Frank Mitchell

Hard bugs and unknown assumptions

Assumptions color everything we do, and I’m pretty sure you can’t function without them. Given how complex the world is, at some point you just have to go, “Yeah, I assume it’s going to work” and move on. But the worst assumptions, the ones that cause you to tear your hair out or cry yourself to sleep in frustration, are the ones you don’t even know you’re making.

Hard bugs

When it comes to programming, the hardest bugs to fix are the ones where a base assumption gets violated. You assume something to be true, only to find out it’s false. Here’s an example with a bit of Ruby I was working on the other day.

results = []
mgram = []
characters.each do |letter|
  mgram << letter
  mgram = mgram[1, size] if mgram.size > size
  results << mgram if mgram.size == size

Every time the loop terminated, the results array would be filled with strings of letters (m-grams) that were longer than the size variable. That wasn’t at all what I intended, and it drove me bonkers. I stepped through the code checking my assumptions.

Which of those assumptions is wrong?

As it turns out, none of them. The place I’d gone wrong was in assuming that appending to an array created a copy. But it didn’t; it created a reference. So when the loop came around again and added another letter to mgram it also put one onto the end of the m-gram I’d just put in results. I fixed it by changing results << mgram to results << mgram.clone.

This kind of thing happens all the time. I’m pretty sure it’s the reason so many programmers distrust documentation and suffer from NIH. There are so many layers between our ideas and what’s really going on, that we have to assume things are working correctly if we want to get anything done.

And when our assumptions go wrong, we’re left with really hard bugs.

Growing persistent assumptions

It’s easy to ditch assumptions you can shine a critical light on. Assumptions you can reason about and test are easy to verify. “Hmm… I turned on the dishwasher and all the lights went out. I’ll assume the fuse is blown.” Easy to think about. Easy to check. Easy to solve.

The assumptions that are hard to let go of are the ones you don’t even know you’re holding. Such base assumptions are typically formed by your first exposure to a new idea.

Here’s an example.

When I first learned to program, it was with the use of XSLT to transform XML files into web pages. Though I didn’t know I was programming, that first act permanently colored my view. Even after getting a Computer Science degree and spending years in industry, I still view programming as the art of writing down information and the functions that transform it into a usable form.

This idea of programming is totally different (and often alien) to someone who grew up programing with punch cards. For them, programing is the art of optimizing for size and speed. Those were the things that were most important when they were exposed to the idea. Those are the things that are most important now.

These base assumptions form without conscious thought because our brains are wonderful pattern recognition engines. They take in new information and form a pattern about it. Any similar information we encounter in the future is automatically filtered through that pattern, which just reinforces it.

While subsequent exposures might modify the pattern, it’s the first exposure that imprints strongest. That exposure doesn’t have to be for any significant duration. An offhand comment by a friend works as well as a college lecture. The only thing that matters is that the information is new.

Continuous learning is an antidote

As soon as you identify a base assumption, you have the opportunity to change it. But how do you recognize it’s there in the first place?

The process that works for me is to keep learning as much as I can about as many different things as possible. I can only keep a finite amount of information in my head. The more I learn, the more assumptions I have to challenge and let go of in order to make room for new ideas.

Then I flop down on the floor, stare at the ceiling, and turn those ideas over in my head. I go through the usual round of questions. “Is this true?”, “Do I believe it?”, and “How does it fit with what I already know?” are all good starting points.

Usually somewhere in that process something will click, and I’ll recognize that I’m holding onto an assumption. When that happens, I can call it out, look it over, and decide wether to keep it or not.