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I'm trying to understand what is the difference between this and using TLA. For example, what is the difference between "robust theoretical basis of the Tempor

I'm trying to understand what is the difference between this and using TLA. For example, what is the difference between "robust theoretical basis of the Temporal Logic of Actions (TLA)" and "state-of-the-art static analysis"? Sorry, I'm not an expert in TLA, but I thought that static analysis was basically what it was used for.

jleahy 3y ago
At very least it looks like it can do what TLA can do whilst being dramatically less painful to learn / work with. That is very much enough to be interesting.
bugarela 3y ago
Hey! We just changed the description (yesterday) to avoid this confusion - sorry! By static analysis there we actually mean things like type and effect checking. With either a TLA+ spec or a Quint spec, you can run a model checker to verify properties or get counterexamples. That's the main similarity. As Quint is based on TLA+, we can atually use the same model checkers (that were originally implemented for TLA+). The main differences between TLA+ and Quint are the surface syntax and the tooling (beyond the model checker). While TLA+ has an indentation-based hard-to-parse mathematical syntax (that looks quite pretty in LaTeX), Quint has a typed programming language styled syntax and a very simple parser, making it easier to develop tools around it. As for tooling, first of all, Quint has type checking, which TLA+ doesn't. Our IDE support is also more similar to that of modern programming languages - with features like "Go to definition". With this, we hope (and have seen many reports of) programmers/engineers having an easier and better time writing Quint specs then they used to have with TLA+ tooling. Quint also has support for execution of specs with random simulation, a testing framework and a REPL. In contrast, TLA+ is a much more permissive language, and you can express more mathematical things that, for instance, could never be executed or are not even supported by TLA+ existing model checkers (TLC and Apalache). TLA+ has a proof system (TLAPS), which Quint does not. Quint imposes many restrictions with the goal of preventing people to write things they don't really understand - which are possible in TLA+. Those restrictions are useful, just as type and effect systems are useful. But mathematicians that really know what they are doing and need more powerful expressivity will likely prefer TLA+ over Quint. Quint is aimed at programmers and engineers. They are complementary, not direct competition.
- pron 3y ago
  > As for tooling, first of all, Quint has type checking, which TLA+ doesn't. I wouldn't put it quite like that. It's not that TLA+ doesn't "do type checking" because TLA+ is just a language for writing mathematical descriptions of things. It doesn't "do" anything (do the formulas of Newtonian mechanics do type-checking?). It's more precise to say that TLA+ is an untyped language. But the model checker does check something like "typing" in the sense of set membership. I.e. the invariant □(x ∈ Int), i.e. "x is always an integer", can be checked with a TLA+ model checker just like many other invariants. Furthermore, a model checker is "static" in the sense that, just like a type-checker, it doesn't "run" a "program". It's easy to see that a model checker doesn't run anything if you consider that a model checker can prove the following "type" on the right hand side of the implication: x ∈ BOOLEAN ∧ □[x' = TRUE ∨ x' = FALSE]_x ⇒ □(x ∈ BOOLEAN) It proves it nearly instantaneously, even though every "execution" of the "program" on the left of the implication is infinite in its duration and there is an uncountably infinite number of such "execution", so clearly nothing is "executed" and the check isn't dynamic. So a TLA+ model checker does do something that's analogous-ish to type-checking. However, it is true that by type-checking we normally mean an automated deductive process, i.e. one that applies inference rules, rather than an automated exhaustive analysis of the semantic domain, which is how a model-checker works. And yes, there are implications to the different ways "type checking" is done, especially when it comes to the algorithmic complexity of the checker. This is just another example where comparisons to programming are unhelpful when describing mathematics that don't "run"; it just is. It may be best to say that a language like Quint is inspired by the TLA logic (the TLA part of TLA+) and is similar to a programming language, whereas TLA+ is something else altogether (that requires learning something that is very much not programming) and leave it at that. I admit that explaining the difference between programming (or something that's programming-like) and specifying a system with mathematics to people who are less familiar with the latter is difficult. It's a little like trying to explain the notion of a physics formula to a catapult builder in ancient Greece. There's clearly a relationship between the two (and some physics formulas may certainly be helpful when designing a catapult and make the catapult builder a better catapult builder), but they're also completely different things operating in two different domains (a formula isn't something that can fire a projectile).
abathologist 3y ago
Those are great questions! Let me add a one addendum to bugarela's excellent explanation, to help disambiguate these three formalism: 1. TLA[0] is a logical calculus invented by Leslie Lamport. 2. TLA+[1] is a formal specification language, also invented by Leslie Lamport, with semantics based on and reducible to TLA. 3. quint is also a formal specification language with semantics based on and reducible to TLA. TLA+ can be understood as TLA extended with syntax sugar to help describe systems. On top of the logical calculus it adds niceties like `LET` bindings, syntax to represent common data structures, `CASE` expressions, a module system, etc. But TLA+ also exposes the underlying logic directly, making the language extremely expressive. The flip side is that nonsensical formulas which may not be checkable are pretty easy to write. quint is a (programmer friendly) syntax that, while also desugaring to TLA, gives up expressivity to add the guardrails that we've found most helpful when specifying the systems we work on. Please let us know if you have any followup questions :) --- [0]: https://en.wikipedia.org/wiki/Temporal_logic_of_actions https://en.wikipedia.org/wiki/Temporal_logic_of_actions [1]: https://en.wikipedia.org/wiki/TLA%2B https://en.wikipedia.org/wiki/TLA%2B

jleahy 3y ago

At very least it looks like it can do what TLA can do whilst being dramatically less painful to learn / work with. That is very much enough to be interesting.

bugarela 3y ago

Hey! We just changed the description (yesterday) to avoid this confusion - sorry! By static analysis there we actually mean things like type and effect checking. With either a TLA+ spec or a Quint spec, you can run a model checker to verify properties or get counterexamples. That's the main similarity. As Quint is based on TLA+, we can atually use the same model checkers (that were originally implemented for TLA+). The main differences between TLA+ and Quint are the surface syntax and the tooling (beyond the model checker). While TLA+ has an indentation-based hard-to-parse mathematical syntax (that looks quite pretty in LaTeX), Quint has a typed programming language styled syntax and a very simple parser, making it easier to develop tools around it. As for tooling, first of all, Quint has type checking, which TLA+ doesn't. Our IDE support is also more similar to that of modern programming languages - with features like "Go to definition". With this, we hope (and have seen many reports of) programmers/engineers having an easier and better time writing Quint specs then they used to have with TLA+ tooling. Quint also has support for execution of specs with random simulation, a testing framework and a REPL. In contrast, TLA+ is a much more permissive language, and you can express more mathematical things that, for instance, could never be executed or are not even supported by TLA+ existing model checkers (TLC and Apalache). TLA+ has a proof system (TLAPS), which Quint does not. Quint imposes many restrictions with the goal of preventing people to write things they don't really understand - which are possible in TLA+. Those restrictions are useful, just as type and effect systems are useful. But mathematicians that really know what they are doing and need more powerful expressivity will likely prefer TLA+ over Quint. Quint is aimed at programmers and engineers. They are complementary, not direct competition.

pron 3y ago
> As for tooling, first of all, Quint has type checking, which TLA+ doesn't. I wouldn't put it quite like that. It's not that TLA+ doesn't "do type checking" because TLA+ is just a language for writing mathematical descriptions of things. It doesn't "do" anything (do the formulas of Newtonian mechanics do type-checking?). It's more precise to say that TLA+ is an untyped language. But the model checker does check something like "typing" in the sense of set membership. I.e. the invariant □(x ∈ Int), i.e. "x is always an integer", can be checked with a TLA+ model checker just like many other invariants. Furthermore, a model checker is "static" in the sense that, just like a type-checker, it doesn't "run" a "program". It's easy to see that a model checker doesn't run anything if you consider that a model checker can prove the following "type" on the right hand side of the implication: x ∈ BOOLEAN ∧ □[x' = TRUE ∨ x' = FALSE]_x ⇒ □(x ∈ BOOLEAN) It proves it nearly instantaneously, even though every "execution" of the "program" on the left of the implication is infinite in its duration and there is an uncountably infinite number of such "execution", so clearly nothing is "executed" and the check isn't dynamic. So a TLA+ model checker does do something that's analogous-ish to type-checking. However, it is true that by type-checking we normally mean an automated deductive process, i.e. one that applies inference rules, rather than an automated exhaustive analysis of the semantic domain, which is how a model-checker works. And yes, there are implications to the different ways "type checking" is done, especially when it comes to the algorithmic complexity of the checker. This is just another example where comparisons to programming are unhelpful when describing mathematics that don't "run"; it just is. It may be best to say that a language like Quint is inspired by the TLA logic (the TLA part of TLA+) and is similar to a programming language, whereas TLA+ is something else altogether (that requires learning something that is very much not programming) and leave it at that. I admit that explaining the difference between programming (or something that's programming-like) and specifying a system with mathematics to people who are less familiar with the latter is difficult. It's a little like trying to explain the notion of a physics formula to a catapult builder in ancient Greece. There's clearly a relationship between the two (and some physics formulas may certainly be helpful when designing a catapult and make the catapult builder a better catapult builder), but they're also completely different things operating in two different domains (a formula isn't something that can fire a projectile).

abathologist 3y ago

Those are great questions! Let me add a one addendum to bugarela's excellent explanation, to help disambiguate these three formalism: 1. TLA[0] is a logical calculus invented by Leslie Lamport. 2. TLA+[1] is a formal specification language, also invented by Leslie Lamport, with semantics based on and reducible to TLA. 3. quint is also a formal specification language with semantics based on and reducible to TLA. TLA+ can be understood as TLA extended with syntax sugar to help describe systems. On top of the logical calculus it adds niceties like `LET` bindings, syntax to represent common data structures, `CASE` expressions, a module system, etc. But TLA+ also exposes the underlying logic directly, making the language extremely expressive. The flip side is that nonsensical formulas which may not be checkable are pretty easy to write. quint is a (programmer friendly) syntax that, while also desugaring to TLA, gives up expressivity to add the guardrails that we've found most helpful when specifying the systems we work on. Please let us know if you have any followup questions :) --- [0]: https://en.wikipedia.org/wiki/Temporal_logic_of_actions https://en.wikipedia.org/wiki/Temporal_logic_of_actions [1]: https://en.wikipedia.org/wiki/TLA%2B https://en.wikipedia.org/wiki/TLA%2B