ReQL is declarative. The queries are compiled into an AST in the client drivers, the AST is shipped to the database, and is then analyzed and executed on the cluster. There is no information loss, and nothing runs on the client. The query language looks operational (as is, just run these commands in order), but is also declarative in the same way as SQL is.

This blog post explains how this works: http://rethinkdb.com/blog/lambda-functions/