[Rd] Dropping RHS of a formula using NULL assignment

[Prof Brian Ripley]

On 14/12/2021 20:26, Blackwell, Matthew wrote:
> 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.

See ?"~", which says

      A formula has mode ‘call’.  It can be subsetted by ‘[[’: the
      components are ‘~’, the left-hand side (if present) and the
      right-hand side _in that order_.

That would suggest that

f <- y ~ x + z
f[[2]] <- NULL

was the documented way (and the one I would have used).   However, ?"[" says

      ‘[’ and ‘[[’ are sometimes applied to other recursive objects such
      as calls and expressions.  Pairlists are coerced to lists for
      extraction by ‘[’, but all three operators can be used for
      replacement.


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

-- 
Brian D. Ripley,                  ripley using stats.ox.ac.uk
Emeritus Professor of Applied Statistics, University of Oxford