​(defn star-seq []
  (iterate #(str % "**") "*"))

(defn pad [s n]
  (let [spaces (repeat (/ (- n (count s)) 2) \space)]
    (apply str (concat spaces s))))

(defn print-pyramid [n]
  (let [stars (take n (star-seq))
        width (count (last stars))
        padded-stars (map #(pad % width) stars)]
    (doseq [row padded-stars]
      (println row))))

;; user=> (print-pyramid 10)
;;          *
;;         ***
;;        *****
;;       *******
;;      *********
;;     ***********
;;    *************
;;   ***************
;;  *****************
;; *******************