[Haskell-cafe] Helper classes for Generics

[Yves Parès]

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 <jpm at 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.pares at 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.pope at 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
>>>
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>>>
>>
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