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

Answering to convey the 'rules' as I know them, rather than to address the
underlying issues that I guess you are really after...

The S4 practice is to use setOldClass() to explicitly treat an S3
character() vector of classes as an assertion of linear inheritance

x <- structure (sqrt (37), class = c ("sqrt.prime", "numeric") )
is(x, "maybeNumber")

[1] FALSE

setOldClass(class(x))
is(x, "maybeNumber")

[1] TRUE

There are some quite amusing things that can go on with S3 classes, since
the class attribute is just a character vector. So

x <- structure ("September", class = c ("sqrt.prime", "numeric") )
is(x, "numeric")  ## similarly, inherits()

[1] TRUE

x <- structure (1, class = c ("numeric", "character"))
is(x, "numeric")

[1] TRUE

is(x, "character")

[1] TRUE

Perhaps the looseness of the S3 system motivated the use of setOldClass()
for anything more than assertion of simple relationships? At least in this
context setOldClass() provides some type checking sanity

setOldClass(c("character", "numeric"))

Error in setOldClass(c("character", "numeric")) :
  inconsistent old-style class information for "character"; the class is
defined but does not extend "numeric" and is not valid as the data part
In addition: Warning message:
In .validDataPartClass(cl, where, dataPartClass) :
  more than one possible class for the data part: using "numeric" rather
than "character"

Martin Morgan

?On 9/24/20, 4:51 PM, "Abby Spurdle" <spurdle.a at gmail.com> wrote:

    Hi Martin,
    Thankyou for your response.

    I suspect that we're not going to agree on the main point.
    Making it trivially simple (as say Java) to set slots to NULL.
    So, I'll move on to the other points here.

    ***Note that cited text uses excerpts only.***

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

    I think the above construct needs to be documented much more clearly.
    i.e. In the introductory and details pages for S4 classes.
    This is something that many people will want to do.
    And BasicClasses or NULL-class, are not the most obvious place to
    start looking, either.

    Also, I'd recommend the S4 authors, go one step further.
    Include character_OR_NULL, numeric_OR_NULL, etc, or something similar,
    in S4's predefined basic classes.
    Otherwise, contributed packages will (eventually) end up with hundreds
    of copies of these.

    > setClassUnion("maybeNumber", c("numeric", "logical"))
    > every instance of numeric _is_ a maybeNumber, e.g.,

    > > is(1, "maybeNumber")

    > [1] TRUE

    > which I think is consistent with the use of 'superclass'

    Not quite.

        x <- structure (sqrt (37), class = c ("sqrt.prime", "numeric") )
        is (x, "numeric") #TRUE
        is (x, "maybeNumber") #FALSE

    So now, an object x, is a numeric but not a maybeNumber.
    Perhaps a class union should be described as a partial imitation of a
    superclass, for the purpose of making slots more flexible.


    B.