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

[chessai .]

Been wanting this for a while now

On Fri, Aug 30, 2019, 4:08 PM chessai . <chessai1996 at gmail.com> wrote:

> +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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>>>
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