[R] as.generic

[ripley@stats.ox.ac.uk]

On Wed, 5 Jun 2002, Roger Koenker wrote:

> I've been writing some matrix-type methods for a new class of sparse matrices
> and for most methods this has been straightforward.  However, there are
> examples, like %*% and chol, that (apparently) R doesn't automatically
> recognize as generic.  What to do in these cases?  At this point, I've
> been writing new generic methods with slightly perturbed names %m% and
> cholesky for example in the expectation that I'll eventually rename them
> when I discover the source of my confusion.  Can someone enlighten me?

chol() is not generic, as you can see from the definition.  It is easy for
a package to define a generic and to capture from base the original as the
default method.  E.g.

chol <- function(x, ...) UseMethod("chol")
chol.default <- get("chol", pos=length(search()))

(There are other ways to specify base, e.g. envir=NULL, but I am not sure
what will still work come namespaces and 1.6.0.)

%*% is a primitive, and so harder to tell if it is generic.  It isn't

> "%*%.foo" <- function(e1, e2) stop("using foo")
> a <- structure(1, class="foo")
> a %*% a

so you need

"%*%.default" <- .Primitive("%*%")
"%*%" <- function(e1, e2) UseMethod("%*%")


Now, other functions in base in R-devel (1.6.0 to be) are sealed and so
will use the original definitions.  That may or may not be what you want.
You can always ask R-core to make functions generic and we usually agree.

All of this is in terms of S3 methods, not the formal methods of package
`methods'.  Another way is to use formal methods and assign a method, when
most of this is automatic: see the Green Book.

Brian

-- 
Brian D. Ripley,                  ripley at stats.ox.ac.uk
Professor of Applied Statistics,  http://www.stats.ox.ac.uk/~ripley/
University of Oxford,             Tel:  +44 1865 272861 (self)
1 South Parks Road,                     +44 1865 272860 (secr)
Oxford OX1 3TG, UK                Fax:  +44 1865 272595

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