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Editorial Team
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Asked: 2026-05-31T15:23:32+00:00

In attempting to Answer this Question I came across this in the output of

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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?

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1 Answer

  1. Editorial Team

    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.

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