Proposal: Add `Generically` (or `WrappedGeneric`) newtype to GHC.Generics

[Oleg Grenrus]

Note, that many Generic-derived instances for newtypes are equivalent with GND instance. Generic mechanism is powerful enough to notice newtype special case.

> On 1 Sep 2019, at 8.56, Dmitrii Kovanikov <kovanikov at gmail.com> wrote:
> 
> +1
> 
> This sounds very useful. I also thought about having this `newtype` in the past and there were even some posts on Reddit about such feature.
> 
> Basically, after this change, we can start recommending using `Generically` + `DerivingVia` instead of `DefaultSignatures`. It always bothered me that you can have only one default signature and for some reasons it's implemented for generics in almost every package. It just seems too arbitrary and ad-hoc to me. `newtype`s feel like a more natural solution. Instead of relying on whatever `default-signature` provides you can explicitly specify your deriving strategy.
> 
>> On Fri, Aug 30, 2019 at 9:16 PM Alexis King <lexi.lambda at gmail.com> wrote:
>> Hi all,
>> 
>> In recent years, the DefaultSignatures extension has seen popular use as a mechanism for providing derived typeclass instances via GHC.Generics. Although undeniably useful, I have always felt it is somewhat ugly: it is mutually exclusive with other, non-Generic default method implementations, and it can only be used for one Generic deriving mechanism, so implementations must choose between GHC.Generics and Data.Data.
>> 
>> Fortunately, with the advent of DerivingVia, there is a better way: simply attach generic instances to a separate newtype, defined like
>> 
>> newtype Generically a = Generically { unGenerically :: a }
>> 
>> instance Generic a => C (Generically a) where
>>   ...
>> 
>> then derive instances using DerivingVia as follows:
>> 
>> data Foo = Bar X | Baz Y Z
>>   deriving C via Generically Foo
>> 
>> The `Generically` name already exists for this purpose in the `generic-data` package, making it a good candidate name for a newtype in GHC.Generics (`generic-data` could simply re-export the type with suitably recent versions of `base`). An alternate name would be the more traditional `WrappedGeneric`. I don’t have much of a preference either way, but I do think the `Generically` name is cute, especially when used with DerivingVia.
>> 
>> I believe this type should be in `base` because it is
>> 
>> (1) clearly generally useful in the same way that similar newtypes in `base` like `WrappedMonad` are (and probably even more so),
>> 
>> (2) extremely lightweight in terms of additional API complexity (it’s just a newtype),
>> 
>> (3) isn’t worth depending on a separate package for, encouraging a proliferation of (possibly name-conflicting) newtypes in individual packages if it isn’t in `base`, and
>> 
>> (4) is an opportunity to add instances based on `Generic` for classes already in `base`.
>> 
>> Overall, it’s something that would feel right at home in GHC.Generics to me.
>> 
>> As a final note, whichever name people prefer, it would of course make sense to provide an analogous `Generically1` or `WrappedGeneric1` type for `Generic1` (as `generic-data` does as well).
>> 
>> Alexis
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