setClassUnion with numeric; extending class union

Dear list:

I am looking for a good way to create an S4 class that would extend
numeric, but would allow NULL instead of data as well. As far as I can
see there is no way at the moment to do that, but please correct me if I
am wrong. The best solution I came up with so far was the following (it
also indicates a problem of using setClassUnion with numeric as one of
the classes):

I define a class union of numeric and NULL:

Unfortunately the following works only with warnings:
setClassUnion("numericOrNULL", c("numeric","NULL"))

So I do a workaround as:

setClass("aNumeric", contains="numeric")
setClassUnion("numericOrNULL", c("aNumeric","NULL"))

Then I cannot really extend the above virtual class and can only use it
in a user-defined slot as follows:

setClass("myClass", representation(data="numericOrNULL"))
new("myClass", data=runif(20))
new("myClass", data=NULL)

and this works.

Obviously it would be nicer to have something like the following:

setClass("myClass", contains="numericOrNULL")
new("myClass", runif(20)) ## .Data is not a slot of myClass
setClass("myClass", representation("numericOrNULL"))
new("myClass", runif(20)) ## ibid

Technically I understand that the reason behind it failing to work is
that the virtual class numericOrNULL has not got the .Data slot from
numeric, but it would be nice to have such a functionality.

Any ideas about better ways for solving such a problem than the one
described above?

Thanks.

Best,
Oleg

Dr Oleg Sklyar
Research Technologist
AHL / Man Investments Ltd
+44 (0)20 7144 3107
osklyar at maninvestments.com

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