Missing values for S4 slots [One Solution]

Ross Boylan <ross at biostat.ucsf.edu> writes:
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
The following might offer one paradigm:

makeC <- function(myarg) {
    new("testc",
        a=if (missing(myarg)) character(0) else myarg)
    }

A couple of things that come to mind, though: 'ANY' kind of defeats
the purpose of having a class with well-defined slots (I realize this
is a 'toy' example); presenting myarg without a default makes it
obscure to the user what the default value might be, or that a default
value will be used; if 'myarg' is an appropriately descriptive
argument for the user, then perhaps it is also appropriately
descriptive for the slot. Thus:

setClass("testc",
         representation(myarg="character"))

makeC <- function(myarg=character(0)) new("testc", myarg=myarg)
....
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
as.list(match.call()[-1]) might help
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.

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Martin T. Morgan
Bioconductor / Computational Biology
http://bioconductor.org

Missing values for S4 slots [One Solution]

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