RFC for a change in EmptyDataDecls instance deriving

[Erik Hesselink]

Well, things made *im*possible: you can't do silly things like use
Voids as keys in a map, or look for Voids in a list, as that will
expose the Voids for the lies that they are. But you're right, I doubt
this is a very common use case. I guess it just feels wrong to me to
consider 'let x = x in x' be equal to 'error "boo"'. It also fits with
all other functions on Void, which, when receiving an argument, use
something like 'absurd' on it to produce their answer. Which leads to
another oddity: given 'v :: Void', if 'v == v = True', what about
something like 'absurd v :: Int == absurd v'? That would be bottom,
which again, seems weird to me.

Note that I have no great arguments and no intention of breaking
anyone's code, quite the opposite. So I'd be very interested to see
if/how defining the Void Eq instance to return True can be benificial.

Erik

On Thu, Jul 16, 2015 at 6:33 PM, Edward Kmett <ekmett at gmail.com> wrote:
> Do you have an example of a thing made possible by making them more strict?
> =)
>
> -Edward
>
> -Edward
>
> On Thu, Jul 16, 2015 at 9:10 AM, Erik Hesselink <hesselink at gmail.com> wrote:
>>
>> Do you have an example of things made possible by the version returning
>> True?
>>
>> Erik
>>
>> On Thu, Jul 16, 2015 at 2:29 PM, 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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>
>