Clarification on when to use ggproto_parent()

[teunbrand]

In chapter 19.4.5 the following is said about using the ggproto_parent() function:

ggplot2-book/internals.qmd

Line 564 in ff77ea9

This pattern is often easier to read than using `ggproto_parent()` and because ggproto objects are stateless it is just as safe.

However, that paragraph doesn't give a lot of guidance about when to use Class$method() and when to use ggproto_parent(Class, self)$method(). I think it would help if it was clearer that the Class$method() approach is safe when no reading or writing to/from self happens (i.e. the Class$method() formals doesn't include self), and less safe otherwise. To give an example a difference occurs:

library(ggplot2)

Drink <- ggproto(
  "Drink",
  property = "cold",
  describe = function(self) {
    print(paste0("This drink is ", self$property))
  }
)

Coffee <- ggproto(
  "Coffee", Drink,
  property = "hot",
  describe_parent = function(self) {
    ggproto_parent(Drink, self)$describe()
  },
  describe_class = function(self) {
    Drink$describe()
  }
)

Coffee$describe_parent()
#> [1] "This drink is hot"
Coffee$describe_class() # Yuck!
#> [1] "This drink is cold"

^{Created on 2023-07-19 with reprex v2.0.2}

In the example Coffee$describe() would of course give the correct answer, but that is besides the point.