R 是否可以参考默认值的参数列表？

Question

跟进这个question about proper checking of parameters against a list of values，有没有办法在不使用外部常量/变量的情况下引用默认值的参数列表？

例如，在下面，是否可以引用默认值的每个参数列表而不将其设置在函数外部？

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

不是语法，但仍然是语言功能，答案在 base::match.arg 代码和 base::formals 中，另请参阅 function components 以获取描述。

因此，如果确实需要参考默认值的参数列表，而不是解决方法，则可以通过base::formals 来解决。如果您只需要像上面的玩具示例中那样进行简单的参数检查，只需使用完成这项工作的base::match.arg：

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)
}