On the mechanics of function evaluation and argument matching

Brian Rowe <rowe <at> muxspace.com> writes:

Thanks for the lead. Given the example in ?missing though,
 wouldn't it be safer to explicitly define a
default value of NULL:

myplot <- function(x, y=NULL) {
  if(is.null(y)) {
    y <- x
    x <- 1:length(y)
  }
  plot(x, y)
}

[snip]

 In my opinion the missing() functionality can indeed be
fragile (for example, I don't know how I can manipulate an
existing call to make an argument be 'missing' when it was
previously 'non-empty') and using an explicit NULL is often
a good idea.  This makes the documentation a tiny bit less
wieldy if you have lots of parameters ...