[Haskell-cafe] Helper classes for Generics

[José Pedro Magalhães]

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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