Re: [Haskell-cafe] Helper classes for Generics

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 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 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 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....). 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

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