monad-logger-prefix

This package provides an easy way to add prefixes to any MonadLogger. Here's a brief example:

{-# LANGUAGE TemplateHaskell #-}

import Control.Monad.Logger
import Control.Monad.Logger.Prefix

main :: IO ()
main = runStdoutLoggingT $ do
    $(logDebug) "This one has no prefix."

    "foo" `prefixLogs` do
        $(logDebug) "This one has a [foo] prefix."

        "bar" `prefixLogs` do
            $(logDebug) "This one has both [foo] and [bar] prefixes."

The package includes a benchmark demonstrating that there is no performance difference with ordinary logging.