Rule of Three

The Rule of Three in software says that you should be willing to duplicate a piece of code once, but on the third copy you should refactor. This is a refinement on DRY (Don’t Repeat Yourself) accounting for the fact that it might not necessarily be obvious how to eliminate a duplication, and waiting until the third occurrence might clarify. (See also The Wrong Abstraction.)

LLMs love to duplicate code. Think about how, absent any other prompting, if you ask an LLM to modify a program, it will spit out a new copy of the program with the changes requested. For an LLM to decide to refactor code to remove duplication requires a moment where the LLM proactively decides to go out of their way to do some cleanup. (Actually, Sonnet 3.7 also has some of this inclination, but if you look at the Claude Code system prompt, they specifically instruct the model to not be too proactive, since it’s easy for the model to lose track of what it’s doing.) To make matters worse, if code is copy pasted everywhere in your codebase already, the LLM will assume that you want it to continue copy pasting the code everywhere. It’s up to you to ask the model to reduce duplication.

Examples

• I asked the LLM to write tests for some functionality, and it ended up duplicating lots of logic between tests. I had to explicitly ask it to factor helper methods when doing so. (Another difficulty: when the LLM is writing a new test, it needs to know about the helper to use it. In agent mode, you better hope it looks at the right file to find out about the helper!)