Show HN: RealDiff – runtime behavior diffing for pull requests (six languages) | Hacker News Reader

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

github.com

40 pointsissacnitin10 comments

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.

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

Sure, appreciate the feedback.

please try speaking in your own words. i still have no idea what this is for. is it two compare two impls? is it to "optimize" a function (i assume not since there's no mutation)?

hi, this is simply put, to compare the behavior diff of two builds. when i say behavior, i mean the return values of functions etc. it'll help with AI-assisted code reviews etc. since AI looks at the text diff only, and misses behavior diff downstream. it complements static analysis and unit tests for a stronger testing strategy.

I love the concept and see the value for refactoring. There is tremendous value in knowing that two implementations are run-time equivalent.

I just wanted to share for others that there is a configuration language call Dhall (https://dhall-lang.org/) that has this kind of guarantee as one of its nicest features. See the "Refactor Fearlessly" section. In Dhall, most behavior preserving refactors produce the exact same hash code so you can know - for an absolute fact - that a refactor is behavior neutral. The point of sharing that other language is just to legitimize that "this is a thing" i.e. it has value recognized by others.

Thanks for letting me know, i'll check it out!

No support for Zig?

Not yet. But feel free to add :)

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