Eight Queens Puzzle (SICP Exercise 2.42) Clojure solution using list comprehension

queens.clj

(ns sicp.queens)

(def empty-board '())

(defn safe?
  "Check if queen in last row is safe"
  [positions]
  (let [[queen-pos & left] positions
        k (count positions)
        diags-up (map - left (range 1 (inc k)))
        diags-down (map + left (range 1 (inc k)))]
    (empty? (filter (partial = queen-pos) (concat diags-up diags-down left)))))

(defn queens [board-size]
  ((fn queen-cols [k]
     (if (zero? k)
       (list empty-board)
       (for [rest-of-queens (queen-cols (dec k))
             new-row (range 1 (inc board-size))
             :let [new-cols (cons new-row rest-of-queens)]
             :when (safe? new-cols)]
         new-cols)))
  board-size))