On Fri, 17 Jan 2003, John Chambers wrote:
> There are two main known differences from having methods attached:
>
> - the definition of class() changes, in particular no object ever has a
> NULL class. If you have code that depends on test such as
> `if(is.null(class(x)))...', there may be problems.
>
> Usually code with those sort of tests is doing a workaround of the fact
> that not all objects had a class before. The best solution is usually
> to ask what the code really wants to do. If you do have to retain
> exactly the old behavior, one solution is to copy the version of class
> and class<- from the base package and use those (as baseClass and
> baseClass<-, e.g.) instead of class and class<-).
Here is one example, which makes the MASS scripts fail.
> library(MASS)
> corresp
function (x, ...)
{
if (is.null(class(x)))
class(x) <- data.class(x)
UseMethod("corresp", x, ...)
}
That used to work with matrices, and dispatch to corresp.matrix.
Now a matrix has reported class "matrix" but dispatch occurs to
corresp.default. The temporary fix is to remove the is.null condition.
That's quite a common construction, and I think I should expect UseMethod
to dispatch on the class class() reports. So it looks to me as if
UseMethod needs to be altered to do so.
Brian
--
Brian D. Ripley, [EMAIL PROTECTED]
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 272866 (PA)
Oxford OX1 3TG, UK Fax: +44 1865 272595
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