Problem 20 turned out to be yet another ridiculously simple problem for Clojure. Here’s how to sum the digits in 100 factorial:

(defn factorial  [n] (reduce * (range 2 (inc n))))
(defn sum-digits [s] (reduce + (map #(Integer/parseInt (str %)) s)))
(defn euler-20   []  (sum-digits (str (factorial 100))))

(euler-20)