Author here. The TL;DR: move rules from prompts into types the compiler refuses to violate, then bounce the AI coding loop off those refusals. The repo is github.com/pyrex41/Shen-Backpressure. Builds a lot on Geoff Huntley's backpressure idea -- none of this is rocket science, just an effort to apply sound programming principles in a world of LLM coding agents.
This is great but keep in mind that Go allows the programmer skip these invariants in various ways.
I wish Go had a serious type system. Never mind algebraic types, but one that fucking respected private values and did things like validating enum values.
TBH something like this sounds useful even without LLMs (although I haven't fully grokked this yet). The problem with the operational level is that you can't express the invariants etc at the type level - not least because you're working across multiple languages - so the kind of dumb issues that we're beginning to rule out at the level of the language at the process level still require lots of diligence in operational code. Some kind of shared "operational type system" that could be integrated into relevant languages would potentially help a lot.
Shen has some really unique properties that are under-developed here. It's type system itself is Turing complete and very flexible / expressive. Also, the Shen kernel is extremely compact, and easy to port into a wide variety of runtime languages (C, Lisp, Ruby, Python, JS, Go, etc https://shen-language.github.io/#downloads). What I discussed about using it as a compile-time gate + codegen is just scratching the surface, I think.
Now, a lot of the ports haven't been maintained. But the underlying Shen kernel is only 4-5k lines of code...remains extremely portable. More discussion here https://news.ycombinator.com/item?id=39602472
I didn't focus a ton on Shen in the blog post, because the underlying principles aren't really about the implementation. Shen is very cool tho.
That's not a good thing! A Turing complete type system means that compilation is potentially undecidable and non-terminating. The whole unintuitive mess in dependently-typed systems about "definitional" equality (loosely speaking, a notion of equalities that are 'trivially' checked as part of evaluation) is entirely about avoiding Turing-complete type checking!
I mean yes, that's a risk, and you are correct. In practice, is your spec about the shape of the app you want to build really going to be that complicated?
But I mentioned its Turing completeness as a lazy shorthand to illustrate that it is far more flexible and expressive than what people think of as a "type system". https://shenlanguage.org/OSM/Recursive.html
Thank you, interesting work. Please, clarify what is possibly a naive question - your README states that the constraints imposed by your tool are weaker than the formal verification guarantees. Why not implement the backpressure as the full formal verification barrier? Too complex to implement?
The distinction worth keeping clean is between the spec (here, written as proofs in Shen) being formally rigorous and the entire codebase being formally verified. Shen-Backpressure does the first: the spec is a sequent-calculus statement of invariants, and shengen lowers it into guard types the target compiler refuses to violate, so within the target language's type discipline you cannot construct a tenant-access (or any other witness) without discharging its premises.
It does not do the second (formally verify the entire codebase). Outside the guard types your Go or TypeScript is just code. It can panic, race, have bugs in unrelated logic, use reflection to forge values inside the guard package, get a wrong answer from the SQL query that fed the predicate, and so on. The proof ends at the projection boundary.
Why not go further? Not really "too complex to implement," in theory; it has been done before. But verifying the whole program is much higher engineering cost, and the trades-offs to do it make sense in a much narrower set of cases than what I'm trying to target: teams shipping production code with AI in the loop, in the language they already ship.
The pragmatic choice is to spend the verification budget on the small set of invariants that genuinely matter and leave the rest as ordinary code with ordinary review and tests. That is why the claim is phrased as "practically impossible to accidentally bypass, not categorically impossible to bypass." Over-claiming "verified" when the host language is unverified would be misleading.
Learning about Shen is what inspired the project for me. Combination of sequent calculus and prolog in a highly portable lightweight kernel that is easy to port to many runtimes (https://shen-language.github.io) gives a ton of expressive power. If you only want compile-time checks, plenty of other notations work. What actually interests me is the possibility of a single spec whose invariants produce enforcement that crosses compile time (guard types + shen tc+) and runtime (generated checks in constructors, plus the spec itself as a test oracle via shen-derive), with no translation layer required between them.
Hi, thanks for the writeup, I wonder for the auth problem what you think about rego and OPA type solutions and their place in world in comparison to generated guards?
Definitely connected; OPA is itself a structural gate, but at runtime. The post focused on compile-time gates, but there's no reason a structural gate can't run at runtime — which means they compose rather than compete.
I didn't get into this in the post, but Shen is extremely portable and has been ported to a lot of different target runtimes (Go, C, Lisp, JS, many others — https://shen-language.github.io/#downloads). But, OPA offers extremely fast runtime execution in a way that would be more difficult to get to in Shen. What the compile-time guard adds is that it can make the runtime invocation non-skippable — so you could have a compile-time assertion that the code calls the runtime assertion, with OPA sitting behind the constructor. The catch is that if you still want all your invariants in Shen but use OPA for the runtime layer, that's another translation layer to keep in sync (Shen → Rego alongside Shen → guards). The alternative is to lean on Shen's portability: you could run the same spec at runtime with no translation layer at all, trading OPA's speed for that simplicity. Either way they're similar concepts run at different times. Integrating both into one high-level spec is mostly a question of which of those tradeoffs you want.
