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

[Martin Maechler]

>>>>> 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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