Coercing a list of variables in a function call

This is hopefully a trivial problem for list subscribers, but I am very
new to writing R functions.
I wish to call an R function written by myself from another program to
fit a model. I need
to tell it which of the independent variables are factors. I need to do
this in a generic way, 
so that when the list is passed, R will work through the variables in
the data frame and coerce them into being factors.

This is what I tried ...

# Fictitious data for illustration
y<-c(1,2.1,3.3,4.3,5,6.5)
x1<-c(1,1,1,2,2,2)
x2<-c(1,2,3,1,2,3)
x3<-c(1,3,2,4,2,4)
d<-as.data.frame(cbind(y,x1,x2,x3))
#Function definition
model_it=function(data=" ", model=" ", factors=list()){
         attach(d) #Attach the data
         formula<-as.formula(model) #Set up the model
#Identify the factors
         if (length(factors)>0) for(i in 1:length(factors)){
                                   factors[i]<-as.factor(factors[i])
                                   }
#Fit the model
         glm(formula, data=data)
}
#Function call attempted
model_it(data=d, model="y~x1+x2+x3", factors=list(x1,x2)) 

And this is the error message I received ...

Error in sort.list(unique.default(x), na.last = TRUE) : 
  'x' must be atomic for 'sort.list'
Have you called 'sort' on a list?

The line factors[i]<-as.factor(factors[i]) is the one responsible for
the error. 

From the word 'atomic', my guess is that I have asked it to set each

value as a factor rather than the variable, but I don't know
what syntax to use to get it to set the variable as a factor. Any
suggestions gratefully appreciated.

For the record, the above was run on windows XP using R2.8.0




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