- 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.

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.

    from z3 import \*

    a, b, c = Ints('a b c')
    x, y = Ints('x y')
    s = Solver()

    s.add(a > 5)
    s.add(a % 2 == 0)
    theorem = Exists([b, c],
                     And(
                         a == b + c,
                         And(
                             Not(Exists([x, y], And(x > 1, y > 1, x \* y == b))),
                             Not(Exists([x, y], And(x > 1, y > 1, x \* y == c))),
                             )
                         )
                     )

    if s.check(Not(theorem)) == sat:
        print(f"Counterexample: {s.model()}")
    else:
        print("Theorem true")

- T550.6. T550.6. Only half a son is born by queen who ate merely half of mango.

- A1066. A1066. Sun will lock moon in deep ditch in earth's bottom and will eat up stars at end of world.

- K87.1. K87.1. Laughing contest: dead horse winner.

Comments URL: https://news.ycombinator.com/item?id=44798766

Points: 6

# Comments: 1