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...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...

...ng, 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 (&quo...

...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") [...

...nderlying 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",...

...d explicitly with metadata objects (hopefully not!) the changes should be largely compatible, but the point of this mail is to alert owners of packages to watch for exceptions. One known incompatibility has to do with setIs relations that extend the basic datatypes. For example, setClass("maybeNumber") setIs("numeric", "maybeNumber") setIs("logical", "maybeNumber") defines a new class and says that "numeric" and "logical" extend it. Once the new code is committed, this will fail, because the definition of "numeric&quot...