[PROPOSAL] Adding Generics-based DefaultSignature to `deepseq` package

[David Feuer]

I'm generally opposed to DefaultSignatures as an upside-down,
insufficiently-general attempt to solve an important problem, and generally
think the less relies on them the better.
On Oct 16, 2014 6:40 AM, "Herbert Valerio Riedel" <hvr at gnu.org> wrote:

>
> The Proposal
> ============
>
> I hereby propose to merge `deepseq-generics`[2] into `deepseq`[1] in
> order to add Generics support to the `NFData` class based on the
> `-XDeriveGenerics` and `-XDefaultSignature` language extensions.
>
> A concrete patch is available for bike-review at [3]
>
>
> Prior Proposal & What's changed
> ===============================
>
> About 2 years ago, I already proposed something similar[4].  Back then
> the major concern was avoiding a conditionally exported API as using the
> (back then) rather young `Generics` extension would leave the Haskell98
> domain.
>
> This lead to me release Generics support as a companion package[2] which
> turns out to have become a rather popular package (judging from the
> Hackage download-count stats).
>
> I only realized after the discussion was effectively finished, that
> having a separate `deepseq-generics` actually does have an IMO
> non-neglectable downside:
>
>   You can't support a `DefaultSignature`-based default implementation,
> as those need to be backed into the `NFData` class.
>
> Missing out on `DefaultSignature` would be a shame IMO, because
>
>  * There's a chance that starting with GHC 7.10 `deriving` may work for
>    arbitrary classes[5], putting `NFData` on equal footing as built-in
>    classes such as `Eq` or `Show`. Specifically, you would be able to
>    write
>
>       data Foo = Foo [Int] String (Bool,Char) | Bar (Maybe Char)
>                  deriving (Show, Generic, NFData)
>
>    instead of having to manually write the following boilerplate
>
>       instance NFData Foo where
>          rnf (Foo x y z) = rnf x `seq` rnf y `seq` rnf z
>          rnf (Bar x)     = rnf x
>
>    which gets tedious rather soon if you have many (and more complex)
>    types and tend to refactor regularly (with a risk of failing to adapt
>    your manual instances if you change the strictness of fields)
>
>
>  * The current default `rnf` implementation, i.e.
>
>      rnf a = a `seq` ()
>
>    is rather error-prone, as it's *very* easy to end up with an
>    incorrect instance. Especially after refactoring a type for which the
>    NF=WHNF assumption was broken after refactoring by adding new fields,
>    or changing the strictness of existing fields.
>
>    The Generics-derived `rnf` implementation does not have such a
>    problem.
>
>
> Moreover, popular packages are starting adopt (and even recommend) the
> use of Generics in combination with `DefaultSignature` to provide
> automatically derived default instances, most notably `hashable`[6],
> `binary`[7], or `aeson`[8] just to name a few. In addition to providing
> a precedence for the use of Generics, I consider those packages evidence
> for Generics to have proven itself to the point of replacing
> TemplateHaskell in these use-cases.
>
>
> Compatibility & Breakage Considerations
> =======================================
>
>  * This change requires a major version bump to deepseq-1.4.0
>
>  * `deepseq` needs to drop GHC 7.0.* support as GHC 7.2 is the first
>    version to support Generics & `DefaultSignature`.
>
>  * Code relying on the current `rnf` default-implementation will most
>    likely break (unless a `Generics` instance happens to be in-place)
>
>    However, it's easy to provide forward/backward-compatibility w/o any
>    CPP, by simply explicitly defining
>
>      instance NFData XYZ where rnf = seq x ()
>
>
>
> Discussion Period: 2 weeks
>
>
>
>  [1]: http://hackage.haskell.org/package/deepseq
>  [2]: http://hackage.haskell.org/package/deepseq-generics
>  [3]: https://github.com/haskell/deepseq/pull/1
>  [4]: http://thread.gmane.org/gmane.comp.lang.haskell.libraries/17940
>  [5]: https://ghc.haskell.org/trac/ghc/ticket/5462
>  [6]: http://hackage.haskell.org/package/hashable
>  [7]: http://hackage.haskell.org/package/binary
>  [8]: http://hackage.haskell.org/package/aeson
> _______________________________________________
> Libraries mailing list
> Libraries at haskell.org
> http://www.haskell.org/mailman/listinfo/libraries
>
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://www.haskell.org/pipermail/libraries/attachments/20141016/677e0851/attachment.html>