Hello all, I was trying to construct a data type "Profile" which allows some set-like behavior. I started with functions to count, add and filter profiles. type Count = Int data Prof a = P { pCount :: Count, pFilter :: (a -> Bool) -> Prof a, pAdd :: (Prof a) -> (Prof a) } | NoProfile When I tried to actually implement a Profile, the first two functions were easy: exProfile :: [Int] -> Prof Int exProfile xs = P { pCount = length xs, pFilter = \p -> exProfile [x | x<-xs, p x], } But I didn't know how to implement pAdd. I was tempted to write something like pAdd = exProfile (xs ++ ys) But I wouldn't know where to take the ys from. I also hesitate to declare a toList function (which would solve the problem) in Prof. I would only do this *because* it is handy in exProfile, not because it is an intrinsic feature of Prof. What would be a good way to work with set-like behavior without being tied to one particular implementation?