default args for bare col names: missing or NULL?

quo-missing-null.R

library(ggplot2)
library(rlang)

f <- function(df, x, y, facet) {
  
  x <- enquo(x)
  y <- enquo(y)
  facet = enquo(facet)
  
  g <- ggplot(df, aes(x = !!x, y = !!y)) +
    geom_point()
  
  if (!quo_is_missing(facet)) g <- g + facet_wrap(vars(!!facet))
  
  g
}

f(mtcars, mpg, hp)
f(mtcars, mpg, hp, cyl)


f2 <- function(df, x, y, facet = NULL) {
  
  x <- enquo(x)
  y <- enquo(y)
  facet = enquo(facet)
  
  g <- ggplot(df, aes(x = !!x, y = !!y)) +
    geom_point()
  
  if (!is.null(quo_get_expr(facet))) g <- g + facet_wrap(vars(!!facet))
  
  g
}

f2(mtcars, mpg, hp)
f2(mtcars, mpg, hp, cyl)