[Rd] Is it possible to simply the use of NULL slots (or at least improve the help files)?

[Martin Morgan]

I did ?"NULL<tab> at the command line and was lead to ?"NULL-class" and the BasicClasses help page in the methods package.

getClass("NULL"), getClass("character") show that these objects are unrelated, so a class union is the way to define a class that is the union of these. The essence of the behavior you would like is

  setClassUnion("character_OR_NULL", c("character", "NULL"))
  .A = setClass("A", slots = c(x = "character_OR_NULL"))

with

> .A(x = NULL)
An object of class "A"
Slot "x":
NULL

> .A(x = month.abb)
An object of class "A"
Slot "x":
 [1] "Jan" "Feb" "Mar" "Apr" "May" "Jun" "Jul" "Aug" "Sep" "Oct" "Nov" "Dec"

> .A(x = 1:5)
Error in validObject(.Object) :
  invalid class "A" object: invalid object for slot "x" in class "A": got class "integer", should be or extend class "character_OR_NULL"

I understand there are situations where NULL is desired, perhaps to indicate 'not yet initialized' and distinct from character(0) or NA_character_, but want to mention those often appropriate alternatives.

With

  setClassUnion("maybeNumber", c("numeric", "logical"))

every instance of numeric _is_ a maybeNumber, e.g.,

> is(1, "maybeNumber")
[1] TRUE
> is(1L, "maybeNumber")
[1] TRUE
> is(numeric(), "maybeNumber")
[1] TRUE
> is(NA_integer_, "maybeNumber")
[1] TRUE

which I think is consistent with the use of 'superclass' on the setClassUnion help page.

Martin Morgan
 

On 9/23/20, 5:20 PM, "R-devel on behalf of Abby Spurdle" <r-devel-bounces using r-project.org on behalf of spurdle.a using gmail.com> wrote:

    As far as I can tell, there's no trivial way to set arbitrary S4 slots to NULL.

    Most of the online examples I can find, use setClassUnion and are
    about 10 years old.
    Which, in my opinion, is defective.
    There's nothing "robust" about making something that should be
    trivially simple, really complicated.

    Maybe there is a simpler way, and I just haven't worked it out, yet.
    But either way, could the documentation for the methods package be improved?
    I can find any obvious info on NULL slots:

    Introduction
    Classes
    Classes_Details
    setClass
    slot

    Again, maybe I missed it.
    Even setClassUnion, which is what's used in the online examples,
    doesn't contain a NULL slot example.

    One more thing:
    The help file for setClassUnion, uses the term "superclass", incorrectly.

    Its examples include the following:
    setClassUnion("maybeNumber", c("numeric", "logical"))

    If maybeNumber was the superclass of numeric, then every instance of
    numeric would also be an instance of maybeNumber...

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