Proposal: Bring the Contravariant class to base

[Daniel Díaz Casanueva]

Dear haskellers,

I admit I might not have the strongest arguments here, but I thought that I
would share my opinion anyway, and maybe get other people's perspectives. I
would like to propose bringing the Contravariant class [1] to base.

I know, base keeps growing, and maybe there is no need for it to grow
further without a strong argument, but I do feel like Contravariant is a
simple, very basic class, that would receive better and greater use if
included in base. Contravariant is very similar to Functor (some people
call it CoFunctor), but in `contramap` (Contravariant's `fmap`) the "arrow"
of the applied function goes in the opposite direction. I think that
`contramap` can be useful for many types, just like `fmap` is for many
others, but we don't use it because it's not yet so popular, or maybe
because it requires the contravariant package to be included as dependency
(although personally I don't think that is a real problem). The
contravariant package itself provides a plentiful of instances, many of
them for types in base.

In a real world scenario I had, it was very useful to add a Contravariant
instance to `Data.Aeson.ToJSONKeyFunction`, that perhaps is not included in
aeson because either it was not desired to add the contravariant package as
dependency, or simply because Contravariant is not so well-known. Note
that, however, `FromJSONKeyFunction` _is_ instance of Functor. Even though
both instances are equally natural and useful in this context, only one of
them was implemented. This probably would not have happened if
Contravariant was in base.

So, in my opinion, for the sake of completeness, I think we should add
Contravariant to base, just as we have Functor. Note that my proposal does
not necessarily include the rest of types and functions defined in the
contravariant package.

Best regards,
Daniel Casanueva

# References

[1]
http://hackage.haskell.org/package/contravariant-1.4/docs/Data-Functor-Contravariant.html#t:Contravariant
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