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> Backwards compatibility is an absolute non-negotiable for programming languages

What programming language(s) satisfy this criteria, if any?

Ada does. It has been through 5 editions so far and backwards compatibility is always maintained except for some small things that are documented and usually easy to update.

I'd normally be inclined to agree that minor things are probably good enough, but "absolute non-negotiable" is a rather strong wording and i think small things technically violate a facial reading, at least.

On the other hand, I did find what I think are the relevant docs [0] while looking more into things, so I got to learn something!

[0]: https://docs.adacore.com/gnat_rm-docs/html/gnat_rm/gnat_rm/c...

> except for some small things that are documented

I can't think of any established language that doesn't fit that exact criteria.

The last major language breakage I'm aware of was either the .Net 2 to 3 or Python 2 to 3 changes (not sure which came first). Otherwise, pretty much every language that makes a break will make it in a small fashion that's well documented.

Go, PHP, Ruby, JavaScript ... I'd say majority, actually.

It's probably borderline due to the opt-in mechanism, but Go did make a technically backwards-incompatible change to how its for loops work in 1.22 [0].

PHP has had breaking changes [1].

Ruby has had breaking changes [2] (at the very least under "Compatibility issues")

Not entirely sure whether this counts, but ECMAScript has had breaking changes [3].

[0]: https://go.dev/blog/loopvar-preview

[1]: https://www.php.net/manual/en/migration80.incompatible.php

[2]: https://www.ruby-lang.org/en/news/2025/12/25/ruby-4-0-0-rele...

[3]: https://tc39.es/ecma262/2025/#sec-additions-and-changes-that...

There is no such thing as perfection in the real world. Close enough is good enough.

I'd normally agree with you in practice, but since "close enough" seems likely to cover most mainstream languages in use today I figured "absolute non-negotiable" probably was intended to mean a stricter standard.

The interesting thing about Go's loopvar change is that nobody was able to demonstrate any real-world code that it broke (*1), while several examples were found of real-world code (often tests) that it fixed (*2). Nevertheless, they gated it behind go.mod specifying a go version >= 1.22, which I personally think is overly conservative.

*1: A great many examples of synthetic code were contrived to argue against the change, but none of them ever corresponded to Go code anyone would actually write organically, and an extensive period of investigation turned up nothing

*2: As in, the original behavior of the code was actually incorrect, but this wasn't discovered until after the loopvar change caused e.g. some tests to fail, prompting manual review of the relevant code; as a tangent, this raises the question of how often tests just conform to the code rather than the other way around

You certainly won't find me arguing against that change, and the conservatism is why I called it borderline. The only reason I bring it up is because of the "absolute non-negotiable" bit, which I took to probably indicate a very exacting standard lest it include most widespread languages anyways.

Yes, I think it's also a good example of how "absolute" backwards compatibility is not necessarily a good thing. Not only was the old loopvar behavior probably the biggest noob trap in Go (*), it turned out not to be what anyone writing Go code in the wild actually wanted, even people experienced with the language. Everyone seems to have: a) assumed it always worked the way it does now, b) wrote code that wasn't sensitive to it in the first place, or c) worked around it but never benefitted from it.

*: strongest competitor for "biggest noob trap" IMO is using defer in a loop/thinking defer is block scoped

Strongly agree there. IMO breaking backwards compatibility is a tradeoff like any other, and the flexibility non-hardline stances give you is handy for real-world situations,

Yes, most of them.

C# for instance isn't such a "small language", it has grown, but code from older versions, that does not use the newer features will almost always compile and work as before.

breaking changes are for corner cases, e.g. https://github.com/dotnet/roslyn/blob/main/docs/compilers/CS...

The thing is that "most of them" seems incongruous with a demand for "absolute non-negotiable" backwards compatibility. If not for that particular wording I probably wouldn't have said anything.

Java rules here. You can take any Java 1.0 (1995) codebase and compile it as-is on a recent JDK. Moreover, you can also use any ancient compiled Java library and link it to modern Java app. Java source and bytecode backward compatibility is fantastic.

Java has had some breaking changes (e.g., [0, 1]), though in practice I have to say my experience tends to agree and I've been fortunate enough to never run into issues.

[0]: https://stackoverflow.com/q/1654923

[1]: https://news.ycombinator.com/item?id=28542853

* Terms and conditions apply

Java is very good here, but (and not totally it's fault) it did expose internal APIs to the userbase which have caused a decent amount of heartburn. If your old codebase has a route to `sun.misc.unsafe` then you'll have more of a headache making an upgrade.

Anyone that's been around for a while and dealt with the 8->9 transition has been bit here. 11->17 wasn't without a few hiccups. 17->21 and 21->25 have been uneventful.

Can confirm; my team spent the past 9 months upgrading an application JDK 8 -> 17, and there were breaking changes even after we got it compiling + running

Rust does. You have editions to do breaking changes at the surface level. But that is per crate (library) and you can mix and match crates with different editions freely.

Thry do reserve the right to do breaking changes for security fixes, soundness fixes and inference changes (i.e. you may need to add an explicit type that was previously inferred but is now ambiguous). These are quite rare and usually quite small.

I'd normally agree that what you say is good enough in practice, but I question whether it meets GP's "absolute non-negotiable" standards. That specific wording is the reason I asked the question in the first place; it seemed to me that there was some standard that apparently wasn't being met and I was wondering where exactly the bar was.

Haskell's user-facing language gets compiled down to Haskell "core" which is what the language actually can do. So any new language feature has a check in with sanity when that first transformation gets written.

I'm not sure what this is arguing against here. Anyone who follows Rust knows that it's relatively modest when it comes to adding new features; most of the "features" that get added to Rust are either new stdlib APIs or just streamlining existing features so that they're less restrictive/easier to use. And Rust has a fantastic backwards compatibility story.

I had C++, python and ruby in mind, but yes, GP also mentioned Rust in the list of 'sprawling' languages, and they are probably right about that: Rust started as a 'better C replacement' but now it is trying to dominate every space for every programming language (and - in my opinion - not being very successful because niche languages exist for a reason, it is much easier to specialize than to generalize).

I wasn't particularly commenting on Rust's backward compatibility story so if you're not sure what I was arguing about then why did you feel the need to defend Rust from accusations that weren't made in the first place?

Tbh I think `rust-toolchain` solves most of these issues

I suspect the problem is that every feature makes it possible for an entire class of algorithms to be implement much more efficiently and/or clearly with a small extension to the language.

Many people encounter these algorithms after many other people have written large libraries and codebases. It’s much easier to slightly extend the language than start over or (if possible) implement the algorithm in an ugly way that uses existing features. But enough extensions (and glue to handle when they overlap) and even a language which was initially designed to be simple, is no longer.

e.g., Go used to be much simpler. But in particular, lack of generics kept coming up as a pain point in many projects. Now Go has generics, but arguably isn’t simple anymore.

Egad, no. This is how you get C++, whose core tenet seems to be “someone used this once in 1994 so we can never change it”.

Even adding a new keyword will break some code out there that used that as a variable name or something. Perfect backward compatibility means you can never improve anything, ever, lest it causes someone a nonzero amount of porting effort.

No, you get C++ because you're Bjarne Stroustrup and trying to get people to sign on to the C++ bandwagon (A better C! Where have I heard that before?) and so you add every feature they ask for in the hope that that will drive adoption. And you call it object oriented (even if it really isn't) because that's the buzz-word du-jour. Just like Async today.

I’ll accept that, too. But from the outside it seems like they do that by finding bizarre, previously invalid syntax and making that the way to spell the new feature. `foo[]#>££{23}` to say “use implicit parallelism on big endian machines with non-power-of-2 word sizes”? Let’s do it!

If you want or have to build a large program, something must be large, be it the language, its standard library, third party code, or code you write.

I think it’s best if it is one of the first two, as that makes it easier to add third party code to your code, and will require less effort to bring newcomers up to speed w.r.t. the code. As an example, take strings. C doesn’t really have them as a basic type, so third party libraries all invent their own, requiring those using them to add glue code.

That’s why standard libraries and, to a lesser extent, languages, tend to grow.

Ideally that’s with backwards compatibility, but there’s a tension between moving fast and not making mistakes, so sometimes, errors are made, and APIs ‘have’ to be deprecated or removed.

It's a balance thing. You can't make a language without any features, but you can be too small ('Brainfuck') and you can definitely be too large ('C++'). There is a happy medium in there somewhere and the lack of a string type was perceived as a major shortcoming of C, but then again, if you realize that they didn't even have structs in the predecessor to C (even though plenty of languages at the time did have similar constructs) they got enough of it right that it ended up taking off.

C and personal computing hit their stride at roughly the same time, your choices were (if you didn't feel like spending a fortune) Assembly, C, Pascal and BASIC for most systems that mere mortals could afford. BASIC was terribly slow, Pascal and C a good match and assembler only for those with absolutely iron discipline. Which one of the two won out (C or Pascal) was a toss up, Pascal had it's own quirks and it was mostly a matter of which of the two won out in terms of critical mass. Some people still swear by Pascal (and usually that makes them Delphi programmers, which will be around until the end because the code for the heat-death of the universe was writting in it).

For me it was Mark Williams C that clinched it, excellent documentation, good UNIX (and later Posix) compatibility and whatever I wrote on the ST could usually be easily ported to the PC. And once that critical mass took over there was really no looking back, it was C or bust. But mistakes were made, and we're paying the price for that in many ways. Ironically, C enabled the internet to come into existence and the internet then exposed mercilessly all of the inherent flaws in C.

> Languages should be small, not large.

Yes. At the very least, features should carry a lot of weight and be orthogonal to other features. When I was young I used to pride myself on knowing all the ins and outs of modern C++, but over time I realized that needing to be a “language lawyer” was a design shortcoming.

All that being said I’ve never seen the functionality of Rust’s borrow checker reduced to a simpler set of orthogonal features and it’s not clear that’s even possible.

George Orwell showed us that small languages constrain our thinking.

A small language but with the ability to extend it (like Lisp) is probably the sweet spot, but lol look at what you have actually achieved - your own dialect that you have to reinvent for each project - also which other people have had to reinvent time after time.

Let languages and thought be large, but only used what is needed.

I can take anything I wrote in C since ~1982 or so and throw it at a modern C compiler and it will probably work, I may have to set some flags but that's about it. I won't have to hunt up a compiler from that era, so the codebase remains unchanged, which increases the chances that I'm not going to introduce new bugs (though the old ones will likely remain).

If I try the same with a python project that I wrote less than five years ago I'm very, very lucky if I don't end up with a broken system by the time all of the conflicts are resolved. For a while we had Anaconda which solved all of the pain points but it too seems to suffer from dependency hell now.

George Orwell was a writer of English books, not a programmer and whatever he showed us he definitely did not show us that small programming languages constrain our thinking. That's just a very strange link to make, programming languages are not easily compared with the languages that humans use.

What you could say is that a programming languages' 'expressivity' is a major factor in how efficient it is in taking ideas and having them expressed in a particular language. If you take that to an extreme (APL) you end up with executable line-noise. If you take it to the other extreme you end up some of the worst of Java (widget factory factories). There are a lot of good choices to be found in the middle.

> What you could say is that a programming languages' 'expressivity' is a major factor in how efficient it is in taking ideas and having them expressed in a particular language

You mean:

> small languages constrain our thinking.

:)