import Control.Exception
import Control.Monad.Fail
import Control.Monad.State
import Control.Monad.IO.Class (liftIO)

newtype AppState = AppState Int

newtype MyApp a = MyApp ((StateT AppState IO) a)
  deriving (Functor, Applicative, Monad, MonadIO, MonadState AppState)

runMyApp :: AppState -> MyApp a -> IO (a, AppState)
runMyApp st (MyApp stT) = runStateT stT st

tryMyApp :: MyApp a -> MyApp (Either SomeException a)
tryMyApp job = do
  st <- get
  res <- liftIO $ try $ runMyApp st job
  case res of
    Left e -> return $ Left e
    Right (v, st) -> do put st ; return $ Right v

throwError :: MyApp ()
-- error is not caught this way:
-- throwError = return $ error "threw error"
throwError = error "threw error"

doStuff :: MyApp ()
doStuff = do
  liftIO $ putStrLn "do stuff ..."
  res <- tryMyApp throwError
  case res of
    Left _ -> liftIO $ putStrLn $ "catching error worked"
    Right v -> liftIO $ putStrLn $ "got: " ++ show v
  
main :: IO ()
main = runMyApp (AppState 42) doStuff >> return ()