S7 support for `tree`

[ltuijnder]

Maybe a bit premature as S7 still in active development.

But currently tree inspection with S7 does not work as the S7 machinery is defined with attributes which are hidden by default. But when shown it is too verbose. It would be great if S7 objects could be printed in the tree structure like how currently the print method of S7 works.

Some examples:

library(S7)
library(lobstr)
T <- new_class('T', properties = list(a=class_numeric))
t <- T(1)
tree(t)
#> S3<T/S7_object>
tree(list(t,t,t))
#> <list>
#> ├─S3<T/S7_object>
#> ├─S3<T/S7_object>
#> └─S3<T/S7_object>
tree(t, show_attributes = TRUE)
#> S3<T/S7_object>
#> ├┄attr(,"class")<chr [2]>: "T", "S7_object"
#> ├┄attr(,"S7_class"): function(a)
#> ┊ ├┄attr(,"name"): "T"
#> ┊ ├┄attr(,"parent"): function()
#> ┊ ┊ ├┄attr(,"name"): "S7_object"
#> ┊ ┊ ├┄attr(,"properties"): <list>
#> ┊ ┊ ├┄attr(,"abstract"): FALSE
#> ┊ ┊ ├┄attr(,"constructor"): function()
#> ┊ ┊ ├┄attr(,"validator"): function(self)
#> ┊ ┊ └┄attr(,"class")<chr [2]>: "S7_class", "S7_object"
#> ┊ ├┄attr(,"properties"): <list>
#> ┊ ┊ └─a: S3<S7_property>
#> ┊ ┊   ├─name: "a"
#> ┊ ┊   ├─class: S3<S7_union>
#> ┊ ┊   │ ├─classes: <list>
#> ┊ ┊   │ ┊ ├─S3<S7_base_class>
#> ┊ ┊   │ ┊ │ ├─class: "integer"
#> ┊ ┊   │ ┊ │ ├─constructor_name: "integer"
#> ┊ ┊   │ ┊ │ ├─constructor: function(.data)
#> ┊ ┊   │ ┊ │ ├─validator: function(object)
#> ┊ ┊   │ ┊ │ ├┄attr(,"names")<chr [4]>: "class", "constructor_...", "constructor", "validator"
#> ┊ ┊   │ ┊ │ └┄attr(,"class"): "S7_base_class"
#> ┊ ┊   │ ┊ └─S3<S7_base_class>
#> ┊ ┊   │ ┊   ├─class: "double"
#> ┊ ┊   │ ┊   ├─constructor_name: "double"
#> ┊ ┊   │ ┊   ├─constructor: function(.data)
#> ┊ ┊   │ ┊   ├─validator: function(object)
#> ┊ ┊   │ ┊   ├┄attr(,"names")<chr [4]>: "class", "constructor_...", "constructor", "validator"
#> ┊ ┊   │ ┊   └┄attr(,"class"): "S7_base_class"
#> ┊ ┊   │ ├┄attr(,"names"): "classes"
#> ┊ ┊   │ └┄attr(,"class"): "S7_union"
#> ┊ ┊   ├─getter: <NULL>
#> ┊ ┊   ├─setter: <NULL>
#> ┊ ┊   ├─validator: <NULL>
#> ┊ ┊   ├─default: <NULL>
#> ┊ ┊   ├┄attr(,"names")<chr [6]>: "name", "class", "getter", "setter", "validator", "default"
#> ┊ ┊   └┄attr(,"class"): "S7_property"
#> ┊ ├┄attr(,"abstract"): FALSE
#> ┊ ├┄attr(,"constructor"): function(a)
#> ┊ └┄attr(,"class")<chr [2]>: "S7_class", "S7_object"
#> └┄attr(,"a"): 1

^{Created on 2025-02-06 with reprex v2.1.1}