[Haskell-cafe] Helper classes for Generics

[José Pedro Magalhães]

It could, yes. Actually, using DefaultSignatures you can probably use SYB
for defining classes with generic default methods, by adding Data and
Typeable constraints instead of Generic.


Cheers,
Pedro

2012/3/12 Yves Parès <yves.pares at gmail.com>

> 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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>>
>
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