data Array i e = Array i i shouldn't the element type appear here? Typo: data Array i e = Array i i [e] --mockup with
Bulat, BZ>AK> class CollectionClass c e where BZ>this works with -fglasgow-exts : Yup. Figured that out shortly after I e-mailed... Thank you for providing the extra detail, though. -fglasgow-exts fixed the problem, but I didn't have a good idea why the problem went away. Daniel, list
Well, the parameter c of CollectionClass has kind (* -> *), Array has kind (* -> * -> *), so it must be instance CollectionClass (Array i) e where ... ... which (I think) was breaking until -fglasgow-exts. Once that was fixed, this seems to be the preferred way to implement this function.
But the element type doesn't really belong in the class, wouldn't class Collection c where toList :: c e -> [e] Hmmm... For example, implementing Array and UArray as subclasses of Collection: class Collection c where toList :: c e -> [e] instance Collection (Array i) where toList a = ... instance Collection (UArray i) where toList a = ...
If Collection is not parameterized on "e", then there is no way to make sure that the "e" in Collection (UArray i):toList is actually "unboxable". As with the current IArray implementation, the following works and isn't particularly limiting in flexibility: class Collection c e where toList :: c e -> [e] instance Collection (Array i) e where toList a = ... instance Collection (UArray i) Int where --note the Int toList a = ... I understand trying to make Collection as general as possible by removing "e", but it look as if doing so makes useful subclasses difficult to implement?
-- or perhaps, better class Collection c => ArrayLike c i | c -> i where look :: Monad m => i -> c e -> m e be preferable? ermm... Need to read up on Functional Dependencies... brb...
Thanks for the help. - Alson