Am Samstag 19 September 2009 20:55:10 schrieb Andy Gimblett:
On 17 Sep 2009, at 18:01, Ryan Ingram wrote:
Here's a way that works more closely to your original version:
instance Enumerated a => Target a where convert n
| n >= 0 && n < numConstrs = Just (constrs !! n) | otherwise = Nothing
where constrs = constructors numConstrs = length constrs
Aha - that's great, and it works without OverlappingInstances (but still with FlexibleInstances and UndecidableInstances - should that worry me?)
FlexibleInstances need not worry anybody. They just remove a fairly arbitrary restriction of Haskell98 for instance declarations. UndecidableInstances can be dangerous, but there are perfectly safe things which reauire UndecidableInstances, too.
Just making sure constructors is only referenced once is the key, it seems.
Just making sure that every time it is referenced, it is referenced at the correct type.
Thanks!
-Andy