Uninformative error for piping into facets

[mitchelloharawild]

Once of my students mistook %>% for + when adding facets and couldn't determine the problem based on the error message.
It would be nice if the hint for piping layers ("Did you use %>% instead of +?") was also provided if the first argument for facets was a ggplot object.

library(ggplot2)
library(magrittr)
ggplot(diamonds) %>% 
  geom_point(aes(price, carat))
#> Error: `mapping` must be created by `aes()`
#> Did you use %>% instead of +?

ggplot(diamonds) %>% 
  facet_wrap(vars(cut))
#> Warning: Coercing `nrow` to be an integer.
#> Error in sanitise_dim(nrow): 'list' object cannot be coerced to type 'integer'

^{Created on 2021-03-18 by the reprex package (v1.0.0)}

Session info

sessioninfo::session_info()
#> ─ Session info ───────────────────────────────────────────────────────────────
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#>  language en_AU:en                    
#>  collate  en_AU.UTF-8                 
#>  ctype    en_AU.UTF-8                 
#>  tz       Australia/Melbourne         
#>  date     2021-03-18                  
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[yutannihilation]

Thanks, I think we can show similar error message on facets too, but,

ggplot(diamonds) %>% 
  facet_wrap(vars(cut))

is this really a common mistake? I think the realistic code would be something like below. But, I think it's only after the user figured out the fact that ggplot2 uses +, not %>%, at least to geom_*. So I'm curious why they still confuses %>% with + here.

p <- ggplot(diamonds) +
  geom_point(aes(price, carat))

# print w/o facetting
p

# print w/ facetting
p %>%
  facet_wrap(vars(cut))

[mitchelloharawild]

In this particular case, we were using an autoplot() method. The output from these functions can be further modified and the student tried to add facets. So the code flow was more like:

<tsibble> %>% 
  autoplot() %>% 
  facet_wrap(vars(col))

[yutannihilation]

Ah, makes sense.

[yutannihilation]

Would you like to contribute a PR? I guess it's as easy as adding the similar check like this

ggplot2/R/layer.r

Lines 153 to 158 in 199be05

	if (inherits(mapping, "ggplot")) {
	msg <- paste0(
	msg, "\n",
	"Did you use %>% instead of +?"
	)
	}

to here

ggplot2/R/facet-.r

Line 278 in 199be05

as_facets_list <- function(x) {

[mitchelloharawild]

PR in #4385. It required a bit of re-ordering in some places, so I collected the other check for using aes() into a validate_facets() helper.

[yutannihilation]

Oh, sorry I didn't notice it needed some more efforts... Thanks for your contribution!