Thanks for the feedback. I have two further questions 1. Why is it that the Containers class signature does not allow instances to be list of list? I suspect it is because x is a constructor. 2. How would I change the Containers class signature to allow instances to be lists of lists. Thanks, Pat -- x is a type constructor, not a type class Containers x y where insert :: y -> x y -> x y remove :: y -> x y -> x y whatsIn :: x y -> [y] instance Containers [] Char where insert y [] = y:[] insert y m = y:m remove _ [] = [] remove x (y:ys) | x == y = remove x ys | otherwise = y : remove x ys whatsIn = id instance Containers [] Integer where insert y [] = y:[] insert y m = y:m remove _ [] = [] remove x (y:ys) | x == y = remove x ys | otherwise = y : remove x ys whatsIn = id -- cannot have containers of containers. -- instance Containers [] [] where -- container.hs:41:23: -- `[]' is not applied to enough type arguments -- Expected kind `*', but `[]' has kind `* -> *' -- In the instance declaration for `Containers [] []' -- Failed, modules loaded: none. This message has been scanned for content and viruses by the DIT Information Services E-Mail Scanning Service, and is believed to be clean. http://www.dit.ie