S-expressions are a nice representation for ASTs in general. At work we needed a query language for an API, so we used the data structures we had (JSON-ish) to

S-expressions are a nice representation for ASTs in general. At work we needed a query language for an API, so we used the data structures we had (JSON-ish) to express a little Lisp and then wrote a parser from an SQL-ish language to this Lisp.