At Mon, 20 Jun 2011 09:57:38 +0200, José Pedro Magalhães wrote: > > class JSON1 a r | a -> r > toJSON1 :: a -> r > > instance (JSON a) => JSON1 (S1 NoSelector (K1 c a)) [Value] where > toJSON1 (M1 (K1 a)) = [toJSON a] > > instance (Selector x, JSON a) => JSON1 (S1 x (K1 c a)) [(String, Value)] > where > toJSON1 s@(M1 (K1 a)) = [(nameOf s undefined, toJSON a)] > where nameOf :: S1 c f p -> c -> String > nameOf _ c -> selName c > > The key piece of magic I need here (and in so many other places) is to > be able to do one thing at the type level if x is a particular type > (e.g., NoSelector) or sometimes one of a small number of types, and to > do something else if x is any other type. > > Right. I think this is often essential.
One thing you could do to help in this specific case would be to use a different M1 tag--e.g., M1 S ... for selectors and M1 NS ... for fields without selectors (or K1 NS). I presume you've already considered this and/or it's too late to make such a change. (Or to move the distinction up to the constructor with two different constructor tags, CR and CN for record and no-record.) Anyway, as you mention, there are other situations where you still need Overlapping/UndecidableInstances. David _______________________________________________ Haskell-prime mailing list Haskell-prime@haskell.org http://www.haskell.org/mailman/listinfo/haskell-prime
