08 Feb 2023

Logging with class

In two previous posts I've described how I currently compose log messages and how I do the actual logging. This post wraps up this particular topic for now with a couple of typeclasses, a default implementation, and an example showing how I use them.

The typeclasses

First off I want a monad for the logging itself. It's just a collection of functions taking a LogMsg and returning unit (in a monad).

class Monad m => LoggerActions m where
    debug :: LogMsg -> m ()
    info :: LogMsg -> m ()
    warn :: LogMsg -> m ()
    err :: LogMsg -> m ()
    fatal :: LogMsg -> m ()

In order to provide a default implementation I also need a way to extract the logger itself.

class Monad m => HasLogger m where
    getLogger :: m Logger

Default implementation

Using the two typeclasses above it's now possible to define a type with an implementation of LoggerActions that is usable with deriving via.

newtype StdLoggerActions m a = MkStdZLA (m a)
    deriving (Functor, Applicative, Monad, MonadIO, HasLogger)

And its implementattion of LoggerActions looks like this:

instance (HasLogger m, MonadIO m) => LoggerActions (StdLoggerActions m) where
    debug msg = getLogger >>= flip debugIO msg
    info msg = getLogger >>= flip infoIO msg
    warn msg = getLogger >>= flip warnIO msg
    err msg = getLogger >>= flip errIO msg
    fatal msg = getLogger >>= flip fatalIO msg

An example

Using the definitions above is fairly straight forward. First a type the derives its implementaiton of LoggerActions from StdLoggerActions.

newtype EnvT a = EnvT {runEnvT :: ReaderT Logger IO a}
    deriving newtype (Functor, Applicative, Monad, MonadIO, MonadReader Logger)
    deriving (LoggerActions) via (StdLoggerActions EnvT)

In order for it to work, and compile, it needs an implementation of HasLogger too.

instance HasLogger EnvT where
    getLogger = ask

All that's left is a function using a constraint on LoggerActions (doStuff) and a main function creating a logger, constructing an EnvT, and then running doStuff in it.

doStuff :: LoggerActions m => m ()
doStuff = do
    debug "a log line"
    info $ "another log line" #+ ["extras" .= (42 :: Int)]

main :: IO ()
main = withLogger $ \logger ->
    runReaderT (runEnvT doStuff) logger

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