Witness me!

MyLogging.hs

import System.Environment (setEnv, lookupEnv)

doWriteLog :: Int -> Int -> String -> IO ()
doWriteLog logLevel messageLevel message
  | messageLevel >= logLevel = putStrLn message
doWriteLog _ _ _ = return ()

setLogLevel :: Int -> IO ()
setLogLevel newLogLevel = do
  setEnv "__loglevel" $ show newLogLevel

getLogLevel :: IO Int
getLogLevel = do
  maybeLevel <- lookupEnv "__loglevel"
  case maybeLevel of
    Nothing      -> return 3
    Just someStr -> return (read someStr :: Int)

mylog :: Int -> String -> IO ()
mylog messageLevel message = do
  logLevel <- getLogLevel
  doWriteLog logLevel messageLevel message

main = do
  mylog 2 "will not be printed"
  mylog 3 "witness me!"
  setLogLevel 2
  mylog 2 "bloodbag!"