S3 objects in S4 slots

Duncan Murdoch <murdoch at stats.uwo.ca>
    on Tue, 15 Sep 2009 06:26:32 -0400 writes:

    > Martin Kober wrote:

    >> Hello,
    >> 
    >> I am the maintainer of the stringkernels package and have come across
    >> a problem with using S3 objects in my S4 classes.
    >> 
    >> Specifically, I have an S4 class with a slot that takes a text corpus
    >> as a list of character vectors. tm (version 0.5) saves corpora as
    >> lists with a class attribute of c("VCorpus", "Corpus", "list"). I
    >> don't actually need the class-specific attributes, I only care about
    >> the list itself.
    >> 
    >> Here's a simplified example of my problem:
    >> 
    >> 

    >>> setClass("testclass", representation(slot="list"))
    >>> 

    >> [1] "testclass"
    >> 

    >>> a = list(a="1", b="2")
    >>> class(a) = c("VCorpus", "Corpus", "list") # same as corpora in tm v0.5
    >>> x = new("testclass", slot=a)
    >>> 

    >> Error in validObject(.Object) :
    >> invalid class "testclass" object: 1: invalid object for slot "slot"
    >> in class "testclass": got class "VCorpus", should be or extend class
    >> "list"
    >> invalid class "testclass" object: 2: invalid object for slot "slot" in
    >> class "testclass": got class "Corpus", should be or extend class
    >> "list"
    >> invalid class "testclass" object: 3: invalid object for slot "slot" in
    >> class "testclass": got class "list", should be or extend class "list"
    >> 
    >> The last line is a bit confusing here (``got class "list", should be
    >> or extend class "list"''). There's an even more confusing error
    >> message when I try to assign the slot later on:
    >> 
    >> 

    >>> y = new("testclass")
    >>> y at slot = a
    >>> 

    >> Error in checkSlotAssignment(object, name, value) :
    >> c("assignment of an object of class \"VCorpus\" is not valid for
    >> slot \"slot\" in an object of class \"testclass\"; is(value, \"list\")
    >> is not TRUE", "assignment of an object of class \"Corpus\" is not
    >> valid for slot \"slot\" in an object of class \"testclass\"; is(value,
    >> \"list\") is not TRUE", "assignment of an object of class \"list\" is
    >> not valid for slot \"slot\" in an object of class \"testclass\";
    >> is(value, \"list\") is not TRUE")
    >> 
    >> The last part of the message claims that ``is(value, "list") is not
    >> TRUE'', but is(a, "list") is certainly TRUE. (??)
    >> 
    >> On a side note, it does work when "list" is the first entry in class().
    >> 
    >> 
    >> I tried to use setOldClass, but seemingly using list is not possible
    >> because it does not extend oldClass, or I didn't find out how to do
    >> it:
    >> 

    >>> setOldClass(c("VCorpus", "Corpus", "list"))
    >>> 

    >> Error in setOldClass(c("VCorpus", "Corpus", "list")) :
    >> inconsistent old-style class information for "list"; the class is
    >> defined but does not extend "oldClass"
    >> 
    >> 
    >> Intuitively I would have thought that, because the underlying data is
    >> of type list, it would "fit" into an object slot requiring a list,
    >> irrespective of S3 class attributes. The only thing I can think of is
    >> a manual solution removing the class attribute.
    >> 
    >> Is there a way to define lists with S3 class attributes such that they
    >> are accepted as lists in S4 object slots? Or any other ways to solve
    >> this?
    >> 

    > A possible workaround is to store unclass(a) in that slot, rather than 
    > a.  You won't be able to use the VCorpus or Corpus methods on it, but it 
    > sounds as though you don't want to.

    > I would say checkSlotAssignment should be using some variation on 
    > inherits(), rather than checking for an exact class match, but probably 
    > the real message is that you shouldn't mix S3 and S4 systems.

I don't think so.
Recent releases of R have had big improvements (instrumented by
John Chambers) in order to make exactly this (S3 classed objects as
slots of S4 objects) possible !!

    >   Convert  the VCorpus objects to S4 objects, or use S3 objects everywhere.

Well, no, at least not at all according to what I think has been the
intentions of the recent years' changes of making S3 objects
better fit into the S4 frame work.

Martin Maechler