[Haskell] Closed Projections on HLists?

[Jared Warren]

> If the inference is unique, then of course, we _can_ infer that the
> empty list can only be projected onto the empty list. However any
> non-empty list can be projected in several ways. How do we infer this
> ambiguous result? Either we have to specify which projection result we
> want, and the typechecker will tell us if that is possible. Or we
> obtain the powerset of a list -- and then enumerate it.

But is there no way things could be changed so we can write (to use an
example without projection to HNil):

*> hProject (hProject (hCons hZero hNil)) :: HCons HZero HNil