Rpart and case weights: working with functions

I can't get rpart accept case weights defined inside a function.
It keeps using the copy defined in the "global" environment (if they exists) 
instead of the function-defined ones.

Here is what I do:

test.function <- function (formula, data) {
 weights <- rep(.1, 100)
 rpart(formula, data, weights)
}

test.function(x~y, data)

And I get an error:

Error in model.frame(formula, rownames, variables, varnames, extras, 
extranames,  :         invalid type (closure) for variable '(weights)'

If I define weights in the global environment, it uses this instead of the 
function-defined one (which can be verified by .Last.value$frame$wt[1]: it 
should equal sum(weights) and actually equals the sum of global weights, not 
the function one).

I tried to pass a named argument (weights=weights), using other names (w, 
foo, ...), or both, but still it doesn't work. I found nothing in rpart doc 
about that behaviour...
It was tested on R 2.7.0 (Windows XP) and R 2.5.1 (Linux kernel 2.6.9).

For the moment, the only workaround I found is to use/modify the global 
weights using <<- (not very satisfying).


What did I wrong? And what should I do to get it work (i.e. using the copy of 
weights defined inside the function)?

Thanks in advance for your help.

Xavier Robin

-- 
Xavier Robin

Biomedical Proteomics Research Group (BPRG)
Department of Structural Biology and Bioinformatics (DBSB)
Geneva University Medical Center (CMU)
1, rue Michel Servet - CH-1211 Gen?ve 4 - Switzerland
Tel: (+41 22) 379 53 21