App a b = App { unApp :: WriterT a IO b }

{-# LANGUAGE GeneralizedNewtypeDeriving #-}

import Control.Monad.Trans.Writer (WriterT, runWriterT)

import Control.Monad.Writer.Lazy (MonadWriter(..), tell)

import Control.Monad.IO.Class (MonadIO, liftIO)

newtype App a b = App { unApp :: WriterT a IO b } deriving (Functor, Applicative, Monad, MonadWriter a, MonadIO)

runApp :: App a b -> IO (b, a)

runApp = runWriterT . unApp

app :: App [String] Int

app = do

tell ["A"]

content <- liftIO $ readFile "./main.hs"

tell ["B"]

return (length content)

main = do

res <- runApp app

print res