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

chessai . chessai1996 at
Sun Sep 1 06:14:56 UTC 2019

Also, default signatures look dreadful in haddocks.

On Sun, Sep 1, 2019, 1:56 AM Dmitrii Kovanikov <kovanikov at> 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> 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
>> _______________________________________________
>> Libraries mailing list
>> Libraries at
> _______________________________________________
> Libraries mailing list
> Libraries at
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <>

More information about the Libraries mailing list