Void type in base

[Michael Snoyman]

I'd recommend including this code in a different module name in base, as
that way there won't be any module name conflicts with the existing void
package. It will hopefully make it a bit easier to have this transition.

On Wednesday, July 17, 2013, Shachaf Ben-Kiki wrote:

> It seems strange that there's a canonical unit type -- () -- which is
> used extensively, but no canonical type for its dual, the uninhabited
> type. The closest we have is in Edward Kmett's void package, but
> several people seem to prefer to write it themselves rather than incur
> an extra dependency, which is a shame.
>
> The proposal is just to copy the Data.Void API into base:
>
>     data Void -- EmptyDataDecls is in Haskell 2010
>     absurd :: Void -> a
>     vacuous :: Functor f => f Void -> f a
>     -- instances for Typeable, Data, Generic, Eq, Ord, Show, Read, Ix,
> Exception
>
> (void also has a vacuousM for Monad, but since Functor will probably
> become a superclass, it might not be necessary to export it... By
> default it probably makes sense to include it, though.)
>
> void could then re-export Data.Void so packages that depend on it will
> keep working.
>
> ("void" also has unsafe functions -- namely unsafeVacuous, which isn't
> safe with an invalid Functor instance -- but those probably don't
> belong in base.)
>
> hashable and semigroups would also need to be updated to add their
> respective instances as non-orphans.
>
> The instances should probably be made compatible with void's. In
> particular note that void's Eq instance has `_ == _ = True`.
>
> Discussion period: 2 weeks.
>
>     Shachaf
>
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