Missing argument vs. empty argument

Hi,

is there an easy, robust, and/or recommended way to distinguish a missing 
argument from an empty argument as in:
An empty argument is a missing argument when argument matching is 
done, e.g.
foo <- function(i,j) match.call()
foo(i)
foo(i = i)
foo(i,)
foo(i = i)
foo(,j)
foo(j = j)

It is rather against the spirit of R to use the actual call rather 
than the matched call.  Unless you are doing this to write a '[' 
method I would suggest you find a different convention, e.g. 
distinguish f(i) and f(i, NULL).  For the exception, look at 
`[.data.frame`, which does use nargs().

(NB: what I have said does not apply to primitives like '[' itself, 
which do not do standard argument matching.)
foo <- function(i, j){
   print(missing(j))
   print(nargs())
}

foo(i)  # TRUE, 1
foo(i,) # TRUE, 2

I know I can work around with nargs, the list of arguments and the names of 
the passed arguments, but I wish there is something already in place for 
this.
This is specially important for '['-like methods where x[i,] is not the same 
as x[i].
What I am looking for is a function that tells me if an argument has actually 
been passed empty:

foo <- function(i, j, k){
   print( empty.arg(j) )
   print(nargs())
}

would result in:

foo(i) # FALSE, 1
foo(i, ) # TRUE, 2
foo(i, j) # FALSE, 2
foo(i, k=2) # FALSE, 2
foo(i, k=2, ) # TRUE, 3

Thank you for any help or pointer.

Bests,
Renaud

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Brian D. Ripley,                  ripley at stats.ox.ac.uk
Professor of Applied Statistics,  http://www.stats.ox.ac.uk/~ripley/
University of Oxford,             Tel:  +44 1865 272861 (self)
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Missing argument vs. empty argument

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