[Rd] Dropping RHS of a formula using NULL assignment

[Duncan Murdoch]

On 14/12/2021 3:26 p.m., 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.

The source "y ~ x + z" parses to a call to the `~` function with 
arguments y and x + z.  Calls have the function as the first element, 
and arguments follow:  so f[1] would be ~, f[2] would be y, f[3] would 
be x + z.

You can see this if you pass f through as.list():

 > as.list(f)
[[1]]
`~`

[[2]]
y

[[3]]
x + z

Setting element 2 to NULL removes it, so you see

 > f[2] <- NULL
 > as.list(f)
[[1]]
`~`

[[2]]
x + z

I think it's safe to make use of this even if it's undocumented.  It's a 
pretty basic aspect of formulas.  I'd guess there are lots of packages 
already using it, but I can't point to any particular examples.

(I've ignored the difference between an unevaluated formula and an 
evaluated one, but they're almost the same, the only important 
difference in the attributes:  evaluating it gives it a class and an 
environment.)

Duncan Murdoch