[Haskell-beginners] Question regarding to type classes

Dear list members, as an exercise, I tried to define a type class for structures with ordered key values. The key values could be numbers, but also strings. For instance, I intend to define an address list containing records with names as key values. So far, my type class definition contains a function "key", which should extract the key values from the structures, and a function which compares the key values. So I tried: class StructsWithOrderedKeys a where -- no default definition for key key :: (Ord b) => a -> b ( a -> Bool x