[R] Pass-through of unevaluated arguments

[fharrell@virginia.edu]

Thanks to Deepayan Sarkar (lattice) and Paul Murrell (grid)
I have nearly finished porting the Hmisc library's xYplot
function to R.  xYplot extends xyplot to allow for error bars,
bands, and a few other features.  In S-Plus I extended the
Trellis function xyplot mainly by writing panel.xYplot.
In R, xyplot contains a lot of code, so I implemented xYplot
by calling xyplot from it.  Things are working quite well but
I would appreciate advice on programming the most elegant
solution to passing unevaluated arguments.

Here's the situation.  xYplot needs to fiddle with
arguments formula and data (which I default to 
sys.frame(sys.parent()) which I believe will pass through
to xyplot OK).  There are several arguments passed
through to xyplot which were the "..." arguments to xYplot.
I want to defer evaluation of these to inside xyplot where
it will evaluate the arguments with respect to the
"data" argument.  A good example is the groups= variable:

xYplot(formula, data=d, groups=g)

where g might be in d or in the frame that invoked
xYplot.  Using for example do.call("xyplot", list(...))
from within xYplot will result in "g not defined".
I can do something like
 
 if(!missing(groups)) groups <- eval(substitute(groups),data)

inside xYplot but I feel there is a more elegant approach
where I would not even have to isolate groups as a separate
argument, that experienced R programmers already know.  I
would appreciate some ideas.

Thanks  -Frank 
-- 
Frank E Harrell Jr              Prof. of Biostatistics & Statistics
Div. of Biostatistics & Epidem. Dept. of Health Evaluation Sciences
U. Virginia School of Medicine  http://hesweb1.med.virginia.edu/biostat


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