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

[Johan Tibell]

Could you elaborate on this? What other existing options are better?

On Thu, Oct 16, 2014 at 3:36 PM, David Feuer <david.feuer at gmail.com> wrote:

> 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
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