Use remote roam objects in packages

# Use remote `roam` objects in packages

`roam` is an R package designed to make it easy for package developers to include “regular looking” R objects in packages, which are active bindings that download data from remote sources. This vignette is to demonstrate how to use `roam` in a package.

A demo package `roam.demo` created with `roam` is available at FinYang/roam.demo. This vignette will use code from `roam.demo` as examples.

## Basics

A “regular looking” remote data set in `roam` is called a (activated) roam object. It is created with the `new_roam()` function. A roam object, before it’s activated, is a function with a class `roam_object`.

    library(roam)

    bee <- new_roam("roam", "bee", \(version) "buzzz")
    class(bee)
    #> [1] "roam_object"

After it is activated, it is an active binding, which is an object that returns a value computed by its defining function. It looks like a regular object, but behaves like a function.

    options(roam.autodownload = TRUE)
    roam_activate(bee)
    bee
    #> [1] "buzzz"

We set `options(roam.autodownload = TRUE)` in this vignette to automatically download the data (execute the function) when the roam object is called the first time.

This establishes the two-step process of creating a roam objection in a package: definition and activation. In the following, we will refer to the “developer” as the developer that uses `roam` in their packages, and the “user” as the user of the packages developed by the developers.

To explicitly download the dataset or delete the local cache, the following two functions can be used.

    roam_install(bee)
    roam_delete(bee)

## Definition

    new_roam(package, name, obtainer, ...)

To define a roam object, we need three pieces of information.

- `package`: the name of the package as a string
- `name`: the name of the roam object. It should be the same as the name to which the roam object is assigned.
- `obtainer`: the function the developer defines to retrieve the dataset from the remote source.

Take the following definition from the `roam.demo` package as an example.

    bee <- new_roam(
      "roam.demo",
      "bee",
      function(version) {
        read.csv(
          "https://raw.githubusercontent.com/finyang/roam/master/demo/bee_colonies.csv"
        )
      }
    )

In the function arguments, the package name is `"roam.demo"`. the roam object is called `"bee"`, which is the same as the object `bee`, and the `obtainer` function simply reads a csv file from a remote source.

The `obtainer` function needs to have (at least) one argument called `version`. This is used for versioning purpose and will be covered in the Versioning section below.

## Activation

Active bindings are not preserved during package installation, thus roam objects need to be activated during package loading.

    #' @import roam
    .onLoad <- function(libname, pkgname) {
      roam_activate_all("roam.demo")
    }

The `.onLoad` function is called during package loading. Here we use `roam_activate_all()` to activate all roam objects in the package `"roam.demo"`. `roam_activate_all()` looks through every object in the package to find roam objects. If the package has lots of objects, use `roam_activate()` to specify roam objects individually to improve performance.

    roam_activate(bee)

Don’t forget to import `roam` or the specific functions to use when the package is loaded but not attached, especially when they are in `.onLoad`, if the package depends on `roam`, like `roam.demo` here. The developer will be prompted to import `roam` in `NAMESPACE` during `R CMD check`, if they choose to depend on `roam`. This should already be done for each function if `roam` is listed under `Imports` in `DESCRIPTION`, like any other imported packages.

## Imports and documentation

The `roam.demo` package “depends” on `roam`, but this is not necessary if the developer prefers not to. But it is recommended to at least re-export helper function like `roam_delete()` or `roam_install()` from `roam`, or create your own wrapper of these helper functions, so the user can properly manage the cache of roam objects.

The documentation in `roam.demo` is generated using `roxygen2`. For each roam object, the `format` tag is explicitly defined.

    #' beeeeeeee
    #'
    #' @format buzzzzzzzz
    #' @export

When `roxygen2` generates the Format section, it evaluates the object and records the structure of the resulting output. If a roam object is not cached locally, this evaluation returns `NULL`, and the documented format will incorrectly reflect a `NULL` object. As a result, the generated documentation may vary across devices depending on their local cache state. To avoid this inconsistency, the `format` tag should always be explicitly specified for roam objects.

## Versioning

`roam` allows user to specify a version of the dataset they want to download using `roam_install()`.

    # tidytuesday2026Jan is another example data in `roam.demo`
    roam_install(tidytuesday2026Jan, version = "latest")
    # roam_update() is a wrapper of roam_install()
    # with the version set to "latest"
    roam_update(tidytuesday2026Jan)

This version specified by the user will be passed to the `obtainer` function where the developer can use decide how to download the data. This is why the `obtainer` function needs to have an argument named `version`.

Again, taking an example from `roam.demo`.

    tidytuesday2026Jan <- new_roam(
      "roam.demo",
      "tidytuesday2026Jan",
      function(version) {
        if ((!is.character(version)) || length(version) > 1) {
          stop("version must be a length character")
        }
        if (
          !is.na(version) && (!version %in% c("latest", "2026-01-20", "2026-01-13"))
        ) {
          stop("invalid version number")
        }
        if (is.na(version) || version %in% c("latest", "2026-01-20")) {
          roam_set_version("2026-01-20")
          read.csv(
            "https://raw.githubusercontent.com/rfordatascience/tidytuesday/refs/heads/main/data/2026/2026-01-20/apod.csv"
          )
        } else {
          roam_set_version("2026-01-13")
          read.csv(
            "https://raw.githubusercontent.com/rfordatascience/tidytuesday/refs/heads/main/data/2026/2026-01-13/africa.csv"
          )
        }
      }
    )

Let’s look at the `obtainer` function. First, the developer checks if the version the user specified follows the correct format.

    if ((!is.character(version)) || length(version) > 1) {
      stop("version must be a length character")
    }
    if (
      !is.na(version) && (!version %in% c("latest", "2026-01-20", "2026-01-13"))
    ) {
      stop("invalid version number")
    }

When the user calls the roam object for the first time without a version, this version input is `NA`. When the user calls the roam object using `roam_update()`, this version is `"latest"`. Apart from that, this obtainer function allows two other version numbers `"2026-01-20"` and `"2026-01-13"`. The validation in the function here returns an error if the input version is not one of the four possibilities.

The format of the version is entirely decided by the developer. It also does not need to be hard coded inside the `obtainer` function. Instead, the developer can retrieve a list of valid version number inside the `obtainer`.

Next, based on the input version, the `obtainer` downloads the corresponding data. Again, this does not need to be hard coded, but it can be a call to an API with the version number. This allows updating of datasets without updating packages.

    if (is.na(version) || version %in% c("latest", "2026-01-20")) {
      roam_set_version("2026-01-20")
      read.csv(
        "https://raw.githubusercontent.com/rfordatascience/tidytuesday/refs/heads/main/data/2026/2026-01-20/apod.csv"
      )
    } else {
      roam_set_version("2026-01-13")
      read.csv(
        "https://raw.githubusercontent.com/rfordatascience/tidytuesday/refs/heads/main/data/2026/2026-01-13/africa.csv"
      )
    }

Note the use of `roam_set_version()`.

    roam_set_version("2026-01-13")

The developer should use `roam_set_version()` to associate a version number with the local cache. This version should be a version number with a valid format that might be different from the version the user specifies. In this example, even if the user specified the version to be `"latest"`, the roam object `tidytuesday2026Jan` will only store the cache with the version number the developer specifies, which is `"2026-01-13"`.

If the developer does not specify a version number with `roam_set_version()` inside the `obtainer` function, the cache will be stored with version `NA`. The `roam_set_version()` function should also be called before the `obtainer` function returns the data. The output value of the `obtainer` function should always be the data itself.

The user can use the `roam_version()` function to check which version of the data is cached locally.

    roam_version("roam.demo", "tidytuesday2026Jan")