Using function arguments in update.formula

Question

I am writing a function that takes two variables and separately regresses each of them on a set of controls expressed as a one-sided formula. Right now I'm using the following to make the formula for one of the regressions, but it feels a bit hacked-up:

foo <- function(x, y, controls) {
    cl <- match.call()
    xn <- cl[["x"]]
    xf <- as.formula(paste(xn, deparse(controls)))
}

I'd prefer to do this using update.formula(), but of course update.formula(controls, x ~ .) and update.formula(controls, as.name(x) ~ .) don't work. What should I be doing?

Answer 1

Here's one approach:

right <- ~ a + b + c
left <- ~ y 
left_2 <- substitute(left ~ ., list(left = left[[2]]))

update(right, left_2)

But I think you'll have to either paste text strings together, or use substitute. To the best of my knowledge, there are no functions to create one two sided formula from two one-sided formulas (or similar equivalents).