Testing a function that uses enquo() for a NULL parameter

Question

I have a function which creates dataframe, but changes names in the process. I am trying to handle empty column names with dplyr quosures. My test suite looks like this:

Answer 1

This problem doesn't really have to do with quo and enquo returning different things, it's really about evaluating objects before you really want to. If you were to use the browser() to step through your function, you'd see the error occurs at the if (is.null(new_var_name)) statement.

When you do is.null(new_var_name), you are evaluating the variable passed as new_var_name so it's too late to enquo it. That's because is.null needs to look at the value of the variable rather than just the variable name itself.

A function that does not evaluate the parameter passed to the function but checks to see if it is there is missing().

my_fun <- function(df, col_name, new_var_name=NULL) {
  target <- enquo(col_name)

  c <- df %>% pull(!!target) * 3 # here may be more complex calculations

  # handling NULL name
  if (missing(new_var_name)) {
    new_name <- "default_name"
  } else{
    new_name <- quo_name(enquo(new_var_name))
  }

  data_frame(
    abc = df %>% pull(!!target),
    !!new_name := c
  )
}

Then you can run both of these

my_fun(dataframe, a)
my_fun(dataframe, a, NEW_NAME)