More idiomatic way of exiting mapM early

Question

Given a list of actions that returns m (Maybe a) I'm trying to return m (Maybe [a]) where if any of the individual results are Nothing the whole result is Nothing. m contains StateT and I want to avoid running any actions after the first Nothing is returned.

Trying to use mapM and then moving the Maybe outside the list results in all the actions being run.

I have this solution but nested case statements with just a lot of wrapping and unwrapping gives me the feeling that there's probably a more elegant way of doing this. Usually when I have this feeling there a one-liner with a more general type that does exactly the same thing.

Any suggestions?

myMapM' :: Monad m => (t1 -> m (Maybe t)) -> [t1] -> m (Maybe [t])
myMapM' f [] = return (Just [])
myMapM' f (a:as) = do
  r <- f a 
  case r of
    Nothing -> return Nothing
    Just g -> do
      rs <- myMapM' f as
      case rs of
        Nothing -> return Nothing
        Just gs -> return (Just (g:gs))

Answer 1

The behavior you want is that of the monad transformer MaybeT.

myMapM :: Monad m => (t1 -> m (Maybe t)) -> [t1] -> m (Maybe [t])
myMapM f = runMaybeT . mapM (MaybeT . f)