Chapter 4. Package Metadata

The job of the DESCRIPTION file is to store important metadata about your package. When you first start writing packages, you’ll just use this metadata to record what packages are needed to run your package. However, as time goes by and you start sharing your package with others, the metadata file becomes increasingly important because it specifies who can use it (the license) and whom to contact (you!) if there are any problems.

Every package must have a DESCRIPTION. In fact, it’s the defining feature of a package (RStudio and devtools consider any directory containing DESCRIPTION to be a package). To get you started, devtools::create("mypackage") automatically adds a bare-bones description file. This allows you to start writing the package without having to worry about the metadata until you need to. The minimal description will vary a bit depending on your settings, but should look something like this:

Package: mypackage
Title: What The Package Does (one line, title case required)
Version: 0.1
Authors@R: person("First", "Last", email = "first.last@example.com",
                  role = c("aut", "cre"))
Description: What the package does (one paragraph)
Depends: R (>= 3.1.0)
License: What license is it under?
LazyData: true

(If you’re writing a lot of packages, you can set global options via devtools.desc.author, devtools.desc.license, devtools.desc.suggests, and devtools.desc. See package?devtools for more details.)

DESCRIPTION uses a simple file format called the Debian control format (DCF). You can see most of the structure in the simple example shown here. Each line consists of a field name and a value, separated by a colon. When values span multiple lines, they need to be indented:

Description: The description of a package is usually long,
    spanning multiple lines. The second and subsequent lines
    should be indented, usually with four spaces.

This chapter will show you how to use the most important Description fields.

Dependencies: What Does Your Package Need?

It’s the job of the Description to list the packages necessary for your package to work. R has a rich set of ways of describing potential dependencies. For example, the following lines indicate that your package needs both ggvis and dplyr to work:

Imports:
    dplyr,
    ggvis

Conversely, the lines here indicate that while your package can take advantage of ggvis and dplyr, they’re not required to make it work:

Suggests:
    dplyr,
    ggvis,

Both Imports and Suggests take a comma-separated list of package names. I recommend putting one package on each line, and keeping them in alphabetical order. That makes it easy to skim.

Imports and Suggests differ in their strength of dependency:

Imports

Packages listed in Imports must be present for your package to work. In fact, any time your package is installed, those packages will, if not already present, be installed on your computer (devtools::load_all() also checks that the packages are installed).

Adding a package dependency here ensures that it’ll be installed. However, it does not mean that it will be attached along with your package (i.e., library(x)). The best practice is to explicitly refer to external functions using the syntax package::function(). This makes it very easy to identify which functions live outside of your package. This is especially useful when you read your code in the future.

If you use a lot of functions from other packages, this is rather verbose. There’s also a minor performance penalty associated with :: (on the order of 5µs, so it will only matter if you call the function millions of times). You’ll learn about alternative ways to call functions in other packages in “Imports”.

Suggests

Your package can use these packages, but doesn’t require them. You might use suggested packages for example datasets, to run tests, build vignettes, or maybe there’s only one function that needs the package.

Packages listed in Suggests are not automatically installed along with your package. This means that you need to check if the package is available before using it (use requireNamespace(x, quietly = TRUE)). There are two basic scenarios:

# You need the suggested package for this function    
my_fun <- function(a, b) {
  if (!requireNamespace("pkg", quietly = TRUE)) {
    stop("Pkg needed for this function to work. Please install it.",
      call. = FALSE)
  }
}

# There's a fallback method if the package isn't available
my_fun <- function(a, b) {
  if (requireNamespace("pkg", quietly = TRUE)) {
    pkg::f()
  } else {
    g()
  }
}

When developing packages locally, you never need to use Suggests. When releasing your package, using Suggests is a courtesy to your users. It frees them from downloading rarely needed packages, and lets them get started with your package as quickly as possible.

The easiest way to add Imports and Suggests to your package is to use devtools::use_package(). This automatically puts them in the right place in your DESCRIPTION, and reminds you how to use them. For example:

devtools::use_package("dplyr") # Defaults to imports
#> Adding dplyr to Imports
#> Refer to functions with dplyr::fun()
devtools::use_package("dplyr", "Suggests")
#> Adding dplyr to Suggests
#> Use requireNamespace("dplyr", quietly = TRUE) to test if package is 
#>  installed, then use dplyr::fun() to refer to functions.

Versioning

If you need a particular version of a package, specify it in parentheses after the package name:

Imports:
    ggvis (>= 0.2),
    dplyr (>= 0.3.0.1)
Suggests:
    MASS (>= 7.3.0)

You almost always want to specify a minimum version rather than an exact version (MASS (== 7.3.0)). Because R can’t have multiple versions of the same package loaded at the same time, specifying an exact dependency dramatically increases the chance of problems.

Versioning is most important when you release your package. Usually people don’t have exactly the same versions of packages installed that you do. If someone has an older package that doesn’t have a function your package needs, they’ll get an unhelpful error message. However, if you supply the version number, they’ll get an error message that tells them exactly what the problem is: an out-of-date package.

Generally, it’s always better to specify the version and to be conservative about which version to require. Unless you know otherwise, always require a version greater than or equal to the version you’re currently using.

Other Dependencies

There are three other fields that allow you to express more specialized dependencies:

Depends: Prior to the rollout of namespaces in R 2.14.0, Depends was the only way to “depend” on another package. Now, despite the name, you should almost always use Imports, not Depends. You’ll learn why, and when you should still use Depends, in Chapter 8.; You can also use Depends to require a specific version of R (e.g., Depends: R (>= 3.0.1)). As with packages, it’s a good idea to play it safe and require a version greater than or equal to the version you’re currently using. devtools::create() will do this for you.; In R 3.1.1 and earlier, you’ll also need to use Depends: methods if you use S4. This bug is fixed in R 3.2.0, so methods can go back to Imports where they belong.
LinkingTo: Packages listed here rely on C or C++ code in another package. You’ll learn more about LinkingTo in Chapter 10.
Enhances: Packages listed here are “enhanced” by your package. Typically, this means you provide methods for classes defined in other packages (a sort of reverse Suggests). But it’s hard to define what that means, so I don’t recommend using Enhances.

You can also list things that your package needs outside of R in the SystemRequirements field. But this is just a plain-text field and is not automatically checked. Think of it as a quick reference; you’ll also need to include detailed system requirements (and how to install them) in your README.

Title and Description: What Does Your Package Do?

The title and description fields describe what the package does. They differ only in length:

Title is a one-line description of the package, and is often shown in the package listing. It should be plain text (no markup), and follow headline-style capitalization; it should not end in a period. Keep it short: listings will often truncate the title to 65 characters.
Description is more detailed than the title. You can use multiple sentences but you are limited to one paragraph. If your description spans multiple lines (and it should!), each line must be no more than 80 characters wide. Indent subsequent lines with four spaces.

The Title and Description for ggplot2 are as follows:

Title: An implementation of the Grammar of Graphics
Description: An implementation of the grammar of graphics in R. It combines 
    the advantages of both base and lattice graphics: conditioning and shared 
    axes are handled automatically, and you can still build up a plot step 
    by step from multiple data sources. It also implements a sophisticated 
    multidimensional conditioning system and a consistent interface to map
    data to aesthetic attributes. See the ggplot2 website for more information, 
    documentation and examples.

A good title and description are important, especially if you plan to release your package to CRAN, because they appear on the CRAN download page, shown in Figure 4-1.

Because Description only gives you a small amount of space to describe what your package does, I also recommend including a README.md file that goes into much more depth and shows a few examples. You’ll learn about that in “README.md”.

Author: Who Are You?

To identify the package’s author, and whom to contact if something goes wrong, use the Authors@R field. This field is unusual because it contains executable R code rather than plain text. Here’s an example:

Authors@R: person("Hadley", "Wickham", email = "hadley@rstudio.com",
  role = c("aut", "cre"))

This command says that both the author (aut) and the maintainer (cre) is Hadley Wickham, and that his email address is hadley@rstudio.com. The person() function has four main arguments:

The name, specified by the first two arguments, given and family (these are normally supplied by position, not name). In Western cultures, given (first name) comes before family (last name), but this convention does not hold in many Eastern cultures.
The email address.
A three-letter code specifying the role. There are four important roles:

cre

The creator or maintainer, the person you should bother if you have problems.

aut

Authors, those who have made significant contributions to the package.

ctb

Contributors, those who have made smaller contributions, like patches.

cph

Copyright holder. This is used if the copyright is held by someone other than the author, typically a company (i.e., the author’s employer).

(The full list of roles is extremely comprehensive. Should your package have a woodcutter (wdc), lyricist (lyr), or costume designer (cst), rest comfortably that you can correctly describe that person’s role in creating your package.)

If you need to add further clarification, you can also use the comment argument and supply the desired information in plain text.

You can list multiple authors with c():

Authors@R: c(
    person("Hadley", "Wickham", email = "hadley@rstudio.com", role = "cre"),
    person("Winston", "Chang", email = "winston@rstudio.com", role = "aut"))

Every package must have at least one author (aut) and one maintainer (cre); they might be the same person. The creator must have an email address. These fields are used to generate the basic citation for the package (e.g., citation("pkgname")). Only people listed as authors will be included in the autogenerated citation. There are a few extra details if you’re including code that other people have written. This typically occurs when you’re wrapping a C library, so it’s discussed in Chapter 10.

In addition to your email address, it’s a good idea to list other resources available for help. You can list URLs in URL. Multiple URLs are separated with a comma. BugReports is the URL where bug reports should be submitted. For example, knitr has:

URL: http://yihui.name/knitr/
BugReports: https://github.com/yihui/knitr/issues

You can also use separate Maintainer and Author fields. I prefer not to use these fields because Authors@R offers richer metadata.

On CRAN

The most important thing to note is that your email address (i.e., the address of cre) is the address that CRAN will use to contact you about your package. Make sure you use an email address that’s likely to be around for a while. Also, because this address will be used for automated mailings, CRAN policies require that this be for a single person (not a mailing list) and that it does not require any confirmation or use any filtering.

License: Who Can Use Your Package?

The License field can be either a standard abbreviation for an open source license, like GPL-2 or BSD, or a pointer to a file containing more information, file LICENSE. The license is really only important if you’re planning on releasing your package. If you don’t, you can ignore this section. If you want to make it clear that your package is not open source, use License: file LICENSE and then create a file called LICENSE, containing, for example:

Proprietary 

Do not distribute outside of Widgets Incorporated.

Open source software licensing is a rich and complex field. Fortunately, in my opinion, there are only three licenses that you should consider for your R package:

YEAR: <Year or years when changes have been made>
COPYRIGHT HOLDER: <Name of the copyright holder>

MIT: This is a simple and permissive license, similar to the BSD 2- and 3-clause licenses. It lets people use and freely distribute your code subject to only one restriction: the license must always be distributed with the code. The MIT license is a “template,” so if you use it, you need License: MIT + file LICENSE, and a LICENSE file.
GPL-2 or GPL-3: These are “copy-left” licenses. This means that anyone who distributes your code in a bundle must license the whole bundle in a GPL-compatible way. Additionally, anyone who distributes modified versions of your code (derivative works) must also make the source code available. GPL-3 is a little stricter than GPL-2, closing some older loopholes.
CC0: This license relinquishes all your rights on the code and data so that it can be freely used by anyone for any purpose. This is sometimes called putting it in the public domain, a term that is neither well defined nor meaningful in all countries. This license is most appropriate for data packages. Data, at least in the United States, is not copyrightable, so you’re not really giving up much. This license just makes this point clear.

If you’d like to learn more about other common licenses, GitHub’s choosealicense.com is a good place to start. Another good resource is tldrlegal.com/, which explains the most important parts of each license. If you use a license other than the three I suggest, make sure you consult the “Writing R Extensions” section on licensing.

If your package includes code that you didn’t write, you need to make sure you’re in compliance with its license. This occurs most commonly when you’re including C source code, so it’s discussed in more detail in Chapter 10.

On CRAN

If you want to release your package to CRAN, you must choose a standard license. Otherwise, it’s difficult for CRAN to determine whether or not it’s legal to distribute your package! You can find a complete list of licenses that CRAN considers valid at https://svn.r-project.org/R/trunk/share/licenses/license.db.

Version

Formally, an R package version is a sequence of at least two integers separated by either . or -. For example, 1.0 and 0.9.1-10 are valid versions, but 1 or 1.0-devel are not. You can parse a version number with numeric_version:

numeric_version("1.9") == numeric_version("1.9.0")
#> [1] TRUE
numeric_version("1.9.0") < numeric_version("1.10.0")
#> [1] TRUE

For example, a package might have a version 1.9. This version number is considered by R to be the same as 1.9.0, less than version 1.9.2, and all of these are less than version 1.10 (which is version “one point ten,” not “one point one zero”). R uses version numbers to determine whether package dependencies are satisfied. A package might, for example, import package devtools (>= 1.9.2), in which case version 1.9 or 1.9.0 wouldn’t work.

The version number of your package increases with subsequent releases of a package, but it’s more than just an incrementing counter—the way the number changes with each release can convey information about what kind of changes are in the package.

I don’t recommend taking full advantage of R’s flexiblity. Instead, always use . to separate version numbers.

A released version consists of three numbers, <major>.<minor>.<patch>. For version number 1.9.2, 1 is the major number, 9 is the minor number, and 2 is the patch number. Never use versions like 1.0; instead, always spell out the three components (i.e., 1.0.0).

An in-development package has a fourth component: the development version. This should start at 9000. For example, the first version of the package should be 0.0.0.9000. There are two reasons for this recommendation: first, it makes it easy to see if a package is released or in-development, and second, the use of the fourth place means that you’re not limited to what the next version will be. 0.0.1, 0.1.0, and 1.0.0 are all greater than 0.0.0.9000.

Increment the development version (e.g., from 9000 to 9001) if you’ve added an important feature that another development package needs to depend on.

If you’re using svn, instead of using the arbitrary 9000, you can embed the sequential revision identifier.

The advice here is inspired in part by Semantic Versioning and by the X.Org versioning schemes. Read them if you’d like to understand more about the standards of versioning used by many open source projects.

We’ll come back to version numbers in the context of releasing your package in “Version Number”. For now, just remember that the first version of your package should be 0.0.0.9000.

Other Components

A number of other fields are described elsewhere in the book:

Collate controls the order in which R files are sourced. This only matters if your code has side effects (most commonly because you’re using S4). This is described in more depth in “S4”.
LazyData makes it easier to access data in your package. Because it’s so important, it’s included in the minimal description created by devtools. It’s described in more detail in Chapter 9.

There are actually many other rarely used fields. A complete list can be found in the “The DESCRIPTION file” section of the R extensions manual. You can also create your own fields to add additional metadata. The only restrictions are that you shouldn’t use existing names, and that, if you plan to submit to CRAN, the names you use should be valid English words (so a spell-checking NOTE won’t be generated).

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R Packages by Hadley Wickham

Chapter 4. Package Metadata

Dependencies: What Does Your Package Need?

Versioning

Other Dependencies

Title and Description: What Does Your Package Do?

Figure 4-1. The CRAN download page for ggplot2

Author: Who Are You?

On CRAN

License: Who Can Use Your Package?

On CRAN

Version

Other Components

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