[ShaneWilton]

I'd imagine it isn't, at least depending on how you define API compatibility, and whether you're only looking at the API interfaces. Imagine two versions of a library that implement the function "add".

  Version 1:
  add Int -> Int -> Int
  add x y = x + y

  Version 2:
  add Int -> Int -> Int
  add x y = x * y

Both versions expose the same API interface, but the functions that conform to that interface are semantically different. A stronger type system could probably differentiate between the two functions, but I doubt you could generally compute whether both functions implement the same behavior.

Perhaps with some sort of functional extensionality you'd be able to compute compatibility perfectly, but I can't imagine that ever being feasible in practice.

That being said, what Elm does offer is still a huge improvement over humans trying to guess whether they made any breaking changes :)

[codebje]

You certainly cannot determine that two programs do or do not have the same behaviour in the general case.

In specific cases, the proofs can be pretty trivial:

https://tinyurl.com/ycx2245q - proof your two programs are different

https://tinyurl.com/y7lcv3re - proof 'y + x' is the same as Version 1.

These proofs weren't automatically discovered, though for such simple programs I'd expect an SMT solver to be able to find the proof or a counterexample easily enough.

But even proving they're the same value for all inputs isn't all that helpful, because of lazy Haskell code like:

    version 1:
    fib :: Int -> Int
    fib n = if n < 3 then 1 else fib (n - 2) + fib (n - 1)

    version 2:
    fib :: Int -> Int
    fib n = let fs = 1:1:zipWith (+) fs (tail fs) in fs !! n

They're (provably) the same for all (positive) input values, but if you call version 1 with n greater than, say, 35, you'll be waiting quite a while for an answer, while version 2 will be very snappy for the first few hundred thousand values of n, at least, after which the size of the answer will be a bottleneck.

If a library switched from the latter to the former, it'd have a good chance of breaking code.

While obviously exponential code is obviously exponential, this sort of behavioural change can show up in less obvious ways - a change in memory usage might blow your heap after a supposedly minor version change, eg.

In the end I don't think the value judgement of 'breaking' or even 'significant' is computable, and you'd need to rely on a human doing something that approximates to 'right' for your world view with their version numbering.

[toast0]

I don't think 'does it work exactly the same' isn't necessarily the right question, given there are expected to be bug fixes which may change the behavior in some functions.

[marcosdumay]

The right question is "what's the difference between a bug-fix and a breaking change?"

[Nadya]

Irrelevant. If fixing a bug is a breaking change then it is a breaking change. This is the importance of pre-releases/"nightly" branches so that you don't have a mistake of addNumbers(5,5) returning 25 instead of 10 and not being caught and then needing to increment a major number to fix a typo of × to +.

A bug fix is a change. A breaking change is a change that breaks the API. Doesn't matter if the previous status was the intended one or not.

SemVer not working is almost entirely people simply not following it correctly. The projects that follow it as best as they can have very few mistakes were "woops that was a major change" occurs while projects that use it as a guideline may as well not be using it at all.

[hluska]

Irrelevant?? That is the most relevant question that a developer could ask in this situation.

Often, there are ways to work around bugs in an API. And, just as often, those workarounds will break as soon as the bug is fixed. The API developer is in a particularly poor place to judge whether a bug fix is breaking or not. Sometimes, they can talk to users to see if a change will break their code, but other times they can't. Thus, it comes down to how well a developer can predict whether a fix will break projects in the wild.

[tremon]

Irrelevant?? That is the most relevant question that a developer could ask in this situation.

No, it isn't. The most relevant question is "can somebody be using the current API". It doesn't matter if your current API matches the documentation, what matters is whether your current API is out there for others to build on.

Don't try to use a crystal ball or other form of divination to predict what your downstream users have been doing with the code; you will always lose. Instead, suck it up, acknowledge the mistake, and signal the breaking change by bumping the major version.

Maybe next time your developers will spend more time validating their public contract, so they won't have to endure the embarrassment of a major version bump.

[hluska]

Using that logic, every single bug fix that even remotely changes how the API operates would be considered a major version. What would be the point of semver?

Semver both acknowledges and is predicated upon the fact that every bug fix has a range of possible impacts. At one end, some bug fixes have almost no chance of breaking an application. At the other end, some bug fixes will almost certainly break applications.

This is not divination. It is understanding your application and its users.

Finally, in regards to your last paragraph, I genuinely hope that you don't speak to developers like that. Mistakes will always happen. Have a little bit of respect and treat people with kindness.

[t3soro]

A bug fix is not a breaking change. You are speaking in tautologies. "A bug fix is a change that is a change to the API". No, bug fix does not change the interface. A bug fix does not change the API. It does not require a semantic version change unless it also changes the expected behavior of the method, in which case it is a change in the documentation, which is part of the application programming interface.

[Nadya]

A bug fix is not always a breaking change but a breaking change is always a breaking change. SemVer doesn't care if it is a bug fix or not. The only single relevant detail is if it broke the behavior of the API for any reason. The reason for the breakage is entirely and completely irrelevant because the only relevant detail in SemVer is if it broke the public API. It's a binary "Yes/No" question.

Doesn't matter if it breaks it in a way that you don't expect users to have been using it - you broke it. Doesn't matter if you were just fixing a typo that changed behavior - you broke it. Doesn't matter if you were fixing a logical error - you broke it. Doesn't matter if you fixed the PRNG causing some results to no longer be possible - you broke it. Doesn't matter if you deleted a deprecated endpoint - you broke it. '

SemVer isn't asking why you broke it - it's asking did you break it.

[lmm]

A bug fix makes the function behave as it's documented to, a breaking change involves a change to the documentation?