[Rd] as.missing

[Paul Gilbert]

(I'm not sure if this is a request for a feature, or another instance 
where a feature has eluded me for many years.)

Often I have a function which calls other functions, and may often use 
the default arguments to those functions, but needs the capability to 
pass along non-default choices. I usually do this with some variation on

foo <- function(x, foo2Args=NULL or a list(foo2defaults),
                    foo3Args=NULL or a list(foo3defaults))

and then have logic to check for NULL, or use the list in combination 
with do.call.  It is also possible to do this with ..., but it always 
seems a bit dangerous passing all the unnamed arguments along to all the 
functions being called, especially when I always seem to be calling 
functions that have similar arguments (maxit, eps, start, frequency, etc).

It is a situation I have learned to live with, but one of my 
co-maintainers just pointed out to me that there should be a good way to 
do this in R.  Perhaps there is something else I have missed all these 
years?  Is there a way to do this cleanly? It would be nice to have 
something like

foo <- function(x, foo2Args=as.missing(),  foo3Args=as.missing())

then the call to foo2 and foo3 could specify  foo2Args and foo3Args, but 
these would get treated as if they were missing, unless they are given 
other values.

Paul Gilbert
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