In a move to finally grok monads and monad stack a bit better, I decided to move the hMollom code to a monad based implementation. The exported functions now execute in the MollomMonad, which at this point is defined as follows:
type MollomState = ReaderT MollomConfiguration (StateT MollomServerList IO)
type MollomMonad = ErrorT MollomError (StateT (Maybe SessionID) MollomState)
The main concept of the code remains the same, each exported function calls the service function. The latter returns a MollomState a, to capture the fact that during the Mollom request, we potentially need to update the list of servers we can query with API calls. This result is then lifted into the MollomMonad stack by using the following function.
returnStateT a = StateT $ \s -> liftM (flip (,) s) a
This is illustrated in the following control flow figure (for a full sized image, follow the link to flickr):
The error handling is done by usage of the ErrorT monad transformer, which add the possibility to return a Left MollomError as function value, indicating a fault during the handling of the request. At this point, I have not added a decent runMollomMonad function, as I am still pondering which functions may be most useful for the users of this library. However, you can easily employ the following function, where defaultMollomConfiguration :: MollomConfiguration and we start without any servers that are available.
(\m -> (runStateT $ (runReaderT $ (runStateT . runErrorT $ m) (Nothing) ) defaultMollomConfig) UninitialisedServerList)
As usual, the code is available from both the GitHub repository, as well as from Hackage. All feedback is welcome. Thanks to #haskell and #ghentfpg for bearing with my questions.
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