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