R help - Aug 2005 - problem with repeated formal arguments and ...

I want to add an argument if it is not present.  Following the Green
Book, p. 337:
test <- function(x, ...){
  dots <- list(...)
  if (!hasArg(from))
    from <- 0
  else
    from <- dots$from
  curve(x, from=from, ...)
}
> test(sin)
> test(sin, from=4)
Error in curve(x, from = from, ...) : formal argument "from" matched
by
multiple actual arguments

The FAQ says, in the section on differences between R and S,
"R disallows repeated formal arguments in function calls."

That seems a perfectly reasonable rule,  but how do I handle this
situation?

-- 
Ross Boylan                                      wk:  (415) 502-4031
530 Parnassus Avenue (Library) rm 115-4          ross at biostat.ucsf.edu
Dept of Epidemiology and Biostatistics           fax: (415) 476-9856
University of California, San Francisco
San Francisco, CA 94143-0840                     hm:  (415) 550-1062

Ross Boylan wrote:
> I want to add an argument if it is not present.  Following the Green
> Book, p. 337:
> test <- function(x, ...){
>   dots <- list(...)
>   if (!hasArg(from))
>     from <- 0
>   else
>     from <- dots$from
>   curve(x, from=from, ...)
> }
> 
> 
>>test(sin)
>>test(sin, from=4)
> 
> Error in curve(x, from = from, ...) : formal argument "from"
matched by
> multiple actual arguments
> 
> The FAQ says, in the section on differences between R and S,
> "R disallows repeated formal arguments in function calls."
> 
> That seems a perfectly reasonable rule,  but how do I handle this
> situation?
> 
Hi, Ross,

Add "from" to your function:

test <- function(x, from = 0, ...) {
   curve(x, from = from, ...)
}

Or another way:

test <- function(x, ...) {
   dots <- list(...)
   if(!hasArg(from)) dots$from <- 0
   dots$expr <- x
   do.call("curve", dots)
}

I actually prefer the latter if I'm changing many arguments. I do this 
quite often when writing custom lattice plots and I want to override 
many of the defaults.

HTH,

--sundar