How confident are you in the soundness of the type system?
Also, are there any Lua constructs that are difficult/impossible to type?
Is type checking decidable? (Is the type system Turing complete?)
replies(3):
Also, are there any Lua constructs that are difficult/impossible to type?
Is type checking decidable? (Is the type system Turing complete?)
* Your type system cannot be sound. It's going to have escape hatches and exceptions because that's how dynamic languages roll.
* There will always be constructs that you can't type. See above.
* If your type system is going to usefully type enough of the ecosystem, it will be Turing complete.
All of these things are the trade-offs you make when you set out to layer types on a dynamic language, and they're well worth it to get 99% of the way to type safety in a language that otherwise couldn't scale. Theoretical purity is meaningless if the language isn't useful.
I think you could prove that you can't construct a sound & complete type system for Lua. But just saying "Your type system cannot be sound" by itself is definitely wrong. I don't understand why people are throwing out both soundness & completeness, instead of at least retaining one (and I think the choice here is pretty obvious, a sound but incomplete type system is much more useful than an unsound one).
From Flow's website[1] (a type checker for JavaScript):
> Flow tries to be as sound and complete as possible. But because JavaScript was not designed around a type system, Flow sometimes has to make a tradeoff. When this happens Flow tends to favor soundness over completeness, ensuring that code doesn't have any bugs.
I don't understand why other type systems for dynamically typed languages cannot strive for that, and in particular I am pretty salty at TypeScript for explicitly not caring about soundness.
[1]: https://flow.org/en/docs/lang/types-and-expressions/#toc-sou...