Clojure with emacs+cider. The ability to interact with a "living" system in a homoiconic language where functional programming allows you to do some neat stuff

Clojure with emacs+cider. The ability to interact with a "living" system in a homoiconic language where functional programming allows you to do some neat stuff was mindblowing. Specially because the LISP guys probably have been doing this for ages. Think React hot reload on steroids, applied to the whole app and with an interactive IDE where you can eval any expression and get immediate feedback, even if the expression interacts with a db (requiring a db connection) or whatever.