fwiw, when faced with a similar problem i've used graph databases (these days i guess neo4j, but at the time i used datalog). you either find errors loading the database, or via queries that parallel the rule-like approach used here. it's not as cool as this, but you end up with something quite similar to what is described (a way to find errors "declaratively") without needing to implement a new language.
also, i wonder if life would have been easier if the implementation had used prolog? not only is it famously nice for supporting dsls (don't ask me how - i've never done it, but you can imagine that parsing is pretty easy, since you get backtracking "for free"), but it might provide much of the scaffolding needed to evaluate queries.
I'm a software engineer at Knewton who worked with Kenny on this project.
We're not using Neo4j because of other unrelated constraints that have led us to write our own graph data store. Kenny's work implemented the "rule paralleling queries" approach you mention.
I'm not sure about Prolog, but Java's Antlr library was really easy to use. Writing the parser+lexer infrastructure only took a few days. It wasn't quite as clean as using Antlr in a language like SML, but for implementing a parser in Java it was pretty great.
I have recently been experimenting with Treetop, a ruby gem that allows you to easily work with parsing expression grammars.
I may move to Common Lisp or Clojure for writing DSLs, but I am pretty happy with my early experience with Treetop. The one downside, especially compared to Antlr which has a couple of books out there, is that the Treetop documentation had me more than once scratching my head and hacking rather than looking up a relevant reference.
I think parsing expression grammars and packrat parsing in general is going to explode the DSL dev scene . . .
interesting, thanks (sorry for misunderstanding slightly). possibly related / useful in future - did you notice that neo4j's licence terms changed a while back? i can't remember the details, but i had avoided it in the past because of licensing and remember that the change seemed to be an improvement.
also, i wonder if life would have been easier if the implementation had used prolog? not only is it famously nice for supporting dsls (don't ask me how - i've never done it, but you can imagine that parsing is pretty easy, since you get backtracking "for free"), but it might provide much of the scaffolding needed to evaluate queries.