Overcoming the Package-Wide Variable Conundrum with R6 and Roxygen2

Introduction

When building an R package, managing dependencies between files can be a daunting task. One common issue is accessing package-wide variables within an R6 class. In this article, we’ll explore solutions to this problem using R6 and Roxygen2.

Background

In R, when you create a package, the package is loaded in a specific order, determined by the Collate section of the DESCRIPTION file. This ordering can affect how your functions are executed and which variables are available at runtime. The issue arises when we need to access package-wide variables within an R6 class, but these variables are not yet loaded due to the loading order.

The Problem

Consider the following example:

#' A class that will need a package-wide variable
#'
#' @export
MyClass <- R6::R6Class(
  "MyClass",
  public = list(
    # This var is a concat between a package-wide variable and a str
    base_url = paste0(.base_url, "me/", "endpoint"),
  )
)

In this example, .base_url is declared in the zzz.R file, but when we try to use it within the MyClass, it’s not available yet.

# Loading test
Error in paste0(.base_url, "me/", "endpoint") :
  object '.base_url' not found

Solution 1: Using @include Roxygen Tag

One way to solve this issue is by using the @include roxygen tag. This tag forces R to load the files listed before it, ensuring that all variables are available at runtime.

#' A class that will need a package-wide variable
#'
#' @include zzz.R
#'
#' @export
MyClass <- R6::R6Class(
  # ... All those fancy class methods
)

By adding the @include tag, we ensure that zzz.R is loaded before the MyClass. This should resolve the issue of .base_url not being available.

Solution 2: Editing the Collate Section

Another approach is to edit the Collate section in the DESCRIPTION file. We can specify the order in which files are loaded, ensuring that zzz.R comes before any other files.

# DESCRIPTION
#' @description My package description.
#'
#' @author [Your Name]
#'
#' @examples
#' # Load package
#' library(MyPackage)
#
#' # Use MyClass
#' obj <- new("MyClass")
#
#' # This will now work as expected
#'
#' obj$base_url

In the Collate section, we can list the files in the order they should be loaded:

## Collate
##  zzz.R
##  Imports: [other packages]

By specifying zzz.R first, we ensure that .base_url is available when we try to use it within MyClass.

Conclusion

Accessing package-wide variables from within an R6 class can be challenging, especially when the loading order of files affects variable availability. By using Roxygen2’s @include tag or editing the Collate section in the DESCRIPTION file, we can overcome this issue and ensure that our code runs smoothly.

Additional Notes

• When working with R packages, it’s essential to understand how the loading order of files affects your code. By specifying the correct loading order, you can avoid issues like the one described above.
• Roxygen2 provides several other tags and options for managing dependencies between files, such as @depends and @export. Familiarize yourself with these tools to create well-structured and maintainable R packages.

Example Use Cases

Here’s an example of how you might use the @include tag in a real-world scenario:

#' A class that will need a package-wide variable
#'
#' @export
MyClass <- R6::R6Class(
  "MyClass",
  public = list(
    # This var is a concat between a package-wide variable and a str
    base_url = paste0(.base_url, "me/", "endpoint"),
    initialize <- function() {
      # Call the function to update .base_url
      update_base_url()
    }
  )
)

#' Function to update .base_url
#'
#' @export
update_base_url <- function() {
  .base_url <- paste0(.base_url, "new", "/endpoint")
}

In this example, we use the @include tag to ensure that zzz.R is loaded before the MyClass. We then update .base_url within the initialize method of MyClass.

Similarly, you might edit the Collate section in your DESCRIPTION file to specify the correct loading order for your files.

Last modified on 2025-02-10