Let bindings

example.clj

(defn get-winner
  [board]
  (let [num-of-rows (get-number-of-rows board)
        spaces-on-board (range (count board))
        horizontal-coords (partition num-of-rows spaces-on-board)
        vertical-coords (apply map list horizontal-coords)
        diagonal-coords (generate-diagonal-coords num-of-rows)
        coords-to-check (concat horizontal-coords vertical-coords diagonal-coords)]
    (loop [coords-to-check coords-to-check
           row-to-check (first coords-to-check)
           winner nil]
      (if (or winner (empty? coords-to-check))
        winner
        (let [this-row-as-set (into #{} (vals (select-keys board (into [] (first coords-to-check)))))
              is-this-a-win (and (seq (first this-row-as-set)) (= 1 (count this-row-as-set)))
              winning-marker (or (and is-this-a-win (:marked (first this-row-as-set))) nil)]
          (recur (rest coords-to-check) (first (rest coords-to-check)) winning-marker))))))