Anyone know how this compares to Fennel, another Lisp-on-Lua language?
AFAIK Urn is effectively its own language with its own semantics, and Fennel I think is better thought of as a lisp-syntax-frontend for Lua. They both compile to Lua, sure, but I believe the Urn compiler is doing a lot more leg work.
That’s correct. The big difference is that Urn does a lot more static analysis; I think at one point they were building a gradual type system into it? But some of that made interop with Lua less direct, and it also made it difficult to support interactive development with a repl. (I started using Urn a bit before I discovered Fennel, but the lack of a repl was rather problematic for me, even tho I liked a lot of other things about it.)
the projects is not being active for the past 2 or 3 years, but if you are interested in lisp like languages that are lean and clean I suggest you take a look at Janet and Fennel (the author of both is a single guy)