Game of Life in 8 lines of Clojure

gistfile1.clj

(defn neighbors-of [cell]
  (set (for [dx [-1 0 1] dy [-1 0 1] :when (not (= [dx dy] [0 0]))]
    [(+ dx (first cell)) (+ dy (second cell))])))

(defn alive? [[cell freqs] world]
  (or (and (= 2 freqs) (contains? world cell)) (= 3 freqs)))

(defn tick [world]
  (let [frequencies (frequencies (reduce #(concat %1 (neighbors-of %2)) [] world))]
    (set (keys (filter #(alive? % world) frequencies)))))

Well, you can move when -- keep #(when (alive? world %) %)
Or rename alive? to something like grow-cell :-)