MaybeT and Transactions in runDb

Question

For my previous question on chaining failures, Michael Snoyman had suggested I use MaybeT to run them so if any of them fails, it will just short-circuit to Nothing.

I was under the impression runDb runs everything in a transaction. So shouldn't a failure at any point in code automatically rollback the transaction?

mauth <- runDb $ runMaybeT $ do
            valid    <- MaybeT $ return $ listToMaybe errs 
            uid      <- MaybeT $ insertUnique u 
            vid     <- MaybeT $ getBy $ UniqueField v -- this step fails but previous insert does not roll back
            auth     <- liftIO $ createAuthToken uid
            return auth

When I run the above code, the getBy fails but user was still inserted. Am I misunderstanding that runDb will rollback on a Nothing inside MaybeT? Do I need to use some other Monad for this to work?

Appreciate your thoughts on how to best rollback on failure.

Update: This is what I ended up doing per Michael's suggestion.

mauth <- runDb $ do
          ma <- runMaybeT $ do
                   valid <- ... 
          case ma of
            Just _ -> return ma
            Nothing -> liftIO $ throwIO MyException

Now I need to figure out how to catch this exception nicely outside and return a proper error message back.

Thanks!

Answer 1

Returning Nothing is not the same thing as a failure. You'd need to throw a runtime exception (via something like throwIO) for Persistent to treat it as a rollback situation.