I love JQ so much we implemented a subset of JQ in Clojure so that our users could use it to munge/filter data in our product (JVM and browser based Kafka tooling). One of the most fun coding pieces I've done, though I am a bit odd and I love writing grammars (big shoutout to Instaparse![1]).
I learned through my implementation that JQ is a LISP-2[2] which surprised me as it didn't feel obvious from the grammar.
[1] https://github.com/Engelberg/instaparse
[2] https://github.com/jqlang/jq/wiki/jq-Language-Description#:~....
It’s not over-promising, either. I went from never having heard of it before to getting complete and correct parse trees of some ancient JSP Expression Language in about 20 minutes. Most of that time was spent typing in the BNF description that I could find only in an image.
(def as-and-bs
(insta/parser
"S = AB*
AB = A B
A = 'a'+
B = 'b'+"))
=> (as-and-bs "aaaaabbbaaaabb")
[:S
[:AB [:A "a" "a" "a" "a" "a"] [:B "b" "b" "b"]]
[:AB [:A "a" "a" "a" "a"] [:B "b" "b"]]]
The only parsing library I’ve played with recently is Instaparse in Clojure, which is apparently GLL. It was a delight to use. I found a PDF of the old version of Apache Expression Language that I needed to interpret, typed it almost verbatim into a quite readable text file, and Instaparse did the rest. It was an exciting experience to have unlocked this embedded DSL with about three lines of code, not counting the grammar definition.