The important problem, as Edward Kmett would put it, is that Haskell is not good at dealing with lots of abstractions. In particular, making the typeclass hierarchy too fine-grained makes it painful to work with, because programmers have to satisfy the tower of superclass constraints in order to write an instance for a class. DefaultSignatures addresses this in a very limited way: If

A a => B a => C a => D a

then I may be able to give A, B, and C methods defaults with signatures so that I can declare an instance of D without needing to declare all the superclass instances. Unfortunately, this breaks down as soon as things branch:

A a => B a => C a => D a

||

V

E a => F a => G a

Both E and B may offer perfectly reasonable default definitions of a method in A, but I can only choose *one* of them. It also fails when class A is in someone else's module, and I'm doing a ton of work with subclasses of B and would like very much to add a default definition of a method in A, but simply can't. The current common use of DefaultSignatures is to use it *only* to provide defaults for Generic instances. While this single use-case works reasonably well, it effectively privileges Generic over everything else and leaves the general problem unsolved.

The sort of general solution I'd hope for would probably look something vaguely like this, but I imagine the type gurus might see problems:

Allow a *subclass* of a class to define (and override) default methods for the superclass. There is, of course, an immediate challenge: a single type could be a member of two subclasses, each of which defines a default for the same superclass method. The best solution I can think of to this is to require that such incoherent defaults be resolved manually by giving an explicit superclass instance declaration; ideally, that declaration would be able to access and choose from one of the available defaults, but that might be more trouble than it's worth.

On Thu, Oct 16, 2014 at 10:39 AM, José Pedro Magalhães <dreixel@gmail.com> wrote:

I'd like to know exactly what is the important problem, and how DefaultSignatures are insufficiently general. Perhaps we can improve them, or come up with something better!

On Thu, Oct 16, 2014 at 2:36 PM, David Feuer <david.feuer@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@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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