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 _______________________________________________ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
