT ..." :: String)

Useful helpers:

• logRaw
• logShow
• logFg
• logBg
• logBold
• toLog

Literal strings can be directly typed in using IsString class, no need to convert using helpers.

Rendering and base loggers

Most applications should use one options-based helper from Module.Logging.Logger:

import Module.Logging
import Module.Logging.Logger
runApp :: EffT '[LogEffect IO LogDoc] NoError IO () -> IO ()
runApp app = do
  stdoutBase <- createSimpleConcurrentStdoutBaseLogger
  fileBase <- createFileLogger "app.log"
  let stdoutLogger = makeLoggerFromBase ( buildLoggerStyle loggerUseAnsi                ) stdoutBase
  let fileLogger   = makeLoggerFromBase ( buildLoggerStyle (loggerJson . loggerNoStyle) ) fileBase
  runEffT00 $ withLoggerCleanup (stdoutLogger <> fileLogger) app

Here each style is a builder function LoggerOptions -> LoggerOptions, and the buildLoggerStyle function is just a composition of them on the defaultLoggerStyle.

For custom pipelines, use the lower-level building blocks:

• renderLogEvent
• loggerFromRenderer
• your own Logger

Styles compose as normal functions:

verboseConsole :: LoggerOptions
verboseConsole =
  buildLoggerStyle
    $ loggerUseAnsi
    . loggerOrder [LogTimeChunk, LogCatChunk, LogLocChunk, LogDocChunk]

Categories

Categories are open and extensible:

data ProxyLog = Bytes | Logic deriving (Lift)
-- The `Lift` class is only necessary if you want to use them inside `logTH` template haskell
-- utilities, otherwise you can remove it.
instance IsLogCat ProxyLog where
  severity Bytes = severity Debug
  severity Logic = severity Info
  logTypeName Bytes = "BYTES"
  logTypeName Logic = "LOGIC"

You can add local categories with effAddLogCat, and filter them with the existing combinators.

MonadLogger compatibility

MonadLogger and MonadLoggerIO are implemented for LogEffect m LogDoc.

Incoming monad-logger messages are wrapped as logRaw, so compatibility fits directly into the unified payload model.

TraceId

This library also provides a super convenient TraceId mechanism that can attach scoped trace IDs to log events. TraceId is just a log category. Use withTraceId or one of the provided generators from Module.Logging.TraceId.

Status

This release presents the library around LogDoc, options-based logger construction, open categories, and boundary-driven rendering.