[R] Extending Matrix class

[Martin Maechler]

This is from a private question which I'm given permission to
answer in public:

>>>>> "IF" == Ingo Feinerer <h0125130 at wu-wien.ac.at>
>>>>>     on Fri, 13 Jul 2007 16:14:07 +0200 writes:

    IF> Hello, We tried to derive a class from Matrix but had
    IF> some problems. Maybe you can help us:

    library("Matrix")
    m <- Matrix(c(1:3,rep(0,12),16:20), nrow = 4, ncol = 5)
    setClass("TermDocMatrix", representation(Weighting = "character"),
            contains = ("Matrix"))

    IF> Now we want to do something like:

    IF> new("TermDocMatrix", .Data = m, weighting = "foobar")

    IF> which obviously does not work due to the missing .Data
    IF> slot.  

yes, obviously, indeed.  There is never any .Data slot  in our
matrices. 

    IF>  Note that we do not know in advance what the
    IF> matrix "m" actually is (we only know it is *some*
    IF> Matrix, e.g., we do not know if it is a dgCMatrix or a
    IF> lgCMatrix or ...).  Is there a (simple) solution?

Well, yes, but probably not the one you had wanted:

setClass("TD_Matrix",
         representation(data = "Matrix", Weighting = "character"))

A <- spMatrix(10,20, i = c(1,3:8),
              j = c(2,9,6:10),
              x = 7 * (1:7))
tdr <- new("TD_Matrix", data = A, Weighting = "foobar")
tdr

----------------

Now I understand that and why you had wanted to do this the
original way you did - which cannot work AFAICS.
OTOH, I wonder if other useRs, particularly those who know about
S4 classes (and the "Matrix" classes), have better proposals
maybe along the following "dream" ..

I think what Ingo would want is to say:
let me  extend the full Matrix class hierarchy (or just the
"dsparseMatrix" sub hierarchy) by a new slot 'Weighting'
and hence by default inherit all methods which I don't
explicitly set myself.
I think this can only partially work, even for a future version
of R, since the inherited methods don't know what to do with
"Weighting", but it would already be interesting if all
necessary new methods could be defined semi-automatically:
 1) call the corresponding Matrix method
 2) pass on all my extra slots

Regards,
Martin