When you write an academic paper with associated code, conferences often require or strongly encourage you to submit a virtual machine image containing your development. This is a good idea for code in general, but in this post I’m going to explain why this is a bad idea for Coq proof developments. Take this with a grain of salt, because I’ve just spent 6 hours trying to get my Coq VM to work, and I’m still not done. Note that everything I say here only applies to Coq artifacts, not other types of artifacts.
Why do conferences require VMs?
There are several arguments for requiring VMs:
- It’s easier for the reviewers to check the proofs if they can just boot up the VM and run them.
- The VM is a self-contained artifact that can be archived and referenced in the future.
- The VM allows the reviewers to verify that the proofs are correct
I don’t think any of these arguments hold up.
It’s easier for the reviewers
Having been an artifact reviewer myself, I strongly disagree. It’s much easier to download a zip file and run
make than to install the VM software of the day, figure out how that works, then download a several gigabyte VM image, boot it up, and figure out how to do the build process inside the VM. The reviewers who choose to review Coq artifacts are likely to be already familiar with Coq. Even if they are not familiar with Coq, it’s still easier to install Coq than to install a VM. Using the VM is extra frustrating because it’s unfamiliar, slow, and you often can’t easily copy-paste or transfer files from the VM to your host machine. That’s bad because the VM probably doesn’t have your favorite text editor, and if you’re using a different keyboard layout than the one configured in the VM, then you’re going to have an extra bad time. I’ve resorted to logging into my email account from inside the VM and emailing myself the files I want to copy over, while looking at a picture of the QWERTY keyboard layout to figure out how to type my password.
The VM is a self-contained artifact
This is true, but not useful. If somebody down the line wants to build on your code, they are not going to do so inside a VM; they will want to build it on their actual computer. Therefore, effort of the authors and the reviewers is better spent on making that easy. If the reviewers want to use their own VM, they are still free to do so.
The VM allows the reviewers to verify that the proofs are correct
Not really. If the authors of the paper are malicious, then they can just put a fake proof in the VM. Realistically, the reviewers aren’t even going to read the makefile, so they could just put a fake script in the makefile that does
sleep 5 and prints the output without actually running the proof. However, if the reviewers are willing to spend actual effort to verify that the authors aren’t malicious, how are they possibly going to do that from inside the VM? The VM is a black box prepared by the authors, so it could have a modified version of Coq or even the OS, that has a backdoor that allows the authors to prove false theorems.
If you don’t trust the authors, then you can’t trust the VM. You can however trust your own installation of Coq, so you might as well just download the zip file and build it yourself. If you do trust the authors, then none of this is necessary, because then you can trust they have indeed run
make and did not lie about the output.
Is Coq code actually getting reused?
Yes, Coq code sometimes gets reused. For instance there is the Iris project. However, people aren’t downloading the VMs archived on Zenodo in order to use Iris. They just install the latest version with
opam. In many other cases, the reuse is purely hypothetical, and even when it does happen, you wouldn’t want to use the VM with an ancient version Coq and other software, anyway.
The advantage of a VM
The VM does have one advantage, and that is that several years down the line, people will still be able to modify and build the code inside the VM. Is this valuable? It is undeniably an advantage of the VM, but is the time invested in making and reviewing the VM really the best use of people’s time, compared to other ways of making the code more reusable? I doubt it, for the reasons outlined above.
It would be interesting to have statistics on how often people actually download and use the VMs. I suspect that the answer is “almost never”.
What is actually happening in the artifact review process
The authors spend time setting up the VM. The reviewers then spend time installing the VM, and running
make. What’s the scientific value of this process?
There is some value in making sure that the code is easy to build and install from scratch, because that’s what people who want to build on your artifact would have to do. So let us do that instead.
But there’s a much more serious problem. I will not name names, but I have seen several papers where the theorem statements in the paper do not match the theorem statements in the Coq code. Wouldn’t the time of the artifact reviewers be better spent reading the theorem statements and checking if they match the theorem statements in the paper, to make sure that the Coq proofs actually prove what they’re claimed to prove?