Proposal: Make `NFData` a fundamental/primitive type-class (by making `deepseq` a GHC-boot package)

[Herbert Valerio Riedel]

Proposal:
~~~~~~~~~

Make `NFData` a fundamental/primitive type-class
(by making `deepseq` a GHC-boot package)

Discussion deadline: 2 weeks.

Note: This is a reboot of a previous unresolved proposal[1] by Johan Tibell
with a slightly different emphasis.

Current Issues:
~~~~~~~~~~~~~~~

 - The `deepseq` package should not depend on `containers` and `array`
   (ideally `deepseq` would be merged with `base` and
   `containers`/`array` would define their own `NFData` instances)

 - Not all data types defined by "core libraries" have `NFData` instances
   defined (e.g. `time` and `bytestring`). The lack of such "standard"
   instances causes users of `deepseq` to define local orphan instances

 - GHC boot packages can't depend on `deepseq` as it is not a GHC-boot 
   package yet, and thus orphan instances are required for those
   packages -- which would benefit in terms of maintainability[4] and 
   efficiency[3] from being defined locally by having access to the
   internal representations

Rationale:
~~~~~~~~~~

The principal idea this proposal is based upon is that the `NFData`
type-class is similarly universal/basic/fundamental as the `Ord`,
`Show`, or `Eq` type-classes are -- it might be even desirable to make
`NFData` part of a future Haskell Prelude at some point and maybe even
allow automatic instance derivation.


Building on this idea that `NFData` is a "primitive" type-class, the
next best thing that can be done relatively easy and short-term as a
transitional step is to

 1) make `deepseq` a GHC-boot package[2],

 2) remove `deepseq`'s dependancy on `array` and `containers`,

 3) move the affected instance definitions to the `array` and
    `containers` packages respectively, and

 4) satisfy the PVP by incrementing package versions where necessary.

This should provide a path of least resistance, as no packages
depending on `deepseq` would have to be adapted except for maybe
PVP-related version constraint widening.


Moreover, this would pave the way for a future proposal to merge the
`deepseq` into `base` at some later point.


References:
~~~~~~~~~~~

 [1]: http://hackage.haskell.org/trac/ghc/ticket/4868
 [2]: http://www.haskell.org/pipermail/libraries/2010-December/015427.html
 [3]: http://www.haskell.org/pipermail/libraries/2010-December/015428.html
 [4]: http://www.haskell.org/pipermail/libraries/2011-January/015488.html


-- hvr