[Rd] stopifnot() does not stop at first non-TRUE argument

[William Dunlap]

While you are fiddling with stopifnot(), please consider changing the form
of the error thrown so that it includes the caller's call.  The change
would be from something like
  stop( <<the message>> )
to
  stop(simpleError( <<the message>>, sys.call(-1)))

For the following code
  f <- function(x, y) {
      stopifnot(x > y)
      x - y
  }
  g <- function(x, y, z) {
      c(f(x, y), f(y, z))
  }
  g(6,3,4)
you would see
  Error in f(y, z) : x > y is not TRUE
instead of the less informative
  Error: x > y is not TRUE



Bill Dunlap
TIBCO Software
wdunlap tibco.com

On Fri, May 19, 2017 at 5:31 AM, Martin Maechler <maechler at stat.math.ethz.ch
> wrote:

> >>>>> Suharto Anggono Suharto Anggono via R-devel <r-devel at 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
>     > <maechler at stat.math.ethz.ch> wrote:
>
>     >  Subject: Re: [Rd] stopifnot() does not stop at first
>     > non-TRUE argument
>
>     >  Cc: r-devel at 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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>
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