<html><head><meta http-equiv="content-type" content="text/html; charset=utf-8"></head><body dir="auto">Note, that many Generic-derived instances for newtypes are equivalent with GND instance. Generic mechanism is powerful enough to notice newtype special case.<br><div dir="ltr"><br>On 1 Sep 2019, at 8.56, Dmitrii Kovanikov <<a href="mailto:kovanikov@gmail.com">kovanikov@gmail.com</a>> wrote:<br><br></div><blockquote type="cite"><div dir="ltr"><div dir="ltr">+1<br><br>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.<br><br>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.</div><br><div class="gmail_quote"><div dir="ltr" class="gmail_attr">On Fri, Aug 30, 2019 at 9:16 PM Alexis King <<a href="mailto:lexi.lambda@gmail.com">lexi.lambda@gmail.com</a>> wrote:<br></div><blockquote class="gmail_quote" style="margin:0px 0px 0px 0.8ex;border-left:1px solid rgb(204,204,204);padding-left:1ex">Hi all,<br>
<br>
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.<br>
<br>
Fortunately, with the advent of DerivingVia, there is a better way: simply attach generic instances to a separate newtype, defined like<br>
<br>
newtype Generically a = Generically { unGenerically :: a }<br>
<br>
instance Generic a => C (Generically a) where<br>
...<br>
<br>
then derive instances using DerivingVia as follows:<br>
<br>
data Foo = Bar X | Baz Y Z<br>
deriving C via Generically Foo<br>
<br>
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.<br>
<br>
I believe this type should be in `base` because it is<br>
<br>
(1) clearly generally useful in the same way that similar newtypes in `base` like `WrappedMonad` are (and probably even more so),<br>
<br>
(2) extremely lightweight in terms of additional API complexity (it’s just a newtype),<br>
<br>
(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<br>
<br>
(4) is an opportunity to add instances based on `Generic` for classes already in `base`.<br>
<br>
Overall, it’s something that would feel right at home in GHC.Generics to me.<br>
<br>
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).<br>
<br>
Alexis<br>
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</blockquote></div>
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