R checking a parameter is defined - r

Question

I'm looking for a general practice how to check a parameter was defined in a function.
I came up with these three ideas. Which one is the proper way of doing it?
Unfortunately, the third one is not working. substitute() is working differently in a function and I couldn't figure it out how to use it properly.
file.names <- list(
cov.value <- "cov.rds",
plot.name <- "plot.pdf"
)
test1 <- function(file.names){
is.save <- !missing(file.names)
}
test2 <- function(file.names = NULL) {
is.save <- !is.null(file.names)
}
test3 <- function(file.names = NULL) {
is.save <- exists(as.character(substitute(file.names)))
}

Answer 1

I personally think the second approach with a default value is MUCH easier to use & understand. (And the third approach is just really bad)
...especially when you are writing a wrapper function that needs to pass an argument to it. How to pass a "missing"-value is not obvious!
wraptest1 <- function(n) {
file.names <- if (n > 0) sample(LETTERS, n)
else alist(a=)[[1]] # Hacky way of assigning 'missing'-value
print(test1(file.names))
}
wraptest1(2) # TRUE
wraptest1(0) # FALSE
wraptest2 <- function(n) {
file.names <- if (n > 0) sample(LETTERS, n)
else NULL # Much easier to read & understand
print(test2(file.names))
}
wraptest2(2) # TRUE
wraptest2(0) # FALSE
[granted, there are other ways to work around passing the missing value, but the point is that using a default value is much easier...]
Some default values to consider are NULL, NA, numeric(0), ''

Answer 2

It's generally a good idea to look at code from experienced coders---and R itself has plenty of examples in the R sources.
I have seen both your first and second example being used. The first one is pretty idiomatic; I personally still use the second more often by force of habit. The third one I find too obscure.