R devel - Nov 2006 - Missing values for S4 slots

Using R 2.4, the following fails:
setClass("testc", representation(a="ANY"))
makeC <- function(myarg) new("testc", a=myarg)
makeC()
-> Error in initialize(value, ...) : argument "myarg" is missing,
   with no default

On the other hand, this is OK:
f <- function(a) g(b=a)
g <- function(b) if(missing(b)) "missing" else "valid
arg"
g()
-> "missing"

By the way, new("testc") succeeds.

Is there a good way around this?  Since my real example has several 4
arguments that might be missing, testing on missingness and calling
"new" with appropriate explicit arguments is not an attractive option.

Passing all the arguments in is also not an option, since my call to
"new" includes an argument not in the constructor (i.e., my equivalent
of makeC). 

I suspect there's something I could do to get the constructor
arguments, modify the list (i.e., delete args that were missing and
insert new ones), and do.call(new, myArgList).  Not only am I unsure
how to do that (or if it would work), I'm hoping there's a better way.

What's the best approach to this problem?

On Fri, Nov 24, 2006 at 11:23:14AM -0800, Ross Boylan
wrote:
> Using R 2.4, the following fails:
> setClass("testc", representation(a="ANY"))
> makeC <- function(myarg) new("testc", a=myarg)
> makeC()
> -> Error in initialize(value, ...) : argument "myarg" is
missing,
>    with no default
....
> 
> I suspect there's something I could do to get the constructor
> arguments, modify the list (i.e., delete args that were missing and
> insert new ones), and do.call(new, myArgList).  Not only am I unsure
> how to do that (or if it would work), I'm hoping there's a better
way.
I didn't find a way to get all the arguments easily(*), but manually
constructing the list works.  Here are fragments of the code:

mspathCoefficients <- function(
                   aMatrix,
                   params,
                   offset=0,
                   baseConstrVec, 
                   covLabels # other args omitted
                   ) {
  # innerArgs are used with do.call("new, innerArgs)
  innerArgs <- list(
                    Class = "mspathCoefficients",
                    aMatrix = aMatrix,
                    baseConstrVec = as.integer(baseConstrVec),
                    params = params
                    )

# the next block inserts the covLabels argument
# only if it is non-missing
  if (missing(covLabels)) {
    # ....
  } else {
    innerArgs$covLabels <- covLabels
  }
#...
  map <- list() 
# fill in the map
# add it to the arguments
  innerArgs$map <- map
  do.call("new", innerArgs)
}

This calls new("mspathCoefficients", ...) with just the non-missing
arguments.  The constructed object has appropriately "missing" values
in the slots (e.g., character(0) given a slot of type "character").


(*) Inside the function, arg <- list(...) will capture the unnamed
arguments, but I don't have any.  as.list(sys.frame(sys.nframe()) is
closer to what I was looking for, though all the values are promises.