[Haskell-cafe] Class and Instance

[Yves Parès]

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 at 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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