[Rd] Dropping RHS of a formula using NULL assignment

Blackwell, Matthew mb|@ckwe|| @end|ng |rom gov@h@rv@rd@edu
Tue Dec 14 21:26:39 CET 2021

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

> 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.


Matthew Blackwell
Associate Professor of Government
Harvard University

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