Am Mittwoch, 1. März 2006 20:20 schrieben Sie:
Daniel,
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 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.
Well, Haskell98 doesn't allow multiparameter type classes, so extensions are required.
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
Ah, I see. I didn't think of that.
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?
Making Collection a single parameter class doesn't increase generality, I believe, but I thought that creating a collection was a property of the type constructor c alone, so the element type didn't really belong to the class. However, the UArray example gives a good reason to include it.
-- 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...
Thus we can declare instance ArrayLike [] Int where look 0 (x:_) = return x ...
Thanks for the help.
- Alson
Cheers, Daniel -- "In My Egotistical Opinion, most people's C programs should be indented six feet downward and covered with dirt." -- Blair P. Houghton