how to write assignment form of function

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Patrick Burns
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Heinz Tuechler wrote:

Dear All,

where can I find information about how to write an assigment form of a
function?
For curiosity I tried to write a different form of the levels()-function,
since the original method for factor deletes all other attributes of a

Of course, the simple method would be to use instead of levels(x) <-
newlevels, attr(x, 'levels') <- newlevels.

I tried the following:
## example
x <- factor(c(1,1,NA,2,3,4,4,4,1,2)); y <- x
attr(x, 'levels') <- c('a', 'b', 'c', 'd')     # does what I want
x
[1] a    a    <NA> b    c    d    d    d    a    b   
Levels: a b c d

'levels.simple<-' <- function (x, value) 
{
     attr(x, 'levels') <- value
}

levels.simple(y) <- c('a', 'b', 'c', 'd')     # does not what I want
y
[1] "a" "b" "c" "d"

Thanks,
Heinz T??chler