[zitterbewegung]

Sure this CVE sounds like a joke but if you create a programming language that is non Turing complete it is much easier to secure than a Turing complete language.

Making a language that have the expressive power of finite state machines could be an example.

[qsort]

We are doing this already.

- Configuration languages (JSON, YAML, XML) are pure combinatorial logic.

- Regular expressions are... mostly not actually regular, but you get the idea.

- Some templating languages are deliberately less powerful than Turing machines, e.g. ST4 is context-free.

- Prepared SQL statements are a similar idea on a different axis.

The real question is whether a non-TC language could be useful for general purpose programming. Such a language might come with very strong guarantees (termination, time complexity, even correctness or a limited form of correctness), but they might be extra-cumbersome for 'normal' workloads.

[ludamad]

A non-TC language can be achieved just by forcing proofs. Oh, you want that program to run? Just prove that it is bounded memory and can't halt, etc

[qsort]

Yeah, but that's the 'cumbersome' part. Could we imagine a non-TC language with simple syntax that's suitable for at least some of the task you'd use a TC language for?

[magmastonealex]

C compiled down to eBPF (as implemented with the Linux kernel) is not turing-complete. eBPF has a wide variety of uses from network filtering to hooking into certain syscalls.

The bytecode is theoretically turing-complete, but the verifier present in the kernel ensures that the code contains no loops and accesses no out-of-bounds memory.

With `clang`, you can compile C down to eBPF bytecode. It's a subset of C, with a lot of restrictions, but it is familiar and capable of some remarkable things past just simple packet parsing & filtering.

[qsort]

I had no idea, this is fascinating. Thanks for sharing.

[UncleMeat]

A useful subset of SQL is not turing-complete and is suitable for meaningful computational tasks. Datalog is also not turing-complete and is similarly useful.

[qsort]

Sure, more examples are CSP solvers and AMPL. They are all somewhat specific though. I was wondering if it would be possible to make one that's "general purpose" in the same sense you call programming languages "general purpose", i.e. one you'd write application software in.

[UncleMeat]

Datalog would be the closest I can think of. A bunch of static analysis engines are implemented in it.

[tester34]

What does Turing completness even mean in practice when it comes to non-theoretical languages?

[zitterbewegung]

You can't do anything like recursion.

[klyrs]

I spent a while trying to parse this, and it doesn't make a lick of sense.

Brainfuck is a classical example of a turing complete language that doesn't have a notion of functions. What does "recursion" mean here? Nothing, I think. So you don't need recursion for Turing completeness.

Some people work on languages that support functions as first class objects, which don't natively support recursion. This is nice from a language design standpoint, because functions only see the scope of their arguments and can't look any further. The "Y Combinator" pattern is used to bootstrap recursion into these languages. You may have heard of it.

Turing-incomplete languages can also support unlimited recursion. For example, take a Turing complete language, and equip it with a state of the art theorem prover. Before entering a loop, or calling a function, the theorem prover is allowed to spend up to 1 minute proving that the loop or function call will terminate. If the theorem prover times out, the program halts with an error. Some functions, such as a naive implementation of factorial, can be easily proved to halt for all inputs. So this language allows recursion, but by construction, all programs terminate, so it's turing incomplete.

[mousepilot]

I'm not that great a programmer, but this one seems easy to me, you write an interpreter in brainfuck that allows function definitions that support recursion, then include your recursive function definitions as an included constant then point your interpreter at the constant and tell it to run it. I assume bf allows constants but like I said I'm not that great a programmer.

Obviously using a recursive descent parser is out but thats probably easy to work around right?

I could be wrong tho so don't base mission critical code on this without rigorous testing!

[klyrs]

> I assume bf allows constants but like I said I'm not that great a programmer.

Key to being a good programmer: at least read a bit about the language before speculating about it. Not sure what you mean by "allows constants", but you're only given increment and decrement operators, and a promise that fresh tape is zeroed out.

Brainfuck supports iteration. Performing recursion in a higher level language will still look like iteration in Brainfuck.

[mousepilot]

100 percent agree, but honestly I couldn't make sense of the examples I looked at. It was probably just me tho.