In oshka: Recursive Quoted Language Expansion

Forwarding NSE Arguments to NSE Functions

If you wish to write a function that uses a programmable NSE function and forwards its NSE arguments to it, you must ensure the NSE expressions are evaluated in the correct environment, typically the parent.frame(). This is no different than with normal NSE functions. An example:

subset3 <- function(x, subset, select, drop=FALSE) {
  frm <- parent.frame()  # as per note in ?parent.frame, better to call here
  sub.q <- expand(substitute(subset), x, frm)
  sel.q <- expand(substitute(select), x, frm)
  eval(bquote(base::subset(.(x), .(sub.q), .(sel.q), drop=.(drop))), frm)
}

We use bquote to assemble our substituted call and eval to evaluate it in the correct frame. The parts of the call that should evaluate in subset3 are escaped with .(). This requires some work from the programmer, but the user reaps the benefits:

col <- quote(Sepal.Length)
sub <- quote(Species == 'setosa')

subset3(iris, sub & col > 5.5, col:Petal.Length)

Notice that we used expand with the base NSE function subset. Because expand just generates language objects, you can use it with any NSE function.

The forwarding is robust to unusual evaluation:

col.a <- quote(I_dont_exist)
col.b <- quote(Sepal.Length)
sub.a <- quote(stop("all hell broke loose"))
threshold <- 3.35

local({
  col.a <- quote(Sepal.Width)
  sub.a <- quote(Species == 'virginica')
  subs <- list(sub.a, quote(Species == 'versicolor'))

  lapply(
    subs,
    function(x) subset3(iris, x & col.a > threshold, col.b:Petal.Length)
  )
})