How to write an R function that evaluates an expression within a data-frame

Question

Puzzle for the R cognoscenti: Say we have a data-frame:

df <- data.frame( a = 1:5, b = 1:5 )

I know we can do things like

Answer 1

The lattice package does this sort of thing in a different way. See, e.g., lattice:::xyplot.formula.

fn <- function(dat, expr) {
  eval(substitute(expr), dat)
}
fn(df, a)             # 1 2 3 4 5
fn(df, 2 * a + b)     # 3 6 9 12 15