Clojure

The language that teaches functional programming through data rather than types. Clojure is a Lisp on the Java platform where nearly everything is an immutable map, vector or set, and the REPL-driven workflow, evaluating code straight from your editor into a running program, is the most fun way on this list to explore an idea. It is proven at serious scale: Nubank, the Brazilian digital bank, runs mission-critical software on Clojure and Datomic and bought Cognitect, the company behind Clojure, in 2020. Full access to Java libraries means you rarely hit a dead end. The trade-offs are the parentheses for the first couple of weeks, dynamic typing that leaves more checking to tests, and JVM startup time for small scripts.