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Comment by gavinhoward | Hacker News Reader

> If turing-completeness is necessary, why should there even exist a build system independent from a programming language now?

This is a great question that I will make sure to answer in my Turing-completeness post in a week.

But to answer now, it's because of multi-project builds, where projects can be in multiple languages.

If you have build systems that are not independent from their language, how do you get them to play nice?

I designed Rig to be able to act as a babysitter and make them play nice.

If I play my cards right, Rig will be the reason other build systems become irrelevant. :)

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I've a similar question to GP, but from a slightly different angle. I can see the attraction of using a Turing complete language, but on the other hand, an important properly of a build system is that it ought to halt at some point. Something like, say, Starlark, which is used in Bazel, is primitive recursive, so it's not Turing complete, but gets you almost all the way to Turing complete while also being guaranteed to halt. It'd be interesting to see this aspect covered.

EDIT: s/Skylark/Starlark/

I'm afraid I have bad news.

The thing about Starlark is that it is effectively Turing complete.

You can simulate a while loop with a long enough foreach, an if statement, and a break statement.

And by "long enough," I mean long enough to make anyone just stop the build, but hey, if I stack the foreach loops, I can make the inner loop exponentially long, even though ints are limited to 32 bits.

By not taking out the break keyword, Starlark remains Turing-complete.

And yes, I tested this.

That's not Turing complete. That's still primitive recursive. You're going out of your way make it loop for a really, really long time, but it's still guaranteed to terminate, and thus isn't Turing complete, even if it'd take a really long time to terminate. This isn't some theoretical thing: it's how you can introduce guarantees into the system that it can't get out of control by accident and it also makes it easier to reason about the system.

A primitive recursive language gets you most of the benefits of a general recursive one _without_ needing to be _actually_ Turing complete. So this isn't "bad news" for me. I'm saying that I think you should avoid using a general recursive (i.e. Turing complete) language for a build system and instead have guarantees that the system will terminate by using a primitive recursive language of some kind because you literally, as you did, need to go out of your way to get code written in such a fashion to run for an inordinate amount of time. And yet, an inordinate amount of time isn't forever and unlike a program written in an actually Turing complete language, you can use static analysis to detect that kind of thing if only primitive recursion is supported.

> And yet, an inordinate amount of time isn't forever

If it's only guaranteed to terminate after the heat death of the universe, it won't terminate, so I would call that "forever."

> unlike a program written in an actually Turing complete language, you can use static analysis to detect that kind of thing if only primitive recursion is supported.

And this is exactly why I said that break was necessary.

Sure your static analysis may detect that the loop goes 2^256 times, which would be "forever," and so it returns the answer, "Does not halt."

Ah, but with the break keyword, it still could. In fact, it might always halt rather quickly.

Rule of thumb is that if static analysis can be wrong, it is Turing-complete.

Not if Hancho beats you to it. ;)

https://github.com/aappleby/Hancho

In all seriousness, I think both our projects are aiming in the right direction, mine is just more focused on minimalism at the cost of having extra-pointy corner cases.

What about a single API for builds, implemented in multiple general purpose languages.

End users get to pick a familiar language to them.

Similar to how Pulumi is available in multiple languages.

Then code both builds and meta build concerns as you would code anything else.

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