[Haskell-cafe] Class and Instance

[Patrick Browne]

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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