Hi, While working on a toy compiler I realized that Data.Set.Set (Set) is not an instance of the Functor class. In other words: 'fmap' is not defined on it. I tried various ways of defining an instance but I failed. The reason is quite interesting: Set is a type constructor (* -> *) so it should qualify it for being a Functor. (In a sense it is very similar to a Map or a list.) However, most Set functions require the elements of the Set to be an instance of Ord. The problem is that this constraint cannot be deduced from the instance declaration for Functor: instance Functor Data.Set.Set where fmap f s = Data.Set.map f s GHCi: Could not deduce (Ord a, Ord b) from the context (Functor Data.Set.Set) arising from use of `Data.Set.map' at ... Probable fix: add (Ord a, Ord b) to the class or instance method `fmap' In the definition of `fmap': fmap f s = Data.Set.map f s In the definition for method `fmap' In the instance declaration for `Functor Data.Set.Set' On the other hand, it seems intuitively natural to make Set an instance of fmap. Any ideas on how to do it? Thanks and Regards, Lajos Nagy