Hi, I recently tried to do the following code: class VertexState s v where new_point :: s -> v -> v -> v next_state :: s -> s -- interleave :: (VertexState s a) => s -> [a] -> [a] -> [a] interleave s (t:ts) (b:bs) = new_point s t b : interleave (next_state s) bs (t:ts) interleave _ [] [b] = [] Firstly, GHC doesn't deal all too gracefully with this if the type signature is uncommented, it tells me: Could not deduce (VertexState s v) from the context (VertexState s a) arising from use of `next_state' at Grid.lhs:25 Probable fix: Add (VertexState s v) to the type signature(s) for `interleave' Unfortunately, doing the probable fix just results in a new request for (VertexState s v1), and so on. I suppose the real problem is that GHC can't decide from next_state that it is supposed to use exactly this instance of VertexState, is there a good way to do that without adding an explicit dummy parameter of type v? (I really would like to be able to have instances with the same s but different v's, which I think adding functional deps would make difficult.) -kzm -- If I haven't seen further, it is by standing in the footprints of giants