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...
On Mon, Mar 12, 2012 at 04:27, Reiner Pope
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....). 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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