[Haskell-cafe] Helper classes for Generics

José Pedro Magalhães jpm at cs.uu.nl
Mon Mar 12 21:04:58 CET 2012


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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