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 <[email protected]>
> 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]
>
> This message has been scanned for content and viruses by the DIT
> Information Services E-Mail Scanning Service, and is believed to be clean.
> http://www.dit.ie
>
> _______________________________________________
> Haskell-Cafe mailing list
> [email protected]
> http://www.haskell.org/mailman/listinfo/haskell-cafe
>
_______________________________________________
Haskell-Cafe mailing list
[email protected]
http://www.haskell.org/mailman/listinfo/haskell-cafe
