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

[Nathan Bouscal]

+1, this is a great idea

On Sat, Aug 31, 2019 at 2:36 AM David Feuer <david.feuer at gmail.com> wrote:

> It sounds like we can probably agree not to include the instance for now.
> But I don't want to just go with my gut feeling, so here's a concrete
> reason:
>
> Generically isn't *just* a DerivingVia target: it's also a perfectly
> reasonable newtype in its own right. Consider something like
> CoercibleUtils.Newtype [*]. If you try to work `under Generically` with
> your instance, then the type checker won't let you. That's rather sad, I
> think, because it seems like a perfectly sensible thing to do.
>
> [*]
> https://github.com/sjakobi/coercible-utils/blob/072c60837059aaaac47628a1822c01fbb2baed6f/src/CoercibleUtils/Newtype.hs
>
> On Fri, Aug 30, 2019, 8:26 PM Alexis King <lexi.lambda at gmail.com> wrote:
>
>> > On Aug 30, 2019, at 17:26, Alexis King <lexi.lambda at gmail.com> wrote:
>> >
>> > If you want to leave that instance out, that’s fine with me; I highly
>> doubt anyone would miss it. As I said before, it’s totally useless.
>>
>> Actually, I take this back. It’s not totally useless, just unlikely to be
>> useful (and ultimately inessential). It’s theoretically useful if you have
>> some function like
>>
>>      foo :: (Show a, Generic a) => a -> Foo
>>
>> and want to use the generic version of the `Show` instance, but for some
>> reason don’t want to actually define that instance on your type. You can
>> write `foo (Generically x)`, and it will work out. That wouldn’t work if
>> `Generic (Generically a)` were the derived instance.
>>
>> On the other hand, you could always get the same behavior in a more
>> flexible way by just defining a separate newtype and deriving the relevant
>> instances as desired, like this:
>>
>>      newtype GenericallyShowable a = GenericallyShowable a
>>          deriving newtype (Generic)
>>          deriving (Show) via Generically a
>>
>> So the instance isn’t necessary by any means, nor does it seem
>> particularly likely to be useful in practice, but it doesn’t seem
>> impossible that it ever could be. My vote is to keep it; I don’t see the
>> harm.
>>
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