What does "hidden list" in the output of `str()` mean? - r

Question

In attempting to Answer this Question I came across this in the output of str()
## R reference
rref <- bibentry(bibtype = "Manual",
title = "R: A Language and Environment for Statistical Computing",
author = person("R Development Core Team"),
organization = "R Foundation for Statistical Computing",
address = "Vienna, Austria",
year = 2010,
isbn = "3-900051-07-0",
url = "http://www.R-project.org/")
> str(rref)
Class 'bibentry' hidden list of 1
$ :List of 7
..$ title : chr "R: A Language and Environment for Statistical Computing"
..$ author :Class 'person' hidden list of 1
.. ..$ :List of 5
.. .. ..$ given : chr "R Development Core Team"
.. .. ..$ family : NULL
.. .. ..$ role : NULL
.. .. ..$ email : NULL
.. .. ..$ comment: NULL
..$ organization: chr "R Foundation for Statistical Computing"
..$ address : chr "Vienna, Austria"
..$ year : chr "2010"
..$ isbn : chr "3-900051-07-0"
..$ url : chr "http://www.R-project.org/"
..- attr(*, "bibtype")= chr "Manual"
In particular, I'm puzzled by this bit:
> str(rref)
Class 'bibentry' hidden list of 1
$ :List of 7
What does the "hidden list" bit refer to? What kind of object is this? Is this just some formatting output from str() when there is only a single component in the object that is itself a list? If so how is there a way to force str() to show the full structure?

Answer 1

This seems like an artefact of str. My interpretation is that the words hidden list are printed in the output of str if the obect is not a pairlist.
Since your object is of class bibtex, and there is no str method for bibtex, the method utils:::str.default is used to describe the structure.
Condensed extract from str.default:
...
if (is.list(object)) {
i.pl <- is.pairlist(object)
...
cat(if (i.pl)
"Dotted pair list"
else if (irregCl)
paste(pClass(cl), "hidden list")
else "List", " of ", le, "\n", sep = "")
...
}
The key bit that defines irregCl is:
....
else {
if (irregCl <- has.class && identical(object[[1L]],
object)) {
....
and that explain the hidden list bit - it hides the outer list if the object has a class and object and object[[1]] are identical. As you showed in the Answer you linked to, the [[ method returns an identical object if the list contains a single "bibentry" object.