Comment by 9rx | Hacker News Reader

Comment by 9rx | Hacker News Reader

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Trouble is that TDD, and formal proofs to much the same extent, assume a model of "double entry accounting". Meaning that you write both the test/proof and the implementation, and then make sure they agree. Like in accounting, the assumption is that the probability of you making the same mistake twice is fairly low, giving high confidence to accuracy when they agree. When there is a discrepancy, then you can then unpack if the problem is in the test/proof or the implementation. The fallible human can easily screw either.

But if you only fill out one side of the ledger, so to speak, an LLM will happily invent something that ensures that it is balanced, even where your side of the entry is completely wrong. So while this type of development is an improvement over blindly trusting an arbitrary prompt without any checks and balances, it doesn't really get us to truly verifying the code to the same degree we were able to achieve before. This remains an unsolved problem.

I don't fully understand what you mean by accounting expects the probability of making the same mistake twice is fairly low? Double-entry bookkeeping can only tell you if the books are balanced or not. We absolutely cannot assume that the books reflect reality just because they're balanced. You don't need to mess up twice to mess up the books in terms of truthness.

Also tests and code are independent while you always affect both sides in double-entry always. Audits exist for a reason.

With double-entry bookkeeping, the only way an error can slip through is if you make the same error on both sides, or else they wouldn’t be balanced. A similar thing is true for testing: If you make both an error in your test and in your implementation, they can cancel out and appear to be error-free.

I don’t quite agree with that reasoning, however, because a test that fails to test the property it should test for is a very different kind of error than having an error in the implementation of that property. You don’t have to make the “same” error on both sides for an error to remain unnoticed. Compared to bookkeeping, a single random error in either the tests or the implementation is more likely to remain unnoticed.

> With double-entry bookkeeping, the only way an error can slip through is if you make the same error on both sides, or else they wouldn’t be balanced. A similar thing is true for testing: If you make both an error in your test and in your implementation, they can cancel out and appear to be error-free.

Yeah but it's very different from tests vrs code though, right? Every entry has two sides at least and you do it together, they are not independent like test and code.

You can easily make a mistake if you write a wrong entry and it will still balance. Balanced books =/= accurate books is my point. And there is no difference between "code" and "tests" in double entry, it's all just "code".

So it seems like the person who made the metaphor doesn't really know how double-entry works or took maybe one accounting class.

> Yeah but it's very different from tests vrs code though, right? Every entry has two sides at least and you do it together, they are not independent like test and code.

The point of the current thread is that the use of AI coding agents threatens to disrupt that. For example, they could observe a true positive test failure and opt to modify the test to ensure a pass instead.

What about the concept of "high confidence" is not understandable?

You can use a little less snark and "high confidence" is pretty easy to understand but your metaphor makes no sense. Balanced books =/= accurate books and it is not at all a sign that the bookkeeping is accurate. The entries are also not independent like code and tests.

> Balanced books =/= accurate books

Naturally. Hence "high confidence" and not "full confidence". But let's not travel too far into the weeds here. Getting us back on track, what about the concept of "high confidence" is not understandable?

That sounds right in theory, but in practice my code is far, far higher quality when I do TDD than when I don't. This applies whether or not I'm using an Ai coding assistant

I don't think GP disagrees. They are (I think) stating that AI-assisted TDD is not as reliable as human TDD, because AI will invent a pointless test just to achieve a passing outcome.

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