[Rd] symbols: xlim and ylim cannot be specified (PR#639)

[Martin Maechler]

[now I'm not CC'ing R-bugs, since this is not about the bug anymore..]
     
>>>>> "BDR" == Prof Brian D Ripley <ripley@stats.ox.ac.uk> writes:

    BDR> On Tue, 22 Aug 2000 maechler@stat.math.ethz.ch wrote:

    .............

    >> yes,
    >> and Uwe's fix cures this problem.
    >> and I have committed it for the release patches (!! i.e. "1.1.1 patched")

which has
		ylab = "", xlim = NULL, ylim = NULL, ...)
and later
		if(is.null(xlim)) { 
		   ... 
		}

    BDR> Why is using NULL preferable here?  The standard paradigm as I
    BDR> understand it is

    BDR> ylab = "", xlim, ylim, ...)
    BDR> ...
    BDR> if(missing(xlim)) {
    BDR>   xlim <- range(x, na.rm = TRUE)
    BDR>   xlim <- xlim + c(-1, 1) * (0.1 * diff(xlim))
    BDR> }

    BDR> In S there is a problem with passing down missingness, but not in R.

I know.  Note however that to be really useful one would have to able to
also *set* missingness for an argument (which I think we can't currently)

I might want to call

	symbols(.very.long.argument.list.etc.etc., 
                xlim = if(<something>) my.xlim else <<DEFAULT>>,
		...)

where with the NULL setup, I can even drop the else clause
{since  if(FALSE)  evaluates to NULL},
but with the   missing(xlim)  paradigm, I should be able to set <<DEFAULT>>
to missing...

In short, we (Uwe, people here and me) *do* prefer NULL to missing...

Martin
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