04 Feb 2023

A take on logging

In my previous post I described a type, with instances and a couple of useful functions for composing log messages. To actually make use of that there's a bit more needed, i.e. the actual logging. In this post I'll share that part of the logging setup I've been using in the Haskell services at $DAYJOB.

The logger type

The logger will be a wrapper around fast-logger's FastLogger, even though that's not really visible.

newtype Logger = Logger (LogMsg -> IO ())

It's nature as a wrapper makes it natural to follow the API of fast-logger, with some calls to liftIO added.

newLogger :: MonadIO m => m (Logger, m ())
newLogger = liftIO $ do
    (fastLogger, cleanUp) <- newFastLogger $ LogStdout defaultBufSize
    pure (Logger (fastLogger . toLogStr @LogMsg), liftIO cleanUp)

The implementation of withLogger is pretty much a copy of what I found in fast-logger, just adapted to the newLogger above.

withLogger :: (MonadMask m, MonadIO m) => (Logger -> m ()) -> m ()
withLogger go = bracket newLogger snd (go . fst)

Logging functions

All logging functions will follow the same pattern so it's easy to break out the common parts.

logIO :: MonadIO m => Text -> Logger -> LogMsg -> m ()
logIO lvl (Logger ls) msg = do
    t <- formatTime defaultTimeLocale "%y-%m-%dT%H:%M:%S%03QZ" <$> liftIO getCurrentTime
    let bmsg = "" :# [ "correlation-id" .= ("no-correlation-id" :: Text)
                     , "timestamp" .= t
                     , "level" .= lvl
                     ]
    liftIO $ ls $ bmsg <> msg

With that in place the logging functions become very short and sweet.

debugIO, infoIO, warnIO, errIO, fatalIO :: MonadIO m => Logger -> LogMsg -> m ()
debugIO = logIO "debug"
infoIO = logIO "info"
warnIO = logIO "warn"
errIO = logIO "error"
fatalIO = logIO "fatal"

Simple example of usage

A very simple example showing how it could be used would be something like this

main :: IO ()
main = withLogger $ \logger -> do
    debugIO logger "a log line"
    infoIO logger $ "another log line" #+ ["extras" .= (42 :: Int)]

Comment here.