“Whenever you see yourself writing the same thing down more than once, there’s something wrong and you shouldn’t be doing it, and the reason is not because it’s a waste of time to write something down more than once. It’s because there’s some idea here, a very simple idea, which has to do with the Sigma notation…not depending upon what it is I’m adding up. And I would like to be able to always…divide the things up into as many pieces as I can, each of which I understand separately. I would like to understand the way of adding things up, independently of what it is I’m adding up.”
- Gerald Sussman, SICP Lecture 2a, “Higher-order Procedures” (emphasis added)

The purpose of abstracting is not to be vague, but to create a new semantic level in which one can be absolutely precise.
- Edsger W. Dijkstra, The Humble Programmer

What Larry Wall said about Perl holds true: “When you say something in a small language, it comes out big. When you say something in a big language, it comes out small.” The same is true for English. The reason that biologist Ernst Haeckel could say “Ontogeny recapitulates phylogeny” in only three words was that he had these powerful words with highly specific meanings at his disposal. We allow inner complexity of the language because it enables us to shift the complexity away from the individual utterance.
- Hal Fulton, The Ruby Way, Introduction (emphasis added)

Programming is our thoughts, and with better ways to express them, we can spend more time thinking them, and less time expressing them.

3 + 3 + 3 + 3 + 3 + 3 is hard…hard to read (how many threes?), hard to get right (I lost count!), hard to reason about (piles of operations!). 3 x 6 is easy, once you learn multiplication. This is a good trade-off. We should look for ways to add abstractions, new semantic levels, to our programs.

If you’re doing the same thing twice, stop, and look for the common idea. Peel the idea away from the context, from the details. Grasp the idea, and then use it over and over. As a bonus, you’ll type less, re-use code, and debug less.

“But I can’t find ways to do that!”

When you look at similar bits of code, and can’t find a good way to remove the duplication, you’re hitting the limits of either your language, or your knowledge.

Programming languages put up very real walls, they force you down their paths, often by leaving out features. A language without recursion puts up a wall in front of recursive solutions; a language without first-class functions makes it tough to write higher-order functions. Language limitations are the cause of Greenspun’s Tenth Rule.

Sometimes, the language is not the problem. Sometimes you just can’t find your way through. This is why you read Refactoring, and Design Patterns, but really, this is why you learn other programming languages. Think about the right way to factor the problem.

If you can’t remove the duplication, you need to work around your language, or learn some new tricks.