Thanks for the clarification. But could not class Data have been used for generic Deriving of classes? I imagine it would have been harder, but I fail to see if would have been possible...
Le 12 mars 2012 16:58, José Pedro Magalhães <j...@cs.uu.nl> a écrit : > Hi Yves, > > GHC.Generics [1] and SYB [2] are two rather different approaches to > generic programming. There are things that can be done in one but not in > the other, and there are things that are easier on one rather than the > other. For instance, SYB tends to be very useful for large AST > transformations, with functions that have a general behaviour but a couple > of particular cases for a few constructors. GHC.Generics, on the other > hand, can encode functions such as generic fmap and traverse. It lends > itself better to optimisation since it doesn't use runtime casts, and as > such tends to be faster than SYB. It isn't planned to replace SYB. > > > Cheers, > Pedro > > [1] http://www.haskell.org/haskellwiki/Generics > [2] http://www.cs.uu.nl/wiki/bin/view/GenericProgramming/SYB > > > On Mon, Mar 12, 2012 at 16:35, Yves Parès <yves.pa...@gmail.com> wrote: > >> I'd have a question concerning GHC.Generics: how does it relate to SYB's >> Data.Generics? >> Is it intended to replace it or complete it? >> In other words: does class Data.Generics.Data class do things that class >> GHC.Generics.Generic can't do? >> >> >> Le 12 mars 2012 04:27, Reiner Pope <reiner.p...@gmail.com> a écrit : >> >>> Hi all, >>> >>> I've been playing with GHC's new generics features (see >>> http://www.haskell.org/ghc/docs/latest/html/users_guide/generic-programming.html). >>> All the documentation I've seen suggests creating a "helper class" -- for >>> instance, the GSerialize class in the above link -- on which one defines >>> generic instances. >>> >>> It seems to me that this isn't necessary. For example, here's the the >>> example from the GHC docs, but without a helper class: >>> >>> > -- set the phantom type of Rep to (), to avoid ambiguity >>> > from0 :: Generic a => a -> Rep a () >>> > from0 = from >>> > >>> > data Bit = O | I >>> > >>> > class Serialize a where >>> > put :: a -> [Bit] >>> > >>> > default put :: (Generic a, Serialize (Rep a ())) => a -> [Bit] >>> > put = put . from0 >>> > >>> > instance Serialize (U1 x) where >>> > put U1 = [] >>> > >>> > instance (Serialize (a x), Serialize (b x)) => Serialize ((a :*: b) x) >>> where >>> > put (x :*: y) = put x ++ put y >>> > >>> > instance (Serialize (a x), Serialize (b x)) => Serialize ((a :+: b) x) >>> where >>> > put (L1 x) = O : put x >>> > put (R1 x) = I : put x >>> > >>> > instance (Serialize (a x)) => Serialize (M1 i c a x) where >>> > put (M1 x) = put x >>> > >>> > instance (Serialize a) => Serialize (K1 i a x) where >>> > put (K1 x) = put x >>> >>> Is there a reason to prefer using helper classes? Or perhaps we should >>> update the wiki page (http://www.haskell.org/haskellwiki/Generics) to >>> avoid using helper classes? >>> >>> Regards, >>> Reiner >>> >>> _______________________________________________ >>> Haskell-Cafe mailing list >>> Haskell-Cafe@haskell.org >>> http://www.haskell.org/mailman/listinfo/haskell-cafe >>> >>> >> >> _______________________________________________ >> Haskell-Cafe mailing list >> Haskell-Cafe@haskell.org >> http://www.haskell.org/mailman/listinfo/haskell-cafe >> >> >
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