Hello, I'm trying to make a simple monad (built on operational's ProgramT) for resource loading. I have classes featuring type families : {-# LANGUAGE TypeFamilies, FlexibleContexts, GADTs #-} -- | A ResourceId is something that identifies a resource. -- It should be unique for one resource, and should be used to find the location (the path) of the resource, -- possibly by using a configuration datatype class (Ord id) => ResourceId id where type LocOf id type CfgOf id retrieveLoc :: CfgOf id -> id -> LocOf id -- | Class describing a resource of type @rsc@ class (ResourceId (IdOf rsc)) => Resource rsc where type IdOf rsc load :: LocOf (IdOf rsc) -> IO (Maybe rsc) -- ^ Called when a resource needs to be loaded unload :: rsc -> IO () -- ^ Idem for unloading -- | Then, the operations that the loader can perform data EDSL id a where Load :: id -> EDSL id () IsLoaded :: id -> EDSL id Bool Unload :: id -> EDSL id () -- | The loader monad itself type RscLoader rsc m a = ProgramT (EDSL (IdOf rsc)) m a -- | And finally, how to run a loader runLoader :: (Monad m, Resource rsc) => CfgOf (IdOf rsc) -> RscLoader rsc m a -> m a runLoader cfg loader = viewT loader >>= eval M.empty where eval :: (Monad m, Resource rsc) => M.Map (IdOf rsc) rsc -> ProgramViewT (EDSL rsc) m a -> m a eval _ (Return x) = return x eval rscs (instr :>>= k) = case instr of Load id -> do let loc = retrieveLoc cfg id -- open and load from loc will go here viewT (k ()) >>= eval rscs -- -- -- Other cases yet to come... Well, there is no way I can get it type-check. I think I must be misusing the type families (I tried with multi-param typeclasses and functional dependencies, but it ends up to be the same kind of nightmare...).