R devel - Sep 2020 - Is it possible to simply the use of NULL slots (or at least improve the help files)?

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...

Sorry, the title should be "simplify", and the third paragraph should
say "I can't".
(Don't know how I missed these).

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 at r-project.org on behalf of spurdle.a at 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...

    ______________________________________________
    R-devel at r-project.org mailing list
    https://stat.ethz.ch/mailman/listinfo/r-devel

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.