The Argument Matcher: A Function for Selecting the Right Arguments {tidyAML}


Steven P. Sanderson II, MPH


February 3, 2023


I am working on finishing up a few things with my new R package {tidyAML} before I release it to CRAN. One of those things is the ability of a user to build a model using a command that might be something like generate_model(). One of the things that is necessary to do is to match the function arguments from the generate_model() to the actual parsnip call.

This is where and argument matcher of sorts may come in handy. I am doing this because it will take one most step of abstraction away, and instead of say calling linear_reg() or mars() or something like that, you can just instead use generate_model() and type in your engine or the parsnip function call there.

Now I am not one hundred percent certain that I’ll actually implement this or not, but the exercise was fun enough that I decided to share it. So let’s get into it.


Here is the current state of the function.

argument_matcher <- function(.f = "linear_reg", .args = list()){
  # TidyEval ----
  fns <- as.character(.f)
  fns_args <- formalArgs(fns)
  fns_args_list <- as.list(fns_args)
  names(fns_args_list) <- fns_args
  arg_list <- .args
  arg_list_names <- unique(names(arg_list))
  l <- list(arg_list, fns_args_list)
  arg_idx <- which(arg_list_names %in% fns_args_list)
  bad_arg_idx <- which(!arg_list_names %in% fns_args_list)
  bad_args <- arg_list[bad_arg_idx]
  bad_arg_names <- unique(names(bad_args))
  final_args <- arg_list[arg_idx]
  # Return ----
  if (length(bad_arg_names > 0)){
      message = paste0("bad arguments passed: ", bad_arg_names),
      use_cli_format = TRUE


When working with R functions, it’s not uncommon to encounter a situation where you need to pass arguments to another function. This can be especially challenging when the arguments are not properly matched. Fortunately, the argument_matcher function provides an elegant solution to this problem.

The argument_matcher function takes two arguments: .f and .args. The .f argument is a string that specifies the name of the function you want to pass arguments to, while the .args argument is a list that contains the arguments you want to pass to the specified function.

The argument_matcher function first uses the formalArgs function to extract the formal arguments of the specified function and store them in fns_args. The names of the formal arguments are then used to create a list, fns_args_list.

Next, the function extracts the names of the arguments in .args and stores them in arg_list_names. It then checks if the names of the arguments in .args match the names of the formal arguments of the specified function, and stores the matching arguments in final_args. Any arguments that don’t match the formal arguments are stored in bad_args, and a warning message is printed indicating that bad arguments were passed.

The final step is to return the final_args list, which contains only the arguments that match the formal arguments of the specified function.

In conclusion, the argument_matcher function is a useful tool for ensuring that arguments are properly matched when passed to another function. Whether you’re working with linear regression models or any other type of function, the argument_matcher function will help you select the right arguments and avoid common errors.


Let’s see a simple example.


  .args = list(
    mode = "regression", 
    engine = "lm",
    cost = 0.5,
    trees = 1, 
    mtry = 1
bad arguments passed: cost
bad arguments passed: trees
bad arguments passed: mtry
[1] "regression"

[1] "lm"