If the proposal made it into base more or less as is, then what I could do
is release a version of void to coincide with it that was conditionally
empty so users of void would feel no change.
e.g. my transformers-compat package uses this technique to make it possible
to use the extra data types added in transformers-0.3 back under
transformers-0.2 (for users who want to span multiple platforms, such as
lens).
-Edward
On Wed, Jul 17, 2013 at 2:34 AM, 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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