Alternative to current evalPopArg

[WetRobot]

...is dividing the large function into methods for new generic evalArg:

evalArg <- function(arg, ...) {
  UseMethod("evalArg")
}


evalArg.default <- function(arg, env, enc) {
  if (is.list(arg)) arg <- as.list(arg)
  return(arg)
  stop("No method found for object having class(es) ", 
       paste0(class(arg), collapse = ", "), " for generic evalArg.")
}

evalArg.name <- function(arg, env, enc) {
  d <- deparse(arg)
  out <- try(get(d, envir = env, inherits = FALSE), silent = TRUE)
  if (inherits(out, "try-error")) {
    out <- try(get(d, envir = enc, inherits = FALSE), silent = TRUE)
  }
  if (inherits(out, "try-error")) {
    stop("Could not find object ", d, ".")
  }
  out <- list(out)
  names(out) <- d
  out
}

evalArg.call <- function(arg, env, enc) {

  out <- eval(arg, env = env, enc = enc)
  if (is.list(out)) {
    out <- as.list(out)
  } else if (mode(out) %in% c("numeric", "character") || is.vector(out)) {
    out <- list(out)
    names(out) <- paste0(deparse(arg), collapse = "")
  }
  out
}

evalArg.character <- function(arg, env) {
  se <- substitute(env)
  out <- try({
    get(arg, envir = as.environment(env), inherits = FALSE)
  }, silent = TRUE)
  if (inherits(out, "try-error")) {
    stop("Could not find object ", arg, ".")
  }
  out <- list(out)
  names(out) <- arg
  out
}

evalArg.formula <- function(arg, env) {


  rhsc <- attr(terms(f), "term.labels")

  rhsl <- lapply(rhsc, function(stri) parse(text = stri)[[1]])

  names(rhsl) <- rhsc

  rhsl <- lapply(rhsl, eval, env = env, enc = attr(arg, ".Environment"))

  ## TODO: handle : operator as if x and y are both factors

  rhsl

}