>>>>> Suharto Anggono Suharto Anggono via R-devel <r-devel@r-project.org>
>>>>> on Thu, 18 May 2017 16:27:09 +0000 writes:
>> From an example in
>>
http://www.uni-muenster.de/ZIV.BennoSueselbeck/s-html/helpfiles/nargs.html
>> , number of arguments in '...' can be obtained by
> (function(...)nargs())(...) .
neat and good. Though really is not exactly "well readable".
In the mean time, there is ...length() in R-devel [somewhat experimentally]
> I now realize that sys.call() doesn't expand '...' when
> the function is called with '...'. It just returns the call as is.
yes.
> If 'stopifnot' uses sys.call() instead of
> match.call() , the following example behaves improperly:
> g <- function(...) stopifnot(...)
> g(TRUE, FALSE)
Indeed. Very improperly (it does not stop).
However, calling stopifnot() with a '...' passed from above is
not a very good idea anyway, because stopifnot has to assume it
is called with explicit expressions.
Hence we have
> g <- function(...) stopifnot(...) ; g(1 == 1, 3 < 1)
Error: ..2 is not TRUE
{and to "fix" this, e.g., with an extra optional argument} would
lead to more complications which I really think we do not want}.
But the example does show we should keep match.call().
Martin
> --------------------------------------------
> On Thu, 18/5/17, Martin Maechler
> <maech...@stat.math.ethz.ch> wrote:
> Subject: Re: [Rd] stopifnot() does not stop at first
> non-TRUE argument
> Cc: r-devel@r-project.org Date: Thursday, 18 May, 2017,
> 3:03 PM
>>>>> Suharto Anggono Suharto Anggono via R-devel <r-devel at r-project.org>
>>>>> on Tue, 16 May 2017 16:37:45 +0000 writes:
>> switch(i, ...) extracts 'i'-th argument in '...'. It is
>> like eval(as.name(paste0("..", i))) .
> Yes, that's neat.
> It is only almost the same: in the case of illegal 'i' the
> switch() version returns invisible(NULL)
> whereas the version we'd want should signal an error,
> typically the same error message as
>> t2 <- function(...) ..2 t2(1)
> Error in t2(1) (from #1) : the ... list does not contain
> 2 elements
>>
>> Just mentioning other things: - For 'n', n <- nargs() can
>> be used.
> I know .. [in this case, where '...' is the only formal
> argument of the function]
>> - sys.call() can be used in place of match.call() .
> Hmm... in many cases, yes.... notably, as we do *not* want
> the argument names here, I think you are right.
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