How to check if any arguments were passed via "..." (ellipsis) in R? Is missing(...) valid?

Question

I'd like to check whether an R function's "..." (ellipsis) parameter has been fed with some values/arguments.

Currently I'm using something like:

test1 <- function(...) {
   if (missing(...)) TRUE
   else FALSE
}

test1()
## [1] TRUE
test1(something)
## [2] FALSE

It works, but ?missing doesn't indicate if that way is proper/valid.

If the above is not correct, what is THE way to do so? Or maybe there are other, faster ways? PS. I need this kind of verification for this issue.

Answer 1

Here's an alternative that will throw an error if you try to pass in a non-existent object.

test2 <- function(...) if(length(list(...))) FALSE else TRUE

test2()
#[1] TRUE
test2(something)
#Error in test2(something) : object 'something' not found
test2(1)
#[1] FALSE

Answer 2

I think match.call is what you need:

test <- function(...) {match.call(expand.dots = FALSE)}

> test()
test()

> test(x=3,y=2,z=5)
test(... = list(x = 3, y = 2, z = 5))

It will give you every time the values passed in the ellipsis, or it will be blank if you won't pass any.

Hope that helps!