Programming is hard. I made some really silly mistakes. Avoid globals, embrace code review and tests.


When I was speaking with my grad school advisor a few weeks ago, we had the idea do some basic analysis on unsafe usage in Rust. It’s a fairly simple concept, so I quickly wrote up something to read a file line-by-line and count lines in unsafe functions or blocks.

Fun story time: Making sure that the script doesn’t think the unsafe block/function ends too early is a simplification of one of my favorite “easy” programming questions – verify that a statement with various delimiters is valid (e.g. that the open and close delimiters match). You can probably see that in my code since I love pushing delimiters to stacks (even when I don’t need to). Enough about my weird hobbies.

I was then distracted by a conference for a week, and when I returned to work yesterday, I finished my script off by writing something to interact with Github. The idea is that you could run this over all Rust projects in Github (or some subset–I just wrote it yesterday people!) and see how people are using unsafe.

Instead of discussing what we might be able to learn from the script’s correct results, this post is about the ways I messed it up and then posted the mistakes to twitter.

Mistake: abuse of global variables

Yup. I stored all of my counts in globals. Yes, I heard my Intro to Programming professor’s voice in my head saying “DON’T use globals unless you have to!!!”

I forgot to clear the counts between repositories.

I noticed this right as I published a tweet about my work. “Why are these numbers monotonically increasing,” I thought. Then I screeched and immediately deleted it.

Globals abuse

Mistake: abuse of endless if/if/elif/else/more ifs

Ok, so that was a pretty gigantic mistake, right? Every real developer knows to avoid globals and if you don’t, make sure you reset them. Oops. But at least I fixed it before I told The Whole World (who reads Rust-related tweets) about this. That’s great–so I tweeted. I even mentioned my near miss!

As it turns out, I had another glaring mistake. This morning, I started adding some stats on how many panic!s there are. As I read over my code, I thought, “something is wrong here…”

 for line in open(filename).readlines():
        # skip comment lines
        if re.match(regex, line):
            comment += 1
        # skip blank lines
        if len(line.strip()) == 0:
            blank += 1

        code += 1

In case you missed it (like I did), I say I’m skipping comments/blank lines and then I don’t! It’s an easy fix, but why did I decide to make such an awful cascade of ifs? No clue. Seemed like a good idea at the time.

Endless if

Mistake ?

So this is where I’m currently at. It doesn’t 100% match up with the comparable cloc stats, but it’s a lot closer.

More mistakes?


Don’t be like me.

Listen to your professors

Avoid globals. Please don’t take my degree away.

Make your code easier to read

If it’s easier to read, it’s easier to catch silly mistakes (this is why I avoid Perl).

Code reviews are the best

Someone else definitely would have caught this in a review (I hope)

Testing is also the best

If I’d initially compared my results to cloc output, I probably would have caught these mistakes earlier and not told all of Twitter.


The errors I detail here are logic errors–I, the programmer, did something horribly wrong. This is a huge problem in security implementations. I work on practical security and implementations…so…buries face in formal verification papers

Rust wouldn’t have saved me from these particular mistakes, but it has saved me from tons of other mistakes, like typos that would result in similar logic errors.

I’ve been programming for 8 years. I’ve had one job or another (that pays me to program!) for 5 of those years. In college, I helped teach intro to programming (do as I say, not as I do, please). I still make really silly mistakes.

Some developers will tell you that real devs don’t make mistakes like this. If that’s the case, then I guess I’m not a real dev (and I’m fine with that). I think it’s probably more likely that they’re lying or they don’t do much work.