RFC for a change in EmptyDataDecls instance deriving

Edward Kmett ekmett at gmail.com
Thu Jul 16 12:36:11 UTC 2015

The Proxy instance for Eq and Ord is deliberately lazy for the same sort of

It is, simply, the maximally defined version of the function and, like it
or not, folks with good intentions but bad design sense often still pass
undefineds for Proxy arguments when they are used to older APIs that didn't
take proxies.

The lazy version of each of these when inlined can eliminate whole code
paths that would otherwise be compiled.


On Thu, Jul 16, 2015 at 8:29 AM, Edward Kmett <ekmett at gmail.com> wrote:

> I'd caution against randomly changing Eq and Ord for Void to be less
> defined in the ill-considered name of consistency.
> We rather deliberately made them as "defined as possible" back in 2012
> after a very long discussion in which the pendulum swung the other way
> using a few examples where folks tied knots with fixed points to get
> inhabitants of Void and it was less consistent to rule them out than it was
> to define equality on _|_ to be True.
> I'd challenge that nothing is gained by making these combinators strict in
> their arguments.
> -Edward
> On Thu, Jul 16, 2015 at 7:14 AM, Herbert Valerio Riedel <hvr at gnu.org>
> wrote:
>> On 2015-07-16 at 05:28:03 +0200, David Feuer wrote:
>> > It's all a bit weird. I think the Proxy instance is lazy too. I would
>> tend
>> > to think that empty types shouldn't have these instances, and that if
>> they
>> > do that should be strict (empty case), but I can't prove that's the
>> right
>> > way.
>> Btw, something similiar came up for deepseq, regarding NFData instances
>> for types only inhabited by ⊥ (and the issue of H2010 forbidding
>> instance auto-derivation for constructor-less types was mentioned too):
>>  https://github.com/haskell/deepseq/pull/1#issuecomment-61914093
>> -- hvr
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