Generics and data families

[Reiner Pope]

Hi all,

I am of the opinion that the approach I described in my previous email is reasonable. To me, the key point is that the Generic class (as opposed to the Generic1 class) is for types of kind *. Since D Bool and D Int are different types of kind *, it is reasonable for them to have unrelated instances for Generic. (The situation would be different for Generic1, once GHC supports deriving that).

A similar question is what the following should do:

> {-# LANGUAGE DeriveGeneric, FlexibleInstances, StandaloneDeriving #-}
> import GHC.Generics
> data T a = T a
> deriving instance Generic (T Bool)

Surprisingly, it currently generates the following instances:

> instance Generic (T Bool) where ...
> type instance Rep (T a) = ...

This means, for instance, that the following does not compile:

> {-# LANGUAGE DeriveGeneric, FlexibleInstances, StandaloneDeriving #-}
> import GHC.Generics
> data T a = T a
> deriving instance Generic (T Bool)
> deriving instance Generic (T Int)

because the "type instance Rep (T a) = ..." declarations conflict.

I would say it should generate the following instances:

> instance Generic (T Bool) where ...
> type instance Rep (T Bool) = ...

Note that these instances are of exactly the same form as the data family instances of my previous email.

Regards,
Reiner

On 14/03/2012, at 9:57 AM, Reiner Pope wrote:

> Hi all,
> 
> The DeriveGeneric language extension in ghc 7.4 does not support data families, so the following will not compile:
> 
>> data family D a
>> data instance D Int = DInt  deriving Generic
>> data instance D Bool = DBool  deriving Generic
> 
> On http://hackage.haskell.org/trac/ghc/ticket/5936, I provided a patch for ghc which makes the above compile, giving the (approximately) the following instances for Rep and Generic:
> 
>> type instance Rep (D Int) = D1 DIntInfo_D (C1 DIntInfo_C U1)
>> type instance Rep (D Bool) = D1 DBoolInfo_D (C1 DBoolInfo_C U1)
>> 
>> instance Generic (D Int) where ...
>> instance Generic (D Bool) where ...
> 
> Note in particular that D Int and D Bool are considered completely distinct types as far as generics are concerned.
> 
> Is this the right approach to take to handle generics for data families? Are there alternative approaches? 
> 
> On the Trac ticket linked above, Pedro suggested we discuss these questions here.
> 
> Regards,
> Reiner