[Rd] External special functions (SPECIALSXP)

[William Dunlap]

> f <- function(...) {
+     dotArgList <- substitute(list(...))
+     dotArgList
+ }
> f(cat("foo\n"), stop("Oops"), warning("Hmm"))
list(cat("foo\n"), stop("Oops"), warning("Hmm"))
> # i.e., no argument was evaluated

Bill Dunlap
Spotfire, TIBCO Software
wdunlap tibco.com  

> -----Original Message-----
> From: r-devel-bounces at r-project.org 
> [mailto:r-devel-bounces at r-project.org] On Behalf Of Peter Danenberg
> Sent: Wednesday, May 25, 2011 11:06 AM
> To: Duncan Murdoch
> Cc: r-devel at r-project.org
> Subject: Re: [Rd] External special functions (SPECIALSXP)
> 
> > However, if you don't want to evaluate the arguments, just pass
> > substitute(arg) to your function instead of arg.
> 
> Thanks, Duncan; the problem is, I'm trying to substitute on `...' and
> I don't think I can access `...' without inadvertently evaluating it.
> 
> I'm trying to write a debugging function which, given any number of
> expressions, prints the expression next to its evaluation.
> 
> This is trivial to do in the single arity case:
> 
>   debug <- function(expression) {
>     cat(substitute(expression), expression, "\n")
>   }
> 
>   a <- 2
>   debug(a)
> 
>   a 2
> 
> but I'm not sure how to make it work with variable arity without
> resorting to SPECIALSXP.
> 
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