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
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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