Hi there haskellers, I have thoroughly confused myself with type-classes in a haskell system I am writing, and I was wondering if anyone had some useful suggestions to get me out of my mess. I apologise if this is all long and rambling, but that maybe why I can't solve it... The system itself performs analyses on data from human eye-tracking experiments, where the tracking data is simplified from raw positional data into "events" which are either fixations or saccades. This is easy to do: data Event = Fixation {...} | Saccade {...} Because, at the basic level all of the experiments share this type of data, it seems that I should be able to write analysis functions that work for any experiment. However, the experiments differ in the stimuli used, and associated with each stimulus set is a set of "milestones" that give times at which important things happen in the stimuli, and "regions of interest" that give areas of the visual scene that are considered important. For a single experiment I would have: data Experiment = Exp [Trial] data Trial = Trial [Event] (Map MileStone Time) data MileStone = M1 | M2 | ... and the analysis functions can take an experiment, use the events and the milestones, and return whatever. The problem arises when I want to represent a second experiment. I assumed the way to solve this was with a type class: class ExperimentClass a data ExperimentOne = Exp [Trial] instance ExperimentClass ExperimentOne and have the analysis functions take (ExperimentClass a) => a -> ... This, however, necessitates a change to Trial as well, because the MileStones for each experiment are different: class MileStoneClass a data (MileStoneClass a) => Trial a = Trial [Event] (Map a Time) which means that: data (MileStoneClass a) => ExperimentOne a = Exp [Trial a] and maybe even: class (MileStoneClass b) => ExperimentClass a b instance ExperimentClass ExperimentOne ExOneMileStones which all seems fine. But then I need to add regions of interest, and a corresponding type parameter, and a couple of more things with type parameters, and I get something like: class (MileStoneClass b, RegionClass c, ... d, ... e) => ExperimentClass a b c d e which is a lot of type parameters that are all dependent on a, since the experiment defines the milestones etc. I can use fundeps to enforce that in the type system, but it is still quite messy. So, I was wondering whether there was something wrong with my basic model which leads to this ugly type class, or whether this is the proper way forward. Either is fine, really, it would just be nice to know for certain. Thanks in advance, Andrew