lifting Either to ErrorT

liftToErrorT.hs

import Control.Monad.Error

execAna :: Int -> Either String (Int, Int)
execAna 0 = Left "ERROR"
execAna n = Right (n - 1, n + 1)

report :: Either String () -> IO ()
report (Left err) = putStrLn $ "failed: " ++ err
report _ = putStrLn "complete"

liftToErrorT :: (Error a, Monad m) => Either a b -> ErrorT a m b
liftToErrorT (Left msg) = throwError msg
liftToErrorT (Right x) = return x

errorRun :: Int -> IO ()
errorRun n = do
    err <- runErrorT $ do
--          Either a b --return--> IO (Either a b) --ErrorT--> ErrorT a IO b
--          phew ;)
            (a, b) <- ErrorT $ return $ execAna n
--            (a, b) <- liftToErrorT $ execAna n
            liftIO $ putStrLn $ show a
            liftIO $ putStrLn $ show b
    report err

main :: IO ()
main = do
    errorRun 1 -- should work and return 0 and 2
    errorRun 0 -- should return an error