Documenting other objects • roxygen2

Datasets

Datasets are stored in data/, not as regular R objects in the package. This means you need to document them in a slightly different way: instead of documenting the data directly, you quote the dataset’s name.

#' Prices of over 50,000 round cut diamonds
#'
#' A dataset containing the prices and other attributes of almost 54,000
#'  diamonds. The variables are as follows:
#'
#' @format A data frame with 53940 rows and 10 variables:
#' \describe{
#'   \item{price}{price in US dollars ($326--$18,823)}
#'   \item{carat}{weight of the diamond (0.2--5.01)}
#'   \item{cut}{quality of the cut (Fair, Good, Very Good, Premium, Ideal)}
#'   \item{color}{diamond colour, from D (best) to J (worst)}
#'   \item{clarity}{a measurement of how clear the diamond is (I1 (worst), SI2,
#'     SI1, VS2, VS1, VVS2, VVS1, IF (best))}
#'   \item{x}{length in mm (0--10.74)}
#'   \item{y}{width in mm (0--58.9)}
#'   \item{z}{depth in mm (0--31.8)}
#'   \item{depth}{total depth percentage = z / mean(x, y) = 2 * z / (x + y) (43--79)}
#'   \item{table}{width of top of diamond relative to widest point (43--95)}
#' }
#' 
#' @source {ggplot2} tidyverse R package.

Note the use of two additional tags that are particularly useful for documenting data:

@format, which gives an overview of the structure of the dataset. This should include a definition list that describes each variable. There’s currently no way to generate this with Markdown, so this is one of the few places you’ll need to Rd markup directly.
@source where you got the data form, often a URL.

Packages

As well as documenting every object inside the package, you can also document the package itself by documenting the special sentinel "_PACKAGE". This automatically includes information parsed from the DESCRIPTION, including title, description, list of authors, and useful URLs.

We recommend placing package documentation in {pkgname}-package.R, and have @keywords internal. Use usethis::use_package_doc() to set up automatically.

Here’s an example:

#' @keywords internal 
"_PACKAGE"

Package documentation is a good place to put # Package options that documents options used by the package.

Some notes:

By default, aliases will be added so that both ?pkgname and package?pkgname will find the package help. If there’s an existing function called pkgname, use @aliases {pkgname}-package NULL to override the default.
Use @references to point to published material about the package that users might find helpful.

S3

S3 generics are regular functions, so document them as such. If necessary, include a section that provides additional details for developers implementing methods.
S3 classes have no formal definition, so document the constructor.
It is your choice whether or not to document S3 methods. Generally, it’s not necessary to document straightforward methods for common generics like print(). (You should, however, always @export S3 methods, even internal ones).

If your method is more complicated, you should document it by setting @rdname or @describeIn. For complicated methods, you might document in their own file (i.e. @name generic.class; for simpler methods you might document with the generic (i.e. @rdname generic). Learn more about these tags in vignette("reuse").
Generally, roxygen2 will automatically figure out the generic that the method belongs to, and you should only need to use @method if there is ambiguity. For example, is all.equal.data.frame() the equal.data.frame method for all(), or the data.frame method for all.equal()?. If this happens to you, disambiguate with (e.g.) @method all.equal data.frame.

S4

S4 generics are also functions, so document them as such.

Document S4 classes by adding a roxygen block before setClass(). Use @slot to document the slots of the class. Here’s a simple example:

#' An S4 class to represent a bank account
#'
#' @slot balance A length-one numeric vector
Account <- setClass("Account",
  slots = list(balance = "numeric")
)

S4 methods are a little more complicated. Unlike S3 methods, all S4 methods must be documented. You can document them in three places:

In the class. Most appropriate if the corresponding generic uses single dispatch and you created the class.
In the generic. Most appropriate if the generic uses multiple dispatches and you control it.
In its own file. Most appropriate if the method is complex. or the either two options don’t apply.

Use either @rdname or @describeIn to control where method documentation goes. See the next section for more details.

R6

R6 methods can be documented in-line, i.e. the method’s documentation comments come right before the definition of the method.
Method documentation can use the @description, @details, @param, @return and @examples tags. These are used to create a subsection for the method, within a separate ‘Methods’ section. All roxygen comment lines of a method documentation must appear after a tag.
@param tags that appear before the class definition are automatically inherited by all methods, if needed.
R6 fields and active bindings can make use of the @field tag. Their documentation should also be in-line.
roxygen2 checks that all public methods, public fields, active bindings and all method arguments are documented, and issues warnings otherwise.
To turn off the special handling of R6 classes and go back to the roxygen2 6.x.x behavior, use the r6 = FALSE option in DESCRIPTION, in the Roxygen entry: Roxygen: list(r6 = FALSE).

roxygen2 automatically generates additional sections for an R6 class:

A section with information about the superclass(es) of the class, with links. In HTML this includes a list of all inherited methods, with links.
An ‘Examples’ section that contains all class and method examples. This section is run by R CMD check, so method examples must work without errors.

An example from the R6 tutorial:

#' R6 Class Representing a Person
#'
#' @description
#' A person has a name and a hair color.
#'
#' @details
#' A person can also greet you.

Person <- R6::R6Class("Person",
public = list(

    #' @field name First or full name of the person.
    name = NULL,

    #' @field hair Hair color of the person.
    hair = NULL,

    #' @description
    #' Create a new person object.
    #' @param name Name.
    #' @param hair Hair color.
    #' @return A new `Person` object.
    initialize = function(name = NA, hair = NA) {
      self$name <- name
      self$hair <- hair
      self$greet()
    },

    #' @description
    #' Change hair color.
    #' @param val New hair color.
    #' @examples
    #' P <- Person("Ann", "black")
    #' P$hair
    #' P$set_hair("red")
    #' P$hair
    set_hair = function(val) {
      self$hair <- val
    },

    #' @description
    #' Say hi.
    greet = function() {
      cat(paste0("Hello, my name is ", self$name, ".\n"))
    }
  )
)