You can also modify you class if your intent is that it always work with polymorph types (And it will save you some trouble with functional dependencies between types x and y. Plus, it will be Haskell98 compliant)

-- insert, remove and whatsIn are then supposed to work forall y, and then impose the kind of x to be * -> * (x is a type constructor, not a mere type)
class Container x where
    insert :: y -> x y -> x y
    remove :: y -> x y -> x y
    whatsIn :: x y -> [y]

Then you can do:
import Data.List (delete)

instance Container [] where
    insert = (:)
    remove = delete  -- removes only the first occurence
    -- remove x = filter (/= x)  -- if you want to remove every occurence of x (there may be a better way)
    whatsIn = id


2011/6/10 Patrick Browne <patrick.browne@dit.ie>
Hi
Below is a class that I wish to create some instances of.
I do not wish to change the class definition.
It is supposed to represent containers of type x that contain things of
type y.

My attempt at the insert function seems ok for Char and lists, but not
ok for Integer. How do I instantiate this class for integers and lists?

Does this class definition permit y to be a list and hence x to be a
list of lists? Something like: instance Containers [] [] where ..

Is this a constructor class?

Thanks,
Pat


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


instance Containers [] Integer where
 insert y [] = y:[]
 insert y m  = y:m


-- OK
-- insert  '1' ['u','u','l','i']

-- Not OK
-- insert 2 [9,2]

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