# Package Building: How DESCRIPTION, NAMESPACE, roxygen, and devtools::document work together

As part of my new year’s resolution to learn new things about R, I’m trying to plug some holes in my R knowledge by writing more vignettes to explain them to myself this year.

This week I finally think I understand more about namespaces in R and why you should use them in your R package.

## Namespaces: Why Bother?

In short, we need namespaces because of the ambiguity of function names. Think of how many packages have a filter() function! How does R know which function from which package you’re talking about? One nightmare case is the Bioconductor exprs() method and the rlang exprs() function. These functions do very, very different things (one of them extracts an expression matrix from a Bioconductor ExpressionSet, and the other one is used for quoting multiple expressions).

What happens when you call library on both of these packages? The worst case scenario is that you mean to call one package function and R executes the other one. This is called a namespace collision, and unfortunately it can break your code.

Enter the namespace, which allows us to be package and function specific. If I want to use the function filter from dplyr, I write it as dplyr::filter. The dplyr:: part is the namespace of the dplyr package.

## DESCRIPTION: The gatekeeper for calling packages

The DESCRIPTION file is one of the gatekeepers in your package. Ever wonder how install.packages knows how to install the packages your package depends on? It’s actually because of a field in the DESCRIPTION file. Here’s the DESCRIPTION file for my burro package:

Package: burro
Type: Package
Title: Shiny App Package for setting up a data exploration session ("burro"w into the data)
Version: 0.1.0
Authors@R: as.person(c(
"Ted Laderas <tedladeras@gmail.com> [aut, cre]",
"Jessica Minnier <minnier@ohsu.edu> [ctb]",
"Gabrielle Choonoo <choonoo@ohsu.edu> [ctb]"
))
Maintainer: Ted Laderas <ted.laderas@gmail.com>
Description: Allows the teacher to deploy a simple data exploration app for exploring a dataset (mostly for teaching purposes).
License: MIT LICENSE
Encoding: UTF-8
LazyData: true
Imports:
dplyr,
shinydashboard,
ggplot2,
visdat,
skimr,
naniar,
data.table,
magrittr,
glue,
usethis,
here,
viridis,
DT
Depends:
shiny
Roxygen: list(markdown = TRUE)
RoxygenNote: 6.1.0
Suggests:
testthat


Look at the Imports: field. You can see a list of all of the packages that burro utilizes. In ye olde days of R, we used the Depends: field. Nowadays we use the Imports: fields. Here’s a Stack Overflow post explaining why. The main reason is that Imports: requires the package to have a namespace.

Modifying the DESCRIPTION file by hand is possible, but I don’t recommend it. Instead, you can use the usethis package to modify it. For example, if I want to use dplyr in my package I can do this in the console, while I am building it.

usethis::use_package("dplyr")


This will add dplyr to the Imports: field of your DESCRIPTION file.

Addition (Thanks Hao Ye, for the suggestion): If the function is in a development version (i.e., hosted on GitHub), you can use usethis::use_dev_package() to add it to your DESCRIPTION file. It will add an additional field called Remotes: to your package:

usethis::use_dev_package("tidyverse/dplyr")


For more info about using remotes, check out the vignette: https://remotes.r-lib.org/articles/dependencies.html

## Using namespaces to call functions from other packages

Now that we have specified the package in our DESCRIPTION file, we can now call any function in dplyr by adding a dplyr:: before the function. So if we wanted to call mutate() in our package function we can do this:

mutate_iris <- function(iris){
dplyr::mutate(iris, sepal_sum = Sepal.Length + Sepal.Width)
}


In many cases, calling a function by specifying its namespace is good practice. For one, there are many functions called filter(): I can think of at least the ones that are in base and dplyr. Using the namespace makes it unambiguous to both R and other developers which filter() function you’re talking about.

## How do I call functions from other packages? Using roxygen docstrings

Ugh, I’ve already written a bunch of code and I don’t want to add the namespaces before all of the functions from other packages! How can I avoid this?

This is where roxygen and devtools::document() come in.

roxygen docstrings are responsible for at least three things in your package: 1) producing the documentation (.Rd) files, but also: 2) specifying what package namespaces you want to utilize, or import in your function, and 3) whether you want to export that function (i.e., make it accessible publicly).

How do they accomplish 2)? When you call devtools::document() to build the documentation, they scan for multiple fields, such as @import and @importFrom in the roxygen doc strings. Then devtools::document() actually modifies the NAMESPACE file in your package.

## Using @importFrom: When you only need one function

Say you just wanted to use filter from dplyr, but didn’t want to write dplyr::filter before all of your functions. You can just import the filter() function by including the following docstring:

#' @importFrom dplyr filter


And then you can just use filter() like normal in your code:

#' @importFrom dplyr filter               #This is where you add the @importFrom
use_filter <- function(df, cutoff=0.5) {

filter(df, value < cutoff)

}


## Using @import: When you need a lot of functions from a package

What if you had a lot of functions from one package, such as shiny, that you want to use? Do you need to add an @importFrom for each of these functions? Nope. You can just use one @import field for the whole package:

# @import shiny


And then code like usual:

# @import shiny            # This is where you add the @import
shinyUI <- function() {
selectInput("")
}


## Importing multiple packages: just don’t do it (UPDATED).

Just a note to not import multiple packages using @import. As Hadley Wickham has noted, you have no control over the development of the packages you import. Just because there are no function collisions right now between the packages doesn’t mean that one of the developers may add a function down the line that might collide. So, if you need to use multiple packages, @import one package and use @importsFrom or namespaces to refer to functions in the other packages.

## Exported versus Internal functions

Remember I mentioned ‘@export’ above? Specifying in a docstring for your function exports it. Specifying it means that ‘devtools::document()’ will add an Export directive for the function in the NAMESPACE file. That means you can access it using ‘::’. Going back to our use_filter example:

#' @export                              #This is where you add the @export
#' @importFrom dplyr filter
use_filter <- function(df, cutoff=0.5) {

filter(df, value < cutoff)

}


If our package name is mypackage, then this function will be accessible if we use library(mypackage) or mypackage::use_filter().

Why is this important? You may write some internal functions that are useful in your package, but they aren’t necessarily ones you want your users to use in their daily use. @export allows you to control which functions you make publicly accessible in your package.

For example, try typing dplyr:: in RStudio and hit the tab key. You’ll see the usual dplyr verbs pop up. But these are only the exported functions. Now try dplyr::: and hit the tab key. You’ll see a list of functions that pop up that’s much longer - these are all the functions, including the internal functions.

So, if there’s a cool bit of internal code you want to use in a package, you can use ‘:::’ to specify it. Just be aware that oftentimes, internal functions may change a lot as code gets refactored, so code that utilizes them may be refactored.

## Remember to run devtools::document()

Once you’ve written code and want to test it in your package, remember to run devtools::document() before you reinstall your package for testing. Otherwise, the NAMESPACE file won’t be modified, and your code won’t work.

You can also modify your Project Options for your package and check the Tools >> Project Options >> Build Tools >> Generate Documentation with Roxygen box. Next to it, there is also a Configure button that lets you select the option to run devtools::document() whenever you build a package for testing.

## Go forth and Package!

TL;DR:

1. Use usethis::use_package() to add packages to your DESCRIPTION file (while in the console) after you’ve created your package skeleton using use_package().
2. Use namespaces (such as mypackage::usefilter()) to refer to external functions where possible to avoid collisions in function names when you are writing code.
3. Use @imports and @importsFrom judiciously in your roxygen documentation for a function if you need to use many extenral package functions within a function. Use only 1 @import statement in a function, use namespaces/@importsFrom for the other packages.
4. Expose functions you want to be made public using @export in your roxygen documentation.
5. Remember to run devtools::document() in the console after you modify/add these fields to the roxygen documentation when you build for testing, or set it up in Tools >> Project Options.

I hope this helps you to understand exactly the relationships between all of the components that are responsible for accessing namespaces in your package. I was confused about this for years, so writing this has helped me understand these relationships.

## Acknowledgements

Thanks to Hao Ye, Sébastien Rochette, Michael Chirico, Tim Hesterberg, and Hadley Wickham for their comments and questions. I’ve incorporated your suggestions.

## Note

I want this to be clear and correct. Please email me if there are any mistakes I’ve made.