which is almost StateT AlexState (Except String), and looks like I can make a MonadError instance out of it.

Therefore I'm curious if there's any existing package that has newtype StateT' s m a = StateT' (s -> m (s, a)) -- note the swapped pair here

so that I can just deriving (MonadError String) via StateT' AlexState (Except String) to implement MonadError.

Thanks!