A small example that shows logging to file and stderr in Haskell.

logging.hs

module Main where

import System.Log.Logger
import System.Log.Handler.Simple
import System.Log.Handler (setFormatter)
import System.Log.Formatter


main = do 
    -- Initialize loggers
    updateGlobalLogger "main" (setLevel DEBUG)
    h <- fileHandler "logfile.log" DEBUG >>= \lh -> return $ 
        setFormatter lh (simpleLogFormatter "[$time : $loggername : $prio] $msg") 
    updateGlobalLogger "main" (addHandler h)

    debugM "main" "RAWR!"
    warningM "main" "ONG HEST!"

Source: http://hackage.haskell.org/packages/archive/hslogger/latest/doc/html/System-Log-Logger.html