The last thing you need to do before your package is a proper R package is to choose a license for your software and specify the license within the DESCRIPTION file.

Software licenses are about two things: copyright, and protecting yourself from being held liable if your software screws something up somewhere down the line.

I know next to nothing about copyright law outside the United States, but in the US, copyright is automatic (you don’t need to write “© 2014 A. Pendantic Person, All Rights Reserved” all over the place, or even once), and it gives you exclusive rights to copy your code. So if you don’t choose a license for your software, no one else can use it!

So, pick a license, any license.

There are lots of different software licenses to choose from. That’s part of why this is such a painful topic. (That they are almost all really boring to read is another part of the pain. The WTFPL is one of the few that is not boring.)

Personally, I choose between the MIT license and the GNU General Public License (GPL). The MIT license is among the more permissive. The GPL is “viral” in that it extends to derivative works: software that incorporates code that was licensed under the GPL must also be licensed under the GPL. So I use the GPL if I have to (that is, if I’ve incorporated others’ GPL code), and I use the MIT license otherwise.

Don’t use a Creative Commons license for software

An important thing to remember: don’t use a Creative Commons (CC) license for software. Creative Commons licenses (like CC-BY) are great, but they’re for things like articles, books, and videos, but not software. As they say in their FAQ:

We recommend against using Creative Commons licenses for software.

Use CC licenses for your lecture notes, slides, and articles, but not for your software.

What about CC0?

CC0 (public domain) seems appropriate for software: you’re just saying that anyone can do anything with the code.

But in some states (e.g., Maryland, I think), software is treated as a “good” (like a car), and so if your code causes something terrible to happen, you could be sued for damages. Using a lenient license, like the MIT license, eliminates that potential problem through the “no warranty” clause.

So, use CC0 for your lecture notes, slides, and web sites, but use a lenient license, like the MIT license, for your software.

Indicating your choice in your package

So, pick a license, any license. And then indicate your choice in the DESCRIPTION file for your R package.

GNU General Public License (GPLv3)

If you choose the GPL, note that there are multiple versions. GPLv2 is the old version. Don’t choose that. Choose the newer one, GPLv3. The newer one fixes some loopholes in the older one.

To use the GPLv3 with your R package, include the following line in your DESCRIPTION file.

License: GPL-3

That’s it.

MIT license

If you’re not incorporating code that is licensed under the GPL, I recommend going with the MIT license.

Unfortunately, and for reasons that I don’t understand, the R Core considers the MIT license to be not a proper license but rather a template for a license. And so if you want to use the MIT license, you must include a LICENSE file in your package that includes just two lines, like this (example here):

YEAR: 2014
COPYRIGHT HOLDER: Karl W Broman

See the license template at http://www.r-project.org/Licenses/MIT.

Then, in your DESCRIPTION file, include the following line.

License: MIT + file LICENSE

The all caps LICENSE in that line is the name of the file (within your package) with the text about year and copyright holder. You can also call the file LICENCE if you want. In this case, the relevant line in your DESCRIPTION file should be the following.

License: MIT + file LICENCE

(I’d thought that you could use a different name for the file, for example License.txt, but the Writing R Extensions manual seems pretty explicit that the file should be either LICENSE or LICENCE.)

With this, our package looks like this, and it is now a proper R package.


Homework

Pick a license for your software, make the appropriate change to your DESCRIPTION file, and if necessary add a LICENSE file.

Then go to the page about checking an R package.