WriterT [a] IO b

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

type App a b = WriterT [a] IO b

app :: App String Int
app = do
  tell ["A"]
  content <- lift $ readFile "./main.hs"
  tell ["B"]
  return (length content)

main = do 
    res <- runWriterT app
    print res