Add dual of vacuous

[treeowl]

There's an obvious

whatever :: Contravariant f => f a -> f Void
whatever = contramap absurd

and I believe this should work as well:

unsafeWhatever :: Contravariant f => f a -> f Void
unsafeWhatever = unsafeCoerce

That way, unsafeVacuous . unsafeWhatever will turn an f a into an f b for free if f is both a Functor and Contravariant.

[treeowl]

If you tell me what to call it and where to put it, I'll make a pull request.

[ekmett]

unsafeVacuous . unsafeWhatever is now phantom, provided by this package.

contramap absurd still doesn't have a name though.

[ekmett]

Given the lack of a good name for it, and the lack of a better than direct definition definition for it, I think that having phantom should be enough.