Monthly Archives: March 2012

The Use/Build Fallacy

Working in the language space, especially in language design, you frequently encounter people who fall victim to what I call the “Use/Build Fallacy.” It goes something like this:

Because I know how to use something, I know how to build it as well.

This fallacy is best illustrated by a story I heard from a friend who’s a teacher (another profession that frequently has to deal with this). She was teaching middle-school when teacher conferences rolled around. Talking to the father of one of her students, she explained to him that his daughter was having a lot of trouble in English class and that, based on her observations of how hard the daughter was working, she was pretty sure that the daughter had some sort of language learning disability. She therefore strongly recommended that he take his daughter to an expert to get tested, and that she be tutored by someone trained to deal with the specific kind of learning disability. The father was nonplussed, mainly because he didn’t like the idea that all this would cost him money. “Can’t you just help her more in class?” he asked. My friend explained that she was helping her all she could, but she wasn’t an expert in diagnosing learning disabilities and his daughter really needed to see someone who had the appropriate training.

After a bit of back-and-forth, the father finally got exasperated and said, “Fine, I’ll just tutor her myself! I mean, how hard could it be? I went to school!” My friend then shot back, “Look, you’re a general contractor, right? What would you think if I came to you and said, ‘I don’t need you to build my house—I’ve lived in a house before, so how hard could it be to build one myself?” This, finally, stumped him. I’m not sure whether he actually got his daughter the help she needed, but the story stuck with me because my friend’s response is the perfect distillation of the Use/Build Fallacy.

Note that I’m not saying that just because you’re not an expert on something you can’t have an opinion. I may not know how to build a house, but that doesn’t mean I have nothing to say to the contractors if I decide to do some renovations on my house. Not falling prey to the fallacy, though, means that I always keep a healthy respect for the expert in a field—as long as they truly seem to know what they’re talking about. (I hear this from friends who are architects all the time—they get hired by someone to build or renovate a house for them, and then their client spends all their time endlessly arguing with everything they do. Why bother to hire an expert if you think you already know how to do it yourself?)

I try to remember this myself every time I encounter some aspect of some programming language that I don’t like. Right now, I’m neck-deep in C++ code and it’s tempting to spend all my time kvetching about how how horrible a job Bjarne has done over the years. And then I try to remember—even as someone who’s actually built a language—that this stuff is hard. A language of any complexity has a huge number of moving parts, all of which interact with each other in an unpredictable manner. Historical choices can come back to bite you in all sorts of unexpected ways. Oftentimes all you have are a bunch of imperfect choices, and you have to simply pick the least bad of them all. And then you get to sit there and listen to everyone on the sidelines complain about how horrible a job you’ve done and how they could do it so much better than you because, hey, they’ve used a programming language before.

So I try to temper my complaints with a little humility, and remember how much different building is from using.