Hi Andres, 2012/2/24 Andres Löh <andres.l...@googlemail.com>
> 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
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