Example of logging in Haskell

logging_example.hs

#!/usr/bin/env runhaskell

-- This example uses the hslogger library.
-- For debugging it may be more convenient to use Debug.Trace instead since that
-- allows you to log debugging output from otherwise pure functions.

import System.IO (stderr, Handle)
import System.Log.Logger (rootLoggerName, setHandlers, updateGlobalLogger,
                          Priority(INFO), Priority(WARNING), infoM, debugM,
                          warningM, errorM, setLevel)
import System.Log.Handler.Simple (fileHandler, streamHandler, GenericHandler)
import System.Log.Handler (setFormatter)
import System.Log.Formatter

main = do
    let logPath = "/tmp/foo.log"
    myStreamHandler <- streamHandler stderr INFO
    myFileHandler <- fileHandler logPath WARNING
    let myFileHandler' = withFormatter myFileHandler
    let myStreamHandler' = withFormatter myStreamHandler
    let log = rootLoggerName
    updateGlobalLogger log (setLevel INFO)
    updateGlobalLogger log (setHandlers [myFileHandler', myStreamHandler'])
    infoM log $ "Logging to " ++ logPath
    debugM log "Hello debug."
    infoM log "Hello info."
    warningM log "Hello warning."
    errorM log "Hello error."

withFormatter :: GenericHandler Handle -> GenericHandler Handle
withFormatter handler = setFormatter handler formatter
    -- http://hackage.haskell.org/packages/archive/hslogger/1.1.4/doc/html/System-Log-Formatter.html
    where formatter = simpleLogFormatter "[$time $loggername $prio] $msg"