R Is it possible to refer to argument list of default values?

Question

Followup to this question about proper checking of parameters against a list of values, is there a way to refer to an argument list of default values without using an external constant/variable?

For example, in the following, is it possible to refer to each argument list of default values without setting it outside the function?

ISO.open_bracket_left <- c("]", ")") # Current workaround
semi_open_intervals <-
  function(a, b, right_half_open=TRUE,
           open_bracket_left= ISO.open_bracket_left,
           open_bracket_right=c("[", "("))
{
  # See https://en.wikipedia.org/wiki/ISO_31-11
  if (missing(open_bracket_left) || !(open_bracket_left %in% ISO.open_bracket_left))
    open_bracket_left <- ISO.open_bracket_left[1]
  if (missing(open_bracket_right) || !(open_bracket_right %in% c("[", "(")))
    open_bracket_right <- "["

  bracket_left <- "["
  bracket_right <- "]"
  if (right_half_open) bracket_right <- open_bracket_right
  else bracket_left <- open_bracket_left
  paste0(bracket_left, a, ", ", b, bracket_right)
}

## Use case
semi_open_intervals(0, 7, right_half_open = FALSE, open_bracket_left = ")") ## ")0, 7]"
semi_open_intervals(0, 7, right_half_open = FALSE, open_bracket_left = "[[") ## "]0, 7]"

Answer 1

Not syntactic but still a language feature, the answer is in base::match.arg code with base::formals, see also function components for a description.

So instead of the workaround, if one really needs to refer to the argument list of default values, the way to do it is through base::formals. If you just need simple argument checking as in the toy example above, just use base::match.arg which does the job:

ISO.open_bracket_left <- c("]", ")") # Current workaround
semi_open_intervals <-
  function(a, b, right_half_open=TRUE,
           open_bracket_left= ISO.open_bracket_left,
           open_bracket_right=c("[", "("))
{
  # See https://en.wikipedia.org/wiki/ISO_31-11
  if (missing(open_bracket_left) || !(open_bracket_left %in% ISO.open_bracket_left))
    open_bracket_left <- ISO.open_bracket_left[1]

  ### Use base, duh!
  open_bracket_right <- match.arg(open_bracket_right)

  bracket_left <- "["
  bracket_right <- "]"
  if (right_half_open) bracket_right <- open_bracket_right
  else bracket_left <- open_bracket_left
  paste0(bracket_left, a, ", ", b, bracket_right)
}