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

[chessai .]

+1

On Fri, Aug 30, 2019, 3:33 PM David Feuer <david.feuer at gmail.com> wrote:

> I do have two concerns:
>
> 1. The Genetically instances for base classes will need to move to base as
> well, including auxiliary classes where those are needed.
>
> 2. The Generic instance of Generically is currently a custom job. That's
> really not the greatest situation in general. Is that essential for some
> reason? If not, I think its Generic instance should just be derived.
>
> On Fri, Aug 30, 2019, 2:16 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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