Explanation: Box modules • rhino

Rationale

With large applications it is critical for maintainability to properly structure your code using files and directories. R comes with the library() and source() functions, but its functionality is limited when it comes to dividing your code into modules and expressing their dependencies.

To address this, Rhino uses the box R package, which allows you to modularize your code in a similar way to languages like Python and Java:

box::use(
  dplyr, # Import dplyr. Its functions can be used via `$`, e.g. `dplyr$filter`.
  shiny[reactive], # Import the `reactive()` function from shiny package.
)
box::use(
  logic/data_validation, # Import the `logic/data_validation.R` module.
)

Box modules force you to be explicit about the dependencies between your files and packages. The graph of dependencies is visible at a glance in an app developed with box, while the traditional approach (global.R, library(), source()) makes it easy to build an app which only the author understands. Introduction of box to existing apps written without it has helped to improve the code structure and find bugs.

Usage

The best place to learn about box is its official documentation. The discussion here will mainly focus on how to use box inside Rhino.

Rhino suggests the use of app/logic and app/view. Rhino creates these directories by default. Code that is independent of Shiny should be kept in app/logic while code using or related to Shiny modules should be kept in app/view. This structure makes it easy to make a nested hierarchy of code with the help of box.

# app/logic/messages.R
#' @export
say_hello <- function(name) {
  paste0("Hello, ", name, "!")
}

#' @export
say_bye <- function(name) {
  paste0("Goodbye, ", name, "!")
}

Both say_hello() and say_bye() can be exported from app/logic/messages.R.

box::use(
  app/logic/messages[say_bye, say_hello],
)

#' @export
greet <- function(name) {
  paste(
    say_hello(name), say_bye(name)
  )
}

Note that box::use() allows for explicit attaching of function names from a module as shown above. Modules can also be imported across directories; use code from app/logic in app/view.

# app/view/greet_module.R
box::use(
  shiny[div, moduleServer, NS, renderText, req, textOutput],
  shiny.semantic[textInput],
)

box::use(
  app/logic/greet[greet],
)

#' @export
ui <- function(id) {
  ns <- NS(id)
  div(
    textInput(ns("name"), "Name"),
    textOutput(ns("message"))
  )
}

#' @export
server <- function(id) {
  moduleServer(id, function(input, output, session) {
    output$message <- renderText({
      req(input$name)
      greet(input$name)
    })
  })
}

With explicit attaching of function names, it is clear from above that the code uses shiny.semantic::textInput() and not shiny::textInput().

# app/main.R
box::use(
  shiny[moduleServer, NS],
  shiny.semantic[semanticPage],
)

box::use(
  app/view/greet_module,
)

#' @export
ui <- function(id) {
  ns <- NS(id)
  semanticPage(
    greet_module$ui(ns("message"))
  )
}

#' @export
server <- function(id) {
  moduleServer(id, function(input, output, session) {
    greet_module$server("message")
  })
}

In main.R, Shiny modules can be attached without attaching the function names. The Shiny module functions are accessed via $.

Advanced Features

Some useful box features are also explained in the sections below.

Init files

Objects exported by an __init__.R file can be imported from its parent directory.

Example

Assume we have an app/foo/__init__.R file with the following content:

#' @export
bar <- "Hello!"

We can now import bar as if it was defined in app/foo.R:

box::use(
  app/foo[bar],
)

This mechanism can be used in combination with reexports to make it easier to import multiple modules from a single directory.

Reexports

A module can reexport objects imported from a different module by applying #' @export to a box::use() statement.

Example

Assume we have modules analysis_tab.R and download_tab.R in the app/view directory. We can reexport them from app/view/__init__.R like this:

#' @export
box::use(
  app/view/analysis_tab,
  app/view/download_tab,
)

The following box::use() statements are now equivalent:

box::use(
  app/view/analysis_tab,
  app/view/download_tab,
)
box::use(
  app/view[analysis_tab, download_tab],
)

Style guide

To enhance the readability and maintainability of code, we suggest following the Rhino style guide.

Known issues

The following issues were fixed in box v1.1.3, which is required by Rhino starting with v1.4.0. This section is left here for reference.

Lazy-loaded data

Prior to v1.1.3 box didn’t support lazy-loaded data, so e.g. box::use(datasets[mtcars]) wouldn’t work (see this issue). It was possible to workaround it by using datasets::mtcars instead.

Trailing commas

Box allows trailing commas in box::use() statements and code, but prior to v1.1.3 they could cause problems in some circumstances:

Reexports (issue).
Functions accessed via $ (issue).