writer_try.hs

import Control.Monad.Writer


sayHello :: Writer (Sum Int, Sum Int) String
sayHello = do
    tell (0,1)
    return "hello"

sayGoodbye :: Writer (Sum Int, Sum Int) String
sayGoodbye = do
    tell (1,0)
    return "goodbye"


tweeting = do
    sayHello
    sayHello
    sayHello
    sayHello
    sayGoodbye
    sayGoodbye

main :: IO ()
main = do
     print $ runWriter tweeting
--    putStrLn $ show $ hello . snd t
--    putStrLn $ show $ goodbye . snd t