errors.hs

import Control.Monad (liftM)
import Control.Monad.Except

type MyMonad = ExceptT String IO

successVal :: MyMonad Int
successVal = return 123

errVal :: MyMonad Int
errVal = throwError "this is an error"

anotherSuccess :: Either String Integer
anotherSuccess = Right 34563456435623423423


doStuff :: MyMonad ()
doStuff = do
  foo <- successVal
  _ <- lift $ putStrLn $ show foo
  baz <- ExceptT $ return anotherSuccess
  _ <- lift $ putStrLn $ show baz
  bar <- errVal
  _ <- lift $ putStrLn $ show bar
  return ()

run :: IO (Either String ())
run = runExceptT doStuff

main :: IO ()
main = do
  result <- run
  case result of
   Right _ -> return ()
   Left err -> putStrLn $ "error: " ++ err