Hi Andres,
2012/2/24 Andres Löh
I don't understand what's going on here. Instances for V1 should of course be defined if they can be! And in this case, a V1 instance makes sense and should be defined. The definition itself doesn't matter, as it'll never be executed.
The definition certainly matters: data Ze deriving Generic
class DeepSeq a where rnf :: a -> () default rnf :: (Generic a, GDeepSeq (Rep a)) => a -> () rnf = grnf . from
instance DeepSeq Ze
class GDeepSeq f where grnf :: f a -> ()
instance GDeepSeq V1 where grnf _ = ()
instance GDeepSeq a => GDeepSeq (M1 i c a) where grnf = grnf . unM1
-- other instances are not relevant now
t :: Ze t = undefined
seq t () == undefined. rnf t == (), because the V1 instance dictates so. Cheers, Pedro