R: Capture prespecified arguments of a function call

Question

I need to capture the prespecified arguments of a function call. This seems easy for the unspecified ones via rlang::list2(...), but more difficult for the specified ones ("specified" meaning defined in the formals of the function).

I have experimented with base::match.call and the functions from https://rlang.r-lib.org/reference/index.html#section-calls, but haven't yet been successful.

library(rlang)

capture_dots <- function(..., arg1) {
  list2(...)
}

capture_dots("abc", arg1 = 1, arg2 = 2)
#> [[1]]
#> [1] "abc"
#> 
#> $arg2
#> [1] 2

Desired output

capture_prespecified("abc", arg1 = 1, arg2 = 2)
#> $arg1
#> [1] 1

Answer 1

If you want to capture all the named parameter to your function, you can use formals() to get their names (and just filter out the "...")

capture_dots <- function(..., arg1) {
  args <- Filter(function(x) x!="...", names(formals()))
  as.list(environment())[args]
}

capture_dots("abc", arg1 = 1, arg2 = 2)

Or, if you run it early in the function, the dots wont expand and there won't be any other variables so you can just grab the current environment with

capture_dots <- function(..., arg1) {
  as.list(environment())
}