Quote While the Promise Is Hot!

October 18, 2018 by Hiroaki Yutani

Suppose we want to quote x when x is not NULL. The naive implementation would be like below. Here, y is for comparison. Do you understand why x and y are quoted differently?

quote_x_and_y <- function(x, y) {
  if (is.null(x)) {
    stop("x is NULL!", call. = FALSE)
  }
  
  x <- rlang::enquo(x)
  y <- rlang::enquo(y)
  
  list(x, y)
}

x <- y <- 1

quote_x_and_y(x, y)
#> [[1]]
#> <quosure>
#>   expr: ^1
#>   env:  empty
#> 
#> [[2]]
#> <quosure>
#>   expr: ^y
#>   env:  global

This is because x is evaluated when is.null() is called before quoting, whereas y is intact. Lionel Henry, the tidyeval super hero, answered my qustion on RStudio Community:

A forced promise can no longer be captured correctly because it no longer carries an environment.

This means we must not touch arguments before quoting. Instead, quote first and check the expression inside quosure by rlang::quo_is_*().

quote_x_and_y2 <- function(x, y) {
  x <- rlang::enquo(x)
  y <- rlang::enquo(y)
  
  if (rlang::quo_is_null(x)) {
    stop("x is NULL!", call. = FALSE)
  }
  
  list(x, y)
}

quote_x_and_y2(x, y)
#> [[1]]
#> <quosure>
#>   expr: ^x
#>   env:  global
#> 
#> [[2]]
#> <quosure>
#>   expr: ^y
#>   env:  global

For more complex checking, we may need to extract the expression from the quosure by rlang::quo_get_expr().

quote_x_and_y_wont_stop <- function(x, y) {
  x <- rlang::enquo(x)
  y <- rlang::enquo(y)

  x_expr <- rlang::quo_get_expr(x)  
  if (rlang::call_name(x) %in% "stop") {
    message("Nothing can stop me!\n")
  }
  
  list(x, y)
}

quote_x_and_y_wont_stop(stop("foo"), "bar")
#> Nothing can stop me!
#> [[1]]
#> <quosure>
#>   expr: ^stop("foo")
#>   env:  global
#> 
#> [[2]]
#> <quosure>
#>   expr: ^"bar"
#>   env:  empty

Anyway, keep in mind to use enquo() (or ensym()) at the very beginning of the function. Quote while the promise is hot.