When an object is modified, R generally copies it (sometimes
lazily) to enforce valuesemantics.
However, some internal types are uncopyable. If you try to copy
them, either with
<- or by argument passing, you actually create
references to the original object rather than actual
copies. Modifying these references can thus have far reaching side
An object to test.
# Let's add attributes with structure() to uncopyable types. Since # they are not copied, the attributes are changed in place: env <- env() structure(env, foo = "bar")#> <environment: 0x7f8291244c28> #> attr(,"foo") #>  "bar"env#> <environment: 0x7f8291244c28> #> attr(,"foo") #>  "bar"# These objects that can only be changed with side effect are not # copyable: is_copyable(env)#>  FALSEstructure(base::list, foo = "bar")#> function (...) .Primitive("list")str(base::list)#> function (...) #> - attr(*, "foo")= chr "bar"