R Proper checking of supplied parameters against a list of values?

Question

In one comment to the accepted answer on how to \"correctly\" specify optional arguments in R, @LouisMaddox said

missing() is useless when y

Answer 1

If you have a small number of options then use match.arg within the function. See ?match.arg for an example.

If the valid argument is all one letter strings then you will need to another approach such as:

# returns logical 
is_one_letter_string <- function(x) {
     !missing(x) && length(x) == 1 && is.character(x) && x %in% c(letters, LETTERS)
}