RFC: (in-principle) native unquoting for standard evaluation

On Mon, Mar 20, 2017 at 7:36 AM, Radford Neal <radford at cs.toronto.edu> wrote:

Michael Lawrence (as last in long series of posters)...

Yes, it would bind the language object to the environment, like an
R-level promise (but "promise" of course refers specifically to just
_lazy_ evaluation).

For the uqs() thing, expanding calls like that is somewhat orthogonal
to NSE. It would be nice in general to be able to write something like
mean(x, extra_args...) without resorting to do.call(mean, c(list(x),
extra_args)). If we had that then uqs() would just be the combination
of unquote and expansion, i.e., mean(x, @extra_args...). The "..."
postfix would not work since it's still a valid symbol name, but we
could come up with something.

I've been trying to follow this proposal, though without tracking down
all the tweets, etc. that are referenced.  I suspect I'm not the only
reader who isn't clear exactly what is being proposed.  I think a
detailed, self-contained proposal would be useful.

We have a working implementation (which I'm calling tidyeval) in
https://github.com/hadley/rlang, but we have yet to write it up. We'll
spend some time documenting since it seems to be of broader interest.