hwayne

There's a huge difference between "Verus can prove the safety of unsafe code" and "Verus can EASILY prove the safety of unsafe code." And I bet it can't prove everything, like calls to a C ABI.

Not the first time Microsoft employees invented a formal methods tool, Microsoft ignored them, and AWS scooped them up. Not the second or third time, either.

The key bit is that specifications don't need to be "obviously computable", so they can be a lot simpler than the code that implements them. Consider the property "if some function has a reference to a value, that value will not change unless that function explicitly changes it". It's simple enough to express, but to implement it Rust needs the borrow checker, which is a pretty heavy piece of engineering. And proving the implementation actually guarantees that property isn't easy, either!

...Whoops. Yup, SMT solvers can famously return `unknown` on top of `sat` and `unsat`. Just added a post addendum about the mistake.

Also:

- Have a clear notion of what part of the specs represents the system under your control (the "machine"), and what part represents the broader world it interacts with. The world can do more than the machine, and the properties on the world are much more serious.

- Make lots of helpers. You need them more than you think.

- Add way more comments than you normally would. Specs are for analyzing very high-level ideas, and you should be explaining your high-level ideas.

- Make the spec assumptions clear. What has to be true about the operating environment for the spec to be sensible in the first place?

- Use lots of model values, use lots of constants, use lots of ASSUME statements. Constrain your fairness clauses as narrowly as possible.

- Understand the difference between the semantics of TLA+ as an abstract notation and the semantics of TLA+ as something concretely model-checked. For example, TLA+ is untyped, but the model checker is typed. Also, a lot of TLA+ features are not available in different model checkers. OTOH, you can break TLA+ semantics with use of TLCSet and TLCGet.

The last tip applies to whatever modeling language you use: most have the same distinction.

I've had to help a client with something not exactly like, but with similar properties as, Google Docs. One of the big properties they had to engineer in was "the doc should eventually look the same for all open browser tabs on the same computer".

The other big question: what happens if user A makes change X and user B makes change Y? There's a lot of outcomes the product can pick between, but whatever they pick it should be consistent. That consistency in conflict resolution is a good property to model.

I think the "high school math" slogan is untrue and ultimately scares people away from TLA+, by making it sound like it's their fault for not understanding a tough tool. I don't think you could show an AP calculus student the equation `<>[](ENABLED <<A>>_v) => []<><<A>>_v` and have them immediately go "ah yes, I understand how that's only weak fairness"

Those things, unlike floats, have approximable-enough facsimiles that you can verify instead. No tools support even fixed point decimals.

This has burned me before when I e.g needed to take the mean of a sequence.

I really do wish that PRISM can one day add some quality of life features like "strings" and "functions"

(Then again, AIUI it's basically a thin wrapper over stochastic matrices, so maybe that's asking too much...)

> No problem with floats or strings as far as specification goes. The particular verification tools you choose to run on your TLA+ spec may or may not have limitations in these areas, though.

I think it's disingenuous to say that TLA+ verifiers "may or may not have limitations" wrt floats when none of the available tools support floats. People should know going in that they won't be able to verify specs with floats!

Thanks for sharing the general term! I didn't know about it.