Suggestion for more informative error message when () missing from geom_*

[mine-cetinkaya-rundel]

If you omit the () at the end of a geom, you get the following error:

library(ggplot2)

ggplot(mtcars, aes(y = mpg, group = vs)) +
  geom_boxplot
#> Error: Don't know how to add geom_boxplot to a plot

I think the error would be more informative if it continued on to say "Did you mean geom_boxplot()?"

[yutannihilation]

This has been discussed on #3106 (comment) and some of us actually agreed the error message can be improved.

[mine-cetinkaya-rundel]

I see, thanks for the link!

This issue wasn't prompted by confusion between %>% and + or people using functions without () in pipelines. In most cases where I've seen code like this it wasn't due to an expectation of geom_point() working without the (), it was just forgetting to add the parantheses. I, like many others, start teaching with visualization/ggplot2 so there are lots of very newbie mistakes as people are learning the syntax.

I also agree that this is not widespread confusion, but more of a nice-to-have to help the few who struggle.

Note that I have not done a cost-benefit analysis of implementation difficulty vs. benefit of having the improved error in place, so I can't really say if it's "worth it" to make the change.

[yutannihilation]

Ah, thanks for the context. I guess it won't be very difficult to add some heuristic here, which is called only when raising this error:

ggplot2/R/plot-construction.r

Lines 90 to 92 in 6f5ffea

	ggplot_add.default <- function(object, plot, object_name) {
	stop("Don't know how to add ", object_name, " to a plot", call. = FALSE)
	}

[clauswilke]

Yes, just check whether object is a function, and if it is suggest that they may have forgotten the parentheses.

[clauswilke]

Actually, I think this will do it. I can prepare a PR.

library(ggplot2)

# ggplot_add variant to be added
ggplot_add.function <- function(object, plot, object_name) { 
  stop(
    "Don't know how to add ", object_name, " to a plot. Did you mean ",
    object_name, "()?", call. = FALSE
  ) 
} 

# new error message
ggplot() + geom_point
#> Error: Don't know how to add geom_point to a plot. Did you mean geom_point()?

^{Created on 2019-10-28 by the reprex package (v0.3.0)}

[clauswilke]

See #3594.

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