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.