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It looks like there are two maintained implementations of this at the moment - one in C# https://github.com/j-brooke/FracturedJson/wiki/.NET-Library https://git

by simonw 10mo ago

It looks like there are two maintained implementations of this at the moment - one in C# https://github.com/j-brooke/FracturedJson/wiki/.NET-Library https://github.com/j-brooke/FracturedJson/wiki/.NET-Library and another in TypeScript/JavaScript https://github.com/j-brooke/FracturedJsonJs https://github.com/j-brooke/FracturedJsonJs. They each have their own test suite. There's an older pure Python version but it's no longer maintained - the author of that recently replaced it with a Python library wrapping the C# code. This looks to me like the perfect opportunity for a language-independent conformance suite - a set of tests defined as data files that can be shared across multiple implementations. This would not only guarantee that the existing C# and TypeScript implementations behaved exactly the same way, but would also make it much easier to build and then maintain more implementations across other languages. Interestingly the now-deprecated Python library does actually use a data-driven test suite in the kind of shape I'm describing: https://github.com/masaccio/compact-json/tree/main/tests/data https://github.com/masaccio/compact-json/tree/main/tests/dat... That new Python library is https://pypi.org/project/fractured-json/ https://pypi.org/project/fractured-json/ but it's a wrapper around the C# library and says "You must install a valid .NET runtime" - that makes it mostly a non-starter as a dependency for other Python projects because it breaks the ability to "pip install" them without a significant extra step.

klysm 10mo ago
This is also basically a pure function which makes it super simple to write a harness.
odyssey7 10mo ago
This is a good idea, though I don’t think it would guarantee program equivalence beyond the test cases.
- rafabulsing 10mo ago
  Well yeah, but then any discrepancies that are found can be discussed (to decide which of the behaviors is the expected one) and then added as a test for all existing and future implementations.
- simonw 10mo ago
  Depends on how comprehensive the test suite is. And OK it's not equivalent to a formal proof, but passing 1,000+ tests that cover every aspect of the specification is pretty close from a practical perspective, especially for a visual formatting tool.
  - boxed 10mo ago
    With mutation testing you can guarantee that all the behavior in the code is tested.
    - odyssey7 10mo ago
      UC Berkeley: “Top-level functional equivalence requires that, for any possible set of inputs x, the two pieces of code produce the same output. … testing, or input-output (I/O) equivalence, is the default correctness metric used by the community. … It is infeasible to guarantee full top-level functional equivalence (i.e., equivalence for any value of x) with testing since this would require testing on a number of inputs so large as to be practically infinite.” https://www2.eecs.berkeley.edu/Pubs/TechRpts/2025/EECS-2025-174.pdf https://www2.eecs.berkeley.edu/Pubs/TechRpts/2025/EECS-2025-...
      - esrauch 10mo ago
        In practice mutation fuzz testers are able to whitebox see where branches are in the underlying code, with a differential fuzz test under that approach its generally able to fuzz over test cases that go over all branches. So I think under some computer science theory case for arbitrary functions its not possible, but for the actual shape of behavior in question from this library I think its realistic that a decent corpus of 'real' examples and then differential fuzzing would give you more confidence that anyone has in nearly any program's correctness here on real Earth.
        odyssey7 10mo ago
        Yes, there are different levels of sureness being described. When I hear guarantee, it makes me think of correctness proofs. Confidence is more of a practical notion for how much you trust the system for a given use case. Testing can definitely provide confidence in this scenario.
EmilStenstrom 10mo ago
Data driven test suites are really good for building trust in a library. Both the html5lib-tests suite and my recent xss-bench are examples of this!
fcoury 10mo ago
Just ported it to rust and plan on maintaining it if you want to add it to your original comment. More details on a sibling comment: https://github.com/fcoury/fracturedjson-rs https://github.com/fcoury/fracturedjson-rs https://crates.io/crates/fracturedjson https://crates.io/crates/fracturedjson Comment with details: https://news.ycombinator.com/item?id=46468641 https://news.ycombinator.com/item?id=46468641
tonixx 10mo ago
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by simonw 10mo ago

klysm 10mo ago

This is also basically a pure function which makes it super simple to write a harness.

odyssey7 10mo ago

This is a good idea, though I don’t think it would guarantee program equivalence beyond the test cases.

rafabulsing 10mo ago
Well yeah, but then any discrepancies that are found can be discussed (to decide which of the behaviors is the expected one) and then added as a test for all existing and future implementations.
simonw 10mo ago
Depends on how comprehensive the test suite is. And OK it's not equivalent to a formal proof, but passing 1,000+ tests that cover every aspect of the specification is pretty close from a practical perspective, especially for a visual formatting tool.
- boxed 10mo ago
  With mutation testing you can guarantee that all the behavior in the code is tested.
  - odyssey7 10mo ago
    UC Berkeley: “Top-level functional equivalence requires that, for any possible set of inputs x, the two pieces of code produce the same output. … testing, or input-output (I/O) equivalence, is the default correctness metric used by the community. … It is infeasible to guarantee full top-level functional equivalence (i.e., equivalence for any value of x) with testing since this would require testing on a number of inputs so large as to be practically infinite.” https://www2.eecs.berkeley.edu/Pubs/TechRpts/2025/EECS-2025-174.pdf https://www2.eecs.berkeley.edu/Pubs/TechRpts/2025/EECS-2025-...
    - esrauch 10mo ago
      In practice mutation fuzz testers are able to whitebox see where branches are in the underlying code, with a differential fuzz test under that approach its generally able to fuzz over test cases that go over all branches. So I think under some computer science theory case for arbitrary functions its not possible, but for the actual shape of behavior in question from this library I think its realistic that a decent corpus of 'real' examples and then differential fuzzing would give you more confidence that anyone has in nearly any program's correctness here on real Earth.
      - odyssey7 10mo ago
        Yes, there are different levels of sureness being described. When I hear guarantee, it makes me think of correctness proofs. Confidence is more of a practical notion for how much you trust the system for a given use case. Testing can definitely provide confidence in this scenario.

EmilStenstrom 10mo ago

Data driven test suites are really good for building trust in a library. Both the html5lib-tests suite and my recent xss-bench are examples of this!

fcoury 10mo ago

Just ported it to rust and plan on maintaining it if you want to add it to your original comment. More details on a sibling comment: https://github.com/fcoury/fracturedjson-rs https://github.com/fcoury/fracturedjson-rs https://crates.io/crates/fracturedjson https://crates.io/crates/fracturedjson Comment with details: https://news.ycombinator.com/item?id=46468641 https://news.ycombinator.com/item?id=46468641

tonixx 10mo ago

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