User defined functions as formula input

Question

Built-in functions in R can be used in formula objects, for example

reg1 = lm(y ~ log(x), data = data1)

How can I write my functions such that they can be used in formula objects?

fnMyFun = function(x) {
  return(x^2)
}
reg2  = lm(y ~ fnMyFun(x), data = data1)

Answer 1

What you've got certainly works. One problem is that different modelling functions handle formulas in different ways. I think that as long as you return something that model.matrix can make sense of, you'll be fine. That would mean

1. The function is vectorised; ie given a vector of length N, it returns a result also of length N

2. It has to return an atomic vector or matrix (but not a list, or of type raw)