[giaour]

> Those aren't "rules engine"s though

Yeah, I don't know why I picked rules engines as an example of things that are hard to configure with JSON. My day job is working on an infrastructure-as-code DSL, and I previously worked on an RPC framework IDL. Both of those projects are replacing an alternative in a traditional serialization language (JSON and XML, respectively) where users spent a lot of their time fighting against JSON or XML gotchas.

The IaC language supports expressions but not named expressions, so we've been able to avoid Turing completeness so far.

[lmm]

That may not be enough; you wouldn't be the first supposedly-non-Turing-complete language where it's actually possible to express a Y combinator and use that to embed arbitrary computations.