by Leon Rosenshein

Chesterton's Fence

I’ve mentioned Chesterton’s Fence in a couple of previous blogs, but I never did define it. Chesterton’s Fence is a parable about how lacking context can be dangerous. In his 1929 book, The Thing, Chesterton wrote:

In the matter of reforming things, as distinct from deforming them, there is one plain and simple principle; a principle which will probably be called a paradox. There exists in such a case a certain institution or law; let us say, for the sake of simplicity, a fence or gate erected across a road. The more modern type of reformer goes gaily up to it and says, “I don’t see the use of this; let us clear it away.” To which the more intelligent type of reformer will do well to answer: “If you don’t see the use of it, I certainly won’t let you clear it away. Go away and think. Then, when you can come back and tell me that you do see the use of it, I may allow you to destroy it.

Or, in other words, if you don’t know why something was done, don’t undo it without figuring out why it was done in the first place. Only when you know why something was put in place can you decide if it’s not be needed any more. Consider the typical bathroom sink.

Many of them have a small hole or two in the near side a little below the top. It just sits there. Nothing comes out of it. Nothing goes into it. So they should be removed, right?

Not so fast. In normal operation those holes are unused. But bathroom sink drains are notorious for being slow. Not blocked, because that’s obvious and gets fixed, but slow. And very often, when in the bathroom using the sink with the water running you’re doing something else. Brushing your teeth, shaving, removing makeup, or just distracted.

That’s when those holes come into play. As the drain runs slowly the water builds up in the sink. Eventually so much water builds up that it starts to run through those holes and down the drain. It’s not perfect, but it helps, and gives you a little more time to correct the problem. But if you never saw them used you might by a sink without them, then find a small lake in your bathroom.

The same thing can happen in code. You want to break long lines at a space if possible. As you’re going through the code you come across a strange byte-counting/examining loops. You’re confused because it doesn’t just start at column 80 and go backward looking for a ``. It starts at the beginning, checks values, skips spaces checks more, and keeps track of the last space it found. Very complicated. So you remove it and go with the simple solution.

You write your tests, deploy, and close out the task. Done and done. Then suddenly you start getting bug reports. Turns out you assumed that all of the input was simple ASCII code, but in reality it’s Unicode/MultiByte characters, so you can’t do the simple thing. Without enough context, you broke the code.

That’s Chesterton’s fence. So the next time you want to remove some seemingly unimportant code, make sure you know why it was added in the first place.