[Haskell-cafe] Help with Type Class
daniel.is.fischer at web.de
Wed Mar 1 09:48:49 EST 2006
Am Mittwoch, 1. März 2006 01:26 schrieb Alson Kemp:
> Although the discussion about Array refactoring died
> down quickly on the Haskell' mailing list, I've been
> noodling on refactoring the various Collections in
> Haskell. In doing so, I've bumped into a problem with
> type classes that I can't resolve. The issue is as
> I'm designing a Collections class heirarchy that is
> a blend between that of Java and of Haskell. The
> problem is that, whereas in OOP it is easy to do so,
> Haskell class mechanism seems to make it difficult to
> add *new* member variable to subclasses.
> Simple goal: create a top-level Collection with an
> *indexed* Array subclass (and various other
> subclasses). The problem I'm running into is
> Collection has no need of an "index" variable and I
> can't seem to figure out how to add an "index" to
> Array when subclassing from Collection Mock up:
> class CollectionClass c e where
> -- every Collection supports toList...
> toList :: c e -> [e]
> class (CollectionClass a e)=> ArrayClass a e where
> data Array i e = Array i i
shouldn't the element type appear here?
> instance CollectionClass Array e where
> -- Since Array is a Collection
> -- toList :: c e -> [e]
> -- but with an Array the type would be
> -- toList :: a i e -> [e]
> toList = ...
Well, the parameter c of CollectionClass has kind (* -> *), Array has kind
(* -> * -> *), so it must be
instance CollectionClass (Array i) e where ...
But the element type doesn't really belong in the class, wouldn't
class Collection c where
toList :: c e -> [e]
instance Collection  where
toList = id
instance Collection (Array i) where
-- or rather instance Ix i => Collection (Array i),
-- if we use Data.Array
toList = elems -- or whatever we'd use for another implementation
class Collection (a i) => ArrayC a i where
item :: i -> a i e -> Maybe e
-- or perhaps, better
class Collection c => ArrayLike c i | c -> i where
look :: Monad m => i -> c e -> m e
> I think that the problem goes away if:
> class CollectionClass c x e where ...
> with "x" used an index for Array or a key for Map or
> () for Set, but it doesn't seem clean to scatter
> member variables in the parent class in case the
> subclass requires them...
> Another possible solution that I couldn't get to
> work would be to use (Array i) as the type for "c" in
I can't imagine why that wouldn't work (apart from the typo above).
It compiles, and if we have a more appropriate Array type, we'd get a decent
> "Collection c":
> instance CollectionClass (Array i) e where
> toList = ... -- (Array i) e -> [e] ?
> This seems clean because it says the Collection
> holds "e"s and is organized by an (Array i).
> Similarly, Set would be a Collection of "e"s organized
> by Set and Map would be a Collection of "e"s organized
> by (Map k).
> Undoubtedly, I've missspoken some crucial aspect of
> the type/kind/class/instance magical incantation.
> - Alson
"In My Egotistical Opinion, most people's C programs should be
indented six feet downward and covered with dirt."
-- Blair P. Houghton
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