Hi all, I have a bunch of data types which are "divided" in other data types as following: type FileName = String data FileWish = File FileName deriving (Show, Eq) data FileFact = FileExist | FileDontExist deriving (Show, Eq) data FileAction = FileCreate deriving (Show, Eq) observe :: FileWish -> [FileFact] observe :: undefined plan :: FileWish -> FileFact -> [FileAction] plan (File _) FileDontExist = [FileCreate] plan (File _) FileExist = [] perform :: FileWish -> [FileAction] -> IO () perform :: undefined I want to have a model as extensible as possible, even if I'm new to Haskell, I think that it's preferable to base parts of system on behavior (type classes) rather than data types. So I have File that give a Wish, a Fact (via observe) and a Action (via plan). I want to put observe, plan and perform in different type classes (let's say Observable, Plannable and Performable). When I create a new thing (like File) I want, for example, that : observe :: Wish e -> [Fact e] where e is File. But I want (Fact e) be an instance of Plannable. And so on. At first glance, I think I have to use this: https://www.fpcomplete.com/school/to-infinity-and-beyond/pick-of-the-week/ty... But it is coupled to data type and one type class. If you have any hints for me. Maybe I don't take the right way to solve my problem. Thanks in advance.