ErrorT usage

errorT.hs

import Control.Monad.Error

execAna :: Int -> ErrorT String IO (Int, Int)
execAna 0 = throwError "ERROR"
execAna n = return (n - 1, n + 1)

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

errorRun :: Int -> IO ()
errorRun n = do
    err <- runErrorT $ do
            (a, b) <- 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 produce an error