Could we promote unlifted tuples?

[David Feuer]

At present, currying and uncurrying at the type level doesn't seem to
work terribly well. In particular, the kinds

(a, b) -> c

and

a -> b -> c

aren't really isomorphic, because (a, b) can be stuck. This makes some
things (like expressing Atkey-style indexed functors in terms of
McBride-style ones) rather awkward, and difficult or impossible to
really get quite right. The thought came to me that maybe we could
allow unlifted tuples to be promoted. Then something of the kind (# a,
b #) would unconditionally unify with '(# a, b #). The restrictions on
how values of unlifted tuple types can be used would presumably
translate directly to restrictions on how types of unlifted tuple kind
can be used.

David Feuer