# A Plant Pathologist's Guide to Writing Your First R Package

## So You Want to Write an R Package?

So you want to write your first R package, but you don’t know where to start? We can help with that! Writing an R package is not difficult per se, but it can be intimidating for a first timer. Why would you want to write an R package? Well, as @PhytophthoraLab says, if you have more than two functions, you have a package, plus it means you can get citations! Yes, if you write a nice R package that people like and use and you have provided the proper information, it can (and should) lead to citations for you.

Before we proceed. I have made some basic assumptions here.

1. You are familiar enough with R to write your own functions.

2. You have RStudio installed.

3. You have read at least one of the next resources mentioned and/or intend to use them in addition to this brief blog post.

4. You intend to use GitHub to host your package’s code.

## Resources

Rather than writing another lengthy blog post about how to create an R package, I’ll just point to some of the resources that are available to get you started and fill in what I see as some gaps.

First, Hadley Wickham has written a book on this topic, “R Packages”, which is free from his website or available from Amazon. That book has everything you need to get started with writing an R package.

Second, I used Hilary Parker’s tutorial, “Writing an R package from scratch“ when I wrote my first R package. I can recommend this one for getting started.

But if that’s not enough, Karl Broman has put together a fantastic primer where he even states that there is value in a diversity of resources, hence this blog post.

The aforementioned resources will help you get your package set up and working. I will not spend too much time covering the ground they cover already. Rather, I will spend some time covering areas that I have found useful, but that they do not cover like CITATION files (and why you need one), setting up a pkgdown website or how to get a DOI for the package.

### Getting Started

The easiest way to get started is with RStudio and the usethis and devtools packages.

Two other packages that you will want to use are available, which you can use to check the availability of your package name and pkgdown for building a documentation website for your package.

If you have not already installed these packages, install them now. They can be installed as follows.

install.packages(c("devtools", "usethis", "available", "pkgdown", "goodpractice"),
dependencies = TRUE)


### Naming

Once you are set up and going, first things first. You need a good name for your new package. Hadley offers his advice here: http://r-pkgs.had.co.nz/package.html#naming While there is no real consensus on naming R packages CRAN does offer a few guidelines that you need to follow.

The mandatory ‘Package’ field gives the name of the package. This should contain only (ASCII) letters, numbers and dot, have at least two characters and start with a letter and not end in a dot.

Also note, that the use of an underscore, “_”, is not allowed in package names.

It is best to keep it short and informative.

Once you have picked a good name, check that it is available and does not have any connotations you might want to convey using available, available::available("spiffypkgname").

### Package Structure

The easiest way to set up your package structure is to either use RStudio’s capability to make an R package when creating a new project (with your spiffy new package name) or use usethis::create_package("spiffypkgname"), which will set up the directories you need go get started and add the skeleton of an R package for you to start filling in.

### What Goes Where?

If you check inside your working directory, you’ll see a file structure like this:

spiffypkgname
| - DESCRIPTION
| - NAMESPACE
| - R/
| - man/


There, you’ve created an R package!

The DESCRIPTION holds the author information, a description of the package itself, and other information on what packages are necessary for yours to work (Imports). There are specific rules about formatting in the DESCRIPTION file that must be followed. Hadley gives a good overview, http://r-pkgs.had.co.nz/description.html, but I suggested that you see https://cran.r-project.org/doc/manuals/R-exts.html#The-DESCRIPTION-file for a much more in-depth guide as well.

Most of the time you will not need to edit the NAMESPACE file directly. The devtools package provides tools that will help maintain that for you. This file ensures that your package plays nice with the others in R.

The R subdirectory is where you put your R scripts. Typically these would be scripts of functions that you wish to share with others.

The man directory is where the help files are stored. Once you start documenting the scripts in R, this will populate.

### Documenting

A properly documented package will make it much easier for the end user, which means it is more likely to be used.

Documenting your package is easiest done using Roxygen2. For each function or dataset that the end user can use you must document it. See http://r-pkgs.had.co.nz/man.html and https://cran.r-project.org/web/packages/roxygen2/vignettes/rd.html for how to write the documentation. As you start getting more advanced in your package development I would refer you to the official “Writing R Extensions” section on documenting your package, https://cran.r-project.org/doc/manuals/R-exts.html#Writing-R-documentation-files.

### Licensing

There are several resources available for helping you pick a license. For my packages I default to the MIT license, but you may chose to use the GPL 2 or 3 or some other license depending on your specific needs and goals.

To generate a license file, again use usethis. ?usethis::use_mit_license() describes both how to use the function, but also gives a quick description of four popular licenses and when they might be used. To generate your LICENSE file type usethis::use_mit_license("Your Name Here").

### Vignettes

Of course you will want to give detailed information on how users can use your spiffy new package, right? That’s where a vignette comes in. Vignettes are long- form documentation that describe how to use the functions in the package in greater detail than the help files or perhaps contain a use case for illustration. You can set up a vignette that’s written using RMarkdown by typing usethis::use_vignette("spiffypkgname"), which creates a vignette simply named “spiffypkgname”, corresponding to your package name and sets up a template for you to edit with your examples.

### Example Data

If you have example data that you wish to include in your package, you may do so. Most of the time you will want to use inst/exdata to include example files to show how to parse data using the package. See Hadley’s book section on data for a thorough discussion on the different ways data can be incorporated and when you might wish to use them.

### Citation

I did mention citations at the beginning. Ideally your package should be easy to cite when your users type citation("spiffypkgname") they’ll get a nicely formatted example of how to cite your package. To do this you need to create a CITATION file with no file extension in the inst subdirectory of your package. The contents of the CITATION file should look something like what follows here. You don’t need to change year or note, these are autofilled with the values taken from the first two lines. The bibtype will always be “Manual” as well.

year <- sub("-.*", "", meta$Date) note <- sprintf("R package version %s", meta$Version)

bibentry(bibtype = "Manual",
title = "{spiffypkgname}: Spiffy Functions for Spiffy Scientists",
author = c(person("Your", "Name")),
year = year,
note = note,
DOI = "DOI goes here",
url = "spiffypkgname website address goes here")


Wait you say! I don’t have a DOI and I don’t know how to make a website! Fear not, for if you are using GitHub you can easily get a DOI to put in that DOI field in your CITATION file. And by using the pkgdown package you can easily create a website for your package that tells users about it.

### DOI

GitHub has teamed up with Zenodo to offer DOIs to help make your package more citeable. Head over to the GitHub Guides and have a read over the Making Your Code Citeable.

Using OSF (Open Science Framework) is also an option but this does not offer DOIs for individual releases like Zenodo does, which will offer a project DOI and a newly minted DOI for each release, e.g. v1.0.0, v1.0.2, etc.

### Website

pkgdown makes deploying a website a very simple exercise. It takes your README file and your documentation (including your vignette) and creates a nicely organised website for users to browse and learn all about spiffypkgname. Simply use usethis::use_pkgdown() once you commit your changes to GitHub, in the repository settings, set the webpage to “master branch/docs folder” in the GitHub Pages section of the Settings.

The link will appear once it is set. Copy and paste that in the repository webpage field next to the title for your repository (use the “Edit” button on the main page of your repository to fill this field in).

### Other Miscellaneous Items

Insert the GitHub links for the repository and for users to file Issues into the DESCRIPTION file, usethis::use_github_links().

Use goodpractice to help you find issues with your package’s code, goodpractice::gp().

# Wrapping Up

This post was not meant to be all-inclusive. There are plenty of other great resources for getting started with making your own R package. My intention here was to specifically point out items that I think are useful for an academic that might be writing an R package for sharing with others and to highlight some of the newer tools that are now available for getting started.

Of course you are free to join our Slack workgroup as well and ask questions. A few of us have experience with different types of R packages and are happy to offer support. If you wish to join, flick us an e-mail at openplantpathology@gmail.com and we will send you an invite link.