[R] Retrieve ... argument values

[Richard A. O'Keefe]

Ben Bolker <bolker at zoo.ufl.edu> wrote:
	  Yes, although this becomes tedious if (e.g.) you have a function that 
	calls two different functions, each of which has many arguments (e.g. 
	plot() and barplot(); then you have to set up a whole lot of arguments 
	that default to NULL and, more annoyingly, you have to document them all 
	in any .Rd file you create -- rather than just having a ... argument which 
	you can say should contain arguments for either of the subfunctions (as 
	long as the arguments don't overlap, of course)
	
Could I suggest a very simple alternative, which extends gracefully to
any number of arguments, and doesn't require you to know anything much
about ... except that you can pass it to another function?

> f <- function(...) {
+     has.ylim <- function(..., ylim) return(!missing(ylim))
+   
+     if (has.ylim(...)) "yes" else "no"
+ } 
> f(xlim=1)
[1] "no"
> f(ylim=2)
[1] "yes"

For each parameter XXX that you want to test the presence of, write
    has.XXX <- function(..., XXX) return(!missing(XXX))
and then use
    has.XXX(...)
in the body of the main function.