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