[Haskell-cafe] Helper classes for Generics

[José Pedro Magalhães]

Hi Reiner,

It is indeed not strictly necessary to define such helper classes for kind
* generic functions. You do need them for kind * -> * functions, though.
Also, I think they should always be used because they help keep things
separate. If we use an implementation of generics with DataKinds [1], then
the helper classes always have a different kind from the user-facing
classes.


Cheers,
Pedro

[1]
http://hackage.haskell.org/trac/ghc/wiki/Commentary/Compiler/GenericDeriving#Kindpolymorphicoverhaul

On Mon, Mar 12, 2012 at 04:27, Reiner Pope <reiner.pope at gmail.com> wrote:

> 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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