list variables in quosure

find_vars.R

# List a variable names (that are not functions) in 
# a quosure
# ideas taken from https://adv-r.hadley.nz/expressions.html

find_vars <- function(x) {
  extr <- function(x) 
    if (is.symbol(x)) {
      as.character(x)
    } else if (is.pairlist(x)) {
      purrr::flatten_chr(purrr::map(as.list(x), find_vars))
    } else if (is.call(x)) {
      purrr::flatten_chr(purrr::map(as.list(x)[-1], find_vars))
    } else {
      NULL
    }
  extr(rlang::quo_squash(x))
}