Obtain all the undefined variables in an expression

find_unbound.r

find_unbound <- function(lang) {
  stopifnot(is.language(lang), !is.expression(lang))

  bound <- character()
  unbound <- character()

  rec_fun <- function(lang) {
    if(is.call(lang)) {
      # These are assignment calls; if any symbols are assigned and have
      # not already been found in a leaf, they are defined as bound

      if((lang[[1]] == as.name("<-") || lang[[1]] == as.name("="))) {
        for(i in as.list(lang)[-(1:2)]) Recall(i)
        if(is.name(lang[[2]]) && !as.character(lang[[2]]) %in% unbound)
          bound <<- c(bound, as.character(lang[[2]]))
      } else for(i in as.list(lang)[-1]) Recall(i)                
    } else if (is.name(lang) && ! as.character(lang) %in% bound)
      # this is a leaf; any previously bound symbols are by definition
      # unbound

      unbound <<- c(unbound, as.character(lang))
  }
  rec_fun(lang)
  unique(unbound)
}
file_unbound(lang)

As defined in this StackOverflow question: http://stackoverflow.com/questions/32363338/how-to-obtain-all-the-variables-that-need-to-be-defined-prior-evaluating-an-expr/32365609#32365609