R package documentation: "Undocumented arguments in documentation object" for a function with no arguments

Question

I receive the following warning when trying to check my package: Undocumented arguments in documentation object '%ni%' '...' 'NA'. The function (is it even a function? or is it just an operator?) doesn't take any arguments, but it appears that R is somehow identifying arguments (... and NA) that are not documented.

Here is the document that I use with roxygen2 to create the package documentation:

#' @title
#' NOTIN Operator.
#'
#' @description
#' NOTIN operator.
#'
#' @details
#' Determine whether values in one vector are not in another vector.
#'
#' @return Vector of \code{TRUE} and \code{FALSE}, indicating whether values in
#' one vector are not in another vector.
#'
#' @family operators
#'
#' @export
#'
#' @examples
#' # Prepare Data
#' v1 <- c("Sally","Tom","Barry","Alice")
#' listToCheckAgainst <- c("Tom","Alice")
#'
#' v1 %ni% listToCheckAgainst
#' v1[v1 %ni% listToCheckAgainst]
#'
#' @seealso
#' \url{https://www.r-bloggers.com/2018/07/the-notin-operator/}

`%ni%` <- Negate(`%in%`)

Here is the relevant output I receive when checking the package:

W  checking for code/documentation mismatches (1.9s)
   Codoc mismatches from documentation object '%ni%':
   %ni%
     Code: function(...)
     Docs: function(..., NA)
     Argument names in docs not in code:
       NA
   
W  checking Rd \usage sections (3.7s)
   Undocumented arguments in documentation object '%ni%'
     '...' 'NA'
   
   Functions with \usage entries need to have the appropriate \alias
   entries, and all their arguments documented.
   The \usage entries must correspond to syntactically valid R code.
   See chapter 'Writing R documentation files' in the 'Writing R
   Extensions' manual.

How can I get rid of the warning?

Answer 1

You can see the arguments to %in% and your %ni% operator using

args(`%in%`)
#> function (x, table) 
#> NULL
`%ni%` <- Negate(`%in%`)
args(`%ni%`)
#> function (...) 
#> NULL

^{Created on 2022-03-01 by the reprex package (v2.0.1.9000)}

So it looks like Negate() replaced the %in% arguments with ..., and Roxygen messed up the help page generation. I think you will need to write the Rd file manually, or use an explicit definition of %ni%, or do something else to tell Roxygen how to get the usage right.

EDITED to add:

Another choice is to use a better Negate function that keeps the arguments, for example

Negate2 <- function (f) 
{
    f <- match.fun(f)
    args <- names(formals(f))
    result <- function() !do.call(f, sapply(args, as.name))
    formals(result) <- formals(f)
    result
}

`%ni%` <- Negate2(`%in%`)
args(`%ni%`)
#> function (x, table) 
#> NULL

^{Created on 2022-03-01 by the reprex package (v2.0.1.9000)}

This would require you to document the x and table arguments, but shouldn't cause problems in Roxygen. I think it should evaluate properly, but I haven't done much testing.