[Rd] Dropping RHS of a formula using NULL assignment

[Blackwell, Matthew]

Hello all,

In attempting to create a one-sided formula from a two-sided formula,
I discovered that the following syntax will successfully complete this
operation:

> f <- y ~ x + z
> f[2] <- NULL
> f
~x + z
> str(f)
Class 'formula'  language ~x + z
  ..- attr(*, ".Environment")=<environment: R_GlobalEnv>

In searching through the formula documentation, I couldn't find this
technique as documented and wondered whether or not it is expected and
if it makes sense to develop a package against the behavior. I'm using
R 4.1.0, but I see the same on R-devel (r81303). I asked on Twitter,
but someone thought this list might be a better venue.

Apologies if I missed some documentation and thanks in advance.

Cheers,
Matt

~~~~~~~~~~
Matthew Blackwell
Associate Professor of Government
Harvard University
https://www.mattblackwell.org