Object Mentor Blog: The Fact/Intention Gap

The Fact/Intention Gap

Michael Feathers — Tue, 21 Oct 2008 11:57:00 -0500

The other day, I was browsing some code with a team and we came across a cluster of static methods. I looked at them and asked whether they were used someplace else without an object reference. I could’ve done a find references in the IDE, but one of the team members had the answer: “No, they’re just used here.”

“Okay, so why are they static?”, I asked.

“Well, the IDE pointed out that they don’t refer to any instance data, so I made them static.”

Technically, there’s nothing wrong with that. I do the same thing sometimes. I make a method static to document its independence of instance data. But, this scenario highlights something important about static and a few other keywords: their uses can be seen as statements of fact or statements of intention. Static can be read as “Hey, this function doesn’t use any instance data, and you should know that.” Or, it can be read as “I am making this static so that it can be used easily anyplace without an instance.” When you’re trying to read a pile of unfamiliar code, it’s nice to know whether you can count on one meaning or the other.

I don’t think I’ve ever heard anyone articulate this directly, but there’s a tendency in a lot of best practice literature to close the gap between fact and intention. Joshua Bloch’s advice to use make fields final whenever you can in Java is a good example. The traditional advice to make methods private whenever they are not used outside of a class is another.

Part of me feels that closing the gap is a good practice but the fact is, there are holes in it. If you are developing a library or a framework, you do have to write code that may not reflect the facts of your code, but rather will reflect the facts of your code and the code that people write to use it or extend it. Beyond that, it might be overly conservative to reduce visibility of things that aren’t used beyond a particular scope. For instance, imagine a method that is used only by other methods of a class. We understand the invariant of the class and we see that the method could be public. We could make it private now, but that means that anyone who wants to make it public later would have to do some re-analysis to arrive at the answer that we have right now.

The Fact/Intention Gap is a very real thing. Whether we know it or not, we confront it every time we try to understand unfamiliar code. I think there’s only one way to solve it, and that’s to try to separate fact from intention in our languages and tooling. Imagine what it would be like if your IDE gave you a visual indicator for all methods which didn’t use instance data. If it did, you could use static on methods only when you want to indicate that the intention is to use them without an instance.

It seems that IDE developers are moving in this direction. I wonder if any language designers will follow suit.

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