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Comment by tikhonj | Hacker News Reader

The rule of 3 is awful because it focuses on the wrong thing. If two instances of the same logic represent the same concept, they should be shared. If 10 instances of the same logic represent unrelated concepts, they should be duplicated.

The goal is to have code that corresponds to a coherent conceptual model for whatever you are doing, and the resulting codebase should clearly reflect the design of the system. Once I started thinking about code in these terms, I realized that questions like "DRY vs YAGNI" were not meaningful.

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Of course, the rule of 3 is saying that you often _can't tell_ what the shared concept between different instances is until you have at least 3 examples.

It's not about copying identical code twice, it's about refactoring similar code into a shared function once you have enough examples to be able to see what the shared core is.

3 just seems arbitrary in practice though. In my job we share code when it makes sense and don’t when it doesn’t, and that serves us just fine

But don’t let the rule of 3 be an excuse for you to not critically assess the abstract concepts that your program is operating upon and within.

I too often see junior engineers (and senior data scientists…) write code procedurally, with giant functions and many, many if statements, presumably because in their brain they’re thinking about “1st I do this if this, 2nd I do that if that, etc”.

I agree. And I think this also distills down to Rob Pike’s rule 5, or something quite like it. If your design prioritizes modeling the domain’s data, shaping algorithms around that model, it’s usually trivial to determine how likely some “duplication” is operating on shared concepts, versus merely following a similar pattern. It may even help you refine the data model itself when confronted with the question.

Agreed. DRY is a compression algorithm. The rule of 3 is a bad compression algorithm. Good abstraction is not compression at all.

The devil’s in the details, as usual. No rule should be followed to the letter, which is what the top comment was initially complaining about.

Yet again, understanding when to follow a rule of thumb or not is another thing that separates the junior from the senior.

> If two instances of the same logic represent the same concept, they should be shared. If 10 instances of the same logic represent unrelated concepts, they should be duplicated.

Exactly.

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