Am Donnerstag 17 September 2009 16:30:14 schrieb Andy Gimblett: > On 17 Sep 2009, at 15:21, José Pedro Magalhães wrote: > > > > E.g. here's a type Bar with three constructors: > > > > > data Bar = X | Y | Z deriving (Show) > > > > > instance Enumerated Bar where > > > > > constructors = [X, Y, Z] > > > > > > > > (This is certainly ugly. Any suggestions?) > > > > > > > |constructors| is expressible in SYB: > > > > Wow. > > > > What about > > > > data Bar = X | Y | Z deriving (Show, Eq, Ord, Enum, Bounded) > > > > instance Enumerated Bar where > > constructors = [minBound .. maxBound] > > > > ? > > > > Oh yes, that will certainly work for this very simple datatype. > > However, one cannot automatically derive instances of |Bounded| for > > datatypes with non-nullary constructors.
Andy's original message hasn't found its way into my inbox yet (neither has yours which Andy here quotes), so I don't know what Andy wants to do. From the part you quoted, I drew the conclusion that one thing Andy wanted was a more elegant way for the case of nullary constructors. For that, SYB is certainly overkill. I haven't looked at your code, I suppose it also does something reasonable in the presence of non-nullary constructors, in which case the separate treatment of only nullary constructors would of course be unnecessary. > > That would be OK in this instance, I think; I'm already dealing with > some of those cases by hand, but there were enough purely nullary ones > that this seemed worth doing. > > I don't know if that will work any better with Foo/convert, > though... :-) I'll probably understand that when your original message arrives :-) > > Thanks though Daniel - it's good to meet Enum and Bounded. > > -Andy _______________________________________________ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
