Show HN: RealDiff – runtime behavior diffing for pull requests (six languages)

Post by vlovich123 » Thu, Aug 27, 2026, 2:39 PM UTC

Post by issacnitin » Thu, Aug 27, 2026, 3:01 PM UTC

vlovich123 wrote: post #2

I’ve read the readme a few times and still have no idea how this is useful. The motivating example of Sort => SortBy has no description of what’s highlighted and no description of how. Is it running the old test suite with the new changes as a weak form of mutation testing? Something else?

Post by issacnitin » Thu, Aug 27, 2026, 3:40 PM UTC

gardnr wrote: post #5

issacnitin wrote: post #4

Earlier quoted context omitted.

Not mutation testing — nothing is mutated. It builds both branches with instrumentation, runs the same test suite on each, and diffs the recorded arguments and return values. In that example, selectDiscount returns a different discount and the order total goes 85 → 60. Neither function is in the diff; only the sort helper is. The tests still pass — one asserts the total is positive, and 60 is positive.

Don’t run your responses through an LLM. People prefer to read your genuine words, even if it may include some incorrect English.

Post by jedisct1 » Thu, Aug 27, 2026, 5:14 PM UTC

Post by ramon156 » Thu, Aug 27, 2026, 10:05 PM UTC

issacnitin wrote: post #4

vlovich123 wrote: post #2

I’ve read the readme a few times and still have no idea how this is useful. The motivating example of Sort => SortBy has no description of what’s highlighted and no description of how. Is it running the old test suite with the new changes as a weak form of mutation testing? Something else?

Not mutation testing — nothing is mutated. It builds both branches with instrumentation, runs the same test suite on each, and diffs the recorded arguments and return values. In that example, selectDiscount returns a different discount and the order total goes 85 → 60. Neither function is in the diff; only the sort helper is. The tests still pass — one asserts the total is positive, and 60 is positive.