The problem is that [] alone is not a concrete type - that is what the error is saying, that [] needs to be applied to another type to yield a concrete type. IE, you need a list of something, not just the idea of a list. That something can be polymorphic, so the following works (note the [a]): class Containers x y where insert :: y -> x y -> x y {-remove :: y -> x y -> x y whatsIn :: x y -> [y]-} instance Containers [] [a] where insert x y = x:y On Jun 11, 2011, at 5:49 AM, Patrick Browne wrote:
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.
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