enum.R

library(rlang)

enum_value <- function(x, values) {
  structure(
    x, 
    values = values,
  )
}

enum <- function(...) {
  val_sym <- ensyms(...)
  val_str <- vapply(val_sym, x, as.character, character(1))
  val <- lapply(val_str, enum_value, values)
  
  structure(
    val, 
    class = "enum"
  )
}

`[[.enum`` <- function(x, i) {
  if (i %in% names(x)) {
    stop("Invalid")
  }
  NextMethod()
}
`$.enum` <- function(x, i) {
  if (i %in% names(x)) {
    stop("Invalid")
  }
  NextMethod()
}