make list() behave as NULL

[aphalo]

To me it feels counter intuitive that NULL is a no-op while list() results in a modified object. Of course the rendered plots do not differ. In the current definition of function add_ggplot() the test for returning p unchanged uses is.null(object), one could use length(object) == 0 instead, but maybe it is just me that is surprised, and there is a good reason behind treating NULL and list() differently.

library(ggplot2)

myplot <- ggplot(data = mtcars,
                 aes(x=disp, y=mpg)) +
  geom_point()

identical(myplot, myplot + NULL)
#> [1] TRUE
identical(myplot, myplot + list())
#> [1] FALSE

[hadley]

Post #2309 it's consistent (the plot is always cloned so identical() is never TRUE).