My take on the print diamond kata https://gist.github.com/trikitrok/378a8af7d9a4295a93f5

core.clj

(ns diamonds.core
  (:require [clojure.string :refer [upper-case]]))


(defn- repeat-space [n]
  "Returns a string of n spaces"
  (apply str (take n (repeat " "))))

(defn- ping-pong [n]
  "Returns a sequence of numbers from 0 to n and back to 0."
  (let [up (range 0 (inc n))
        down (drop 1 (reverse up))]
    (concat up down)))

(defn- diamond-width [row]
  "Returns the diamond width for the given row number."
  (inc (* 2 row)))

(defn- diamond-indent [max row]
  "Returns the left spaces needed to indent the specified row/max."
  (repeat-space (- max row)))

(defn- diamond-body [row]
  "Returns a string with the diamond body for the specified row."
  (let [letter (char (+ (int \A) row))]
      (if (= row 0)
        (str letter)
        (str letter
             (repeat-space (- (diamond-width row) 2))
             letter))))

(defn- diamond-row [max row]
  "Returns a string with both the indentation and the body for the given
  row/max."
  (str (diamond-indent max row)
       (diamond-body row)))

(def ^:private diamond-row-memo (memoize diamond-row))

(defn- diamond [c]
  "Returns the list of strings needed to render the diamond for c."
  (let [width (- (int c) (int \A))]
  (map (partial diamond-row-memo width)
       (ping-pong width))))

(defn print-diamond [s]
  (let [c (first (upper-case s))]
    (doseq [l (diamond c)]
      (println l))))

(defn -main
  [& args]
  (print-diamond (first args)))