I tried to make my previous example a bit more flexible, but I guess I was over-confident. Here's my code: ----- {-# LANGUAGE MultiParamTypeClasses, FlexibleInstances, FunctionalDependencies, UndecidableInstances #-} class Eq a => Graph g a | g -> a where nodes :: g a -> [a] neighbours :: g a -> a -> [a] class (Graph g a, Eq a, Ord w) => WeightedGraph g w a | g -> w, g -> a where edges :: g w -> [(a, a, w)] data MyGraph w a = MyGraph [(a, a, w)] instance Eq a => Graph (MyGraph w) a where nodes _ = [] --stub neighbours _ _ = [] --stub instance (Graph g a, Eq a, Ord w) => WeightedGraph (MyGraph w a) where edges = [] -- stub ----- The error message is temp2.hs:16:53: Kind mis-match The first argument of `WeightedGraph' should have kind `* -> *', but `MyGraph w a' has kind `*' In the instance declaration for `WeightedGraph (MyGraph w a)' Failed, modules loaded: none. It seems to me that in the WeightedGraph class, g should have the kind * -> * -> *, and MyGraph has the kind * -> * -> *, so I'm not sure why I have a kind mismatch, but I suspect that I've written the instance declaration wrong. Any ideas how to fix it?