logger.hs

module Logger where

import System.IO

type Logger = String -> IO ()

data Logging a = Logging { unLogging :: Logger -> IO a }

logLine :: String -> Logging ()
logLine s = Logging $ \l -> l s

runLogging :: Logging a -> IO ()
runLogging (Logging logging) = do
	log <- openFile "log.txt" WriteMode
	logging $ hPutStrLn log
	hClose log

(>>>=) :: Logging a -> (a -> Logging b) -> Logging b
(Logging l1) >>>= f = Logging $ \l -> l1 l >>= \x -> unLogging (f x) l

wrap :: a -> Logging a
wrap x = Logging $ \_ -> return x

instance Monad Logging where
	return = wrap
	(>>=) = (>>>=)

-- even better, we can just use standard monad functions, or even do syntax:

main :: IO ()
main = runLogging doSomething 

doSomething :: Logging ()
doSomething = do 
	logLine "Hello Again" 
	x <- calcSomething "1234"
	logLine (show x)

calcSomething :: String -> Logging Int
calcSomething s = do
	logLine "calculating length" 
	return $ length s