Hi all, I am not a Haskell expert, and I am currently exploring type classes and need some advice. I want to define a family of mutually recursive types as a collection of type classes and then I want to be able to map these collections of types to a set of other types using instance declarations. For example, I have a type family for graphs, consisting of the types "Node" and "Edge". In another part of my application I have the types "Person" and "Likes" (a pair of persons), and I want to map the roles "Node" and "Edge" to "Person" and "Likes", respectively. It seems to me that functional dependencies could be a way to model it (maybe it can also be done much simpler, but I don't know how). Here is what I tried: class (Node n, Edge e) => Graph g n e | g -> n, g -> e where class Node n where isConnectedTo :: Graph g n e => g -> n -> e -> Bool class Edge e where n1 :: Graph g n e => g -> e -> n n2 :: Graph g n e => g -> e -> n type Person = String type Likes = (Person, Person) data DummyGraph = DummyGraph String instance Graph DummyGraph Person Likes where instance Node Person where isConnectedTo g n (p1,p2) = (p1 == n) || (p2 == n) instance Edge Likes where n1 g (p1,p2) = p1 n2 g (p1,p2) = p2 This "DummyGraph" thing is supposed to be used as a kind of "family object" which stands for a particular type class family. However, this is not yet quite right because I get the error message Couldn't match the rigid variable `e' against `(a, b)' `e' is bound by the type signature for `isConnectedTo' Expected type: e Inferred type: (a, b) When checking the pattern: (p1, p2) In the definition of `isConnectedTo': isConnectedTo g n (p1, p2) = (p1 == n) || (p2 == n) Similar error messages occur in the instance declaration for Edge/Likes. I don't understand exactly what my error is. Maybe I would need a completely different strategy to model this. Any help would be appreciated! Regards, Klaus