Questions about calls and formulas

I am trying to adapt boxplot.formula (in graphics) to accept an
additional parameter, weights.
I already managed to adapt boxplot.default to do this.

boxplot.formula prepares the data for a call to boxplot.default and to
achieve that does the following: It takes a formula like

    x~g*h

as the first argument, and then by using

    m <- match.call(expand.dots = FALSE)

saves the call. It transforms the call

    m$na.action <- na.action # force use of default for this method
    m[[1]] <- as.name("model.frame")

and then  evaluates the modified call
    mf <- eval(m, parent.frame())

    print(m)
gives
    model.frame(formula = x ~ g * h)

Then it uses components of mf for the call to boxplot.default.

m has a component m$formula containing the parsed model formula.
mode(m$formula) is "call".
In our case, deparse(m$formula) gives a string representation of the
formula: "x~g*h".
I want to replace the response variable (in our case x) by the weights
variable, which in the string expression can be done easily with
strsplit and paste. Then I need to reconvert the modified string to a call.

So I create newmodelstring<-"weights~g*h" and try

    m$formula<-as.call(parse(newmodelstring))

    print(m)
gives
    model.frame(formula = weights ~ g * h())


When I try to evaluate the modified m this does not work. When I try to
evaluate m with this modification I get

Error in model.frame(formula = weights ~ g * h()) :
        attempt to apply non-function

Is there a way to get rid of the empty parentheses at the
end of the formula? I think then my code could work.

--
Erich Neuwirth, Didactic Center for Computer Science
University of Vienna
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(foo <- eval(substitute(x ~ g * h, list(x = as.name("weights")))))

class(foo)