tree doesn't print S4 class object structure

[royfrancis]

Just wondering why lobstr::tree() doesn't show a breakdown of S4 class objects like str() does.

# create a dummy s4 class
setClass(
  "Foo",
  slots = c(
    df = "data.frame",
    ll = "list",
    mat = "matrix"
  )
)

# create a dummy s4 class object
foo <- new(
  "Foo",
  df = data.frame(
    id = 1:3,
    name = c("A", "B", "C"),
    val = c(10.5, 20.2, 15.3),
    stringsAsFactors = FALSE
  ),
  ll = list(
    group1 = list(a = 1, b = 2),
    group2 = list(x = "hello", y = "world")
  ),
  mat = matrix(1:6, nrow = 2, ncol = 3)
)

str(foo)
#> Formal class 'Foo' [package ".GlobalEnv"] with 3 slots
#>   ..@ df :'data.frame':  3 obs. of  3 variables:
#>   .. ..$ id  : int [1:3] 1 2 3
#>   .. ..$ name: chr [1:3] "A" "B" "C"
#>   .. ..$ val : num [1:3] 10.5 20.2 15.3
#>   ..@ ll :List of 2
#>   .. ..$ group1:List of 2
#>   .. .. ..$ a: num 1
#>   .. .. ..$ b: num 2
#>   .. ..$ group2:List of 2
#>   .. .. ..$ x: chr "hello"
#>   .. .. ..$ y: chr "world"
#>   ..@ mat: int [1:2, 1:3] 1 2 3 4 5 6

library(lobstr)
tree(foo)
#> S4<Foo>

^{Created on 2025-08-22 with reprex v2.1.1}

Session info

sessionInfo()
#> R version 4.4.3 (2025-02-28)
#> Platform: aarch64-apple-darwin20.0.0
#> Running under: macOS Sequoia 15.6
#> 
#> Matrix products: default
#> BLAS:   /opt/homebrew/Caskroom/miniforge/envs/r-4.4-arm/lib/libblas.3.9.0.dylib 
#> LAPACK: /opt/homebrew/Caskroom/miniforge/envs/r-4.4-arm/lib/liblapack.3.9.0.dylib
#> 
#> locale:
#> [1] C
#> 
#> time zone: Europe/Stockholm
#> tzcode source: system (macOS)
#> 
#> attached base packages:
#> [1] stats     graphics  grDevices utils     datasets  methods   base     
#> 
#> other attached packages:
#> [1] lobstr_1.1.2
#> 
#> loaded via a namespace (and not attached):
#>  [1] digest_0.6.37     fastmap_1.2.0     xfun_0.52         glue_1.8.0       
#>  [5] knitr_1.50        htmltools_0.5.8.1 rmarkdown_2.29    lifecycle_1.0.4  
#>  [9] cli_3.6.5         reprex_2.1.1      withr_3.0.2       compiler_4.4.3   
#> [13] tools_4.4.3       evaluate_1.0.4    yaml_2.3.10       crayon_1.5.3     
#> [17] rlang_1.1.6       fs_1.6.6