Understanding other people's code


A few days ago I decided to move away from my old Joomla-based blog site and set up my own static web site. After some investigation I settled on using Hugo. I was impressed by some of the web sites I'd found that were using it. Also my expertise in CSS, and JavaScript were pretty limited and Hugo didn't appear to need much of either.

As I started to build the site I found myself treading a famililar path. Every example I looked at did things in different ways so it was hard to work out how to put the learnings together in a coherent way. As is the norm for busy engineers, the developers got things to work and then moved on to other things. "Documentation is boring!"

In some instances I could look at git histories and discern more of the developers' intents, but all too often I ended up with a frustrating effort to reverse engineer what they'd ended up building. It's not like this was a surprise - I've done this dozens, maybe hundreds of times. But I wish, as an industry, we could do better. I've worked with many engineers and tried to persuade them we should, so maybe I can persuade you too?

Names matter

As the joke goes, "there are two hard problems in computer sciences, cache invalidation, naming things, and off-by-one errors".

When it comes to understanding something, however, names really matter. We want them to make sense and not be surprising. The original theme files I'd picked up had some rather odd names. For example the <head> tag had a partial HTML snippet file called header.html while the HTML header was called head.html. These are things that confused me over the last couple of days and would have done so again. They weren't big sources of confusion, but every incremental time would have been more time wasted for me. More importantly, they'd have been an incremental source of confusion to anyone else who read the code for this site.

Good source code is as simple as possible

Developers seem to love to go to one of two extremes. They either refactor nothing and leave large rambling blocks of code that are impossible to follow, or they refactor everything, turning trivial one and two line snippets into functions or methods.

On the first extreme, large amounts of duplicate code is just a recipe for future problems. Bugs don't end being fixed everywhere, or there's just way more code to understand than we should want. If two blocks of code do the same thing then make it one block of code and save future maintainers from having to guess if they're the same or not.

In the other extreme, if refactoring will result in more lines of source code than not, then is it really making life easier for anyone?

Great source code is consistent

One of the things that's most frustrating to me is lack of consistency. Doing the same thing in different ways just makes it harder for everyone who comes after you. Future readers will wonder what makes one different to the other and worry about what they've not understood.

Great software, like great architecture, or art, should feel coherent. It should appear to be the work of a single mind. It has a singular style.

It doesn't really matter what the style is, it should just always feel the same. When addFoo(), subtractFoo(), and multiplyFoo(), are suddenly joined by FooDivide(), the inconsistency is grating. We're left wondering how and why this last one is different from the others. Even if we eventually conclude it isn't, we've expended unecessary time and mental effort thinking about it.

Self documenting code completely misses the point of comments

This also brings me to a rant about commenting of code. There's a school of thought that “self documenting” code is the best approach, but I'd argue that that's ridiculous. How do we tell the difference between buggy code and correct code? How do we tell our future selves about things that we shouldn't do because they don't work well? How do we hint at things that might want to happen in the future?

Yes, the commit logs in git, or whatever revision control system we're using, might provide some hints, but when was the last time you saw a long commentary about why code is the way it is in those logs?

There are certainly bad comments. There are those that don't match up with the code, or just describe obvious syntactic characteristics. But just because it's possible to write bad comments doesn't offset the huge benefit of good ones.

Great comments explain why the code is the way it's written. They describe the paths not taken and the dragons that might be lurking there. They provide the backstory for the code and can offer references to papers, articles, or standards. They help a future reader understand all the thinking that went into it.

Unconvinced? Here's a test

I don't expect to have convinced anyone yet, but here's a test that might help persuade you. Take any significant piece of software that you've written and find another developer who doesn't already understand it. Your role is to answer any questions about your software, while their role is to read it and explain back to you how each part works.

What I've always find interesting about doing this (and it works much better in person) is the dialogue that invariably happens. You reader will likely seek clarifications. Often they'll end up saying to themselves things like “err, oh, hang on, no, that's not what it's doing”. Sometimes they're completely baffled. Occasionally they'll find bugs. Almost never do they scroll quickly through the code just describing it!

Of course, in this test, you're the one that wrote the code, so you understand it completely. As you watch your counterpart struggle, though, you may find yourself offering insights that are neither obvious from the code structure, nor in the comments.

Source code needs to be written for people

So, finally, here's the thing that I wish more software developers thought about.

The objective of writing good source code isn't to make a compiler or interpreter happy, it's for the benefit of future readers. Those people are our most important audience.