isbn.clj

(defn- valid-pattern [s] (boolean (re-matches #"\d-\d{3}-\d{5}-[\dX]" s)))
(defn- valid-checksum [s]
  (let [csum (->> s
                  (map {\0 0 \1 1 \2 2 \3 3 \4 4 \5 5 \6 6 \7 7 \8 8 \9 9 \X 10})
                  (keep identity)
                  (map * (range 10 0 -1))
                  (reduce +))]
    (zero? (rem csum 11))))
(def isbn? (every-pred valid-pattern valid-checksum))

twelve.clj

(ns twelve
  (:require [clojure.string :as str]))

;; lazy interpose with a special last separator
(defn interpose-last
  [sep sep-last coll]
  (if (next coll)
    `(~@(interpose sep (butlast coll)) ~sep-last ~(last coll))
    coll))

;; data 
(def data
  {:sts ["first" "second" "third" "fourth" "fifth" "sixth" "seventh" "eighth" "ninth" "tenth" "eleventh" "twelfth"]
   :verses ["a Partridge in a Pear Tree"
            "two Turtle Doves"
            "three French Hens"
            "four Calling Birds"
            "five Gold Rings"
            "six Geese-a-Laying"
            "seven Swans-a-Swimming"
            "eight Maids-a-Milking"
            "nine Ladies Dancing"
            "ten Lords-a-Leaping"
            "eleven Pipers Piping"
            "twelve Drummers Drumming"]})

;; recite one verse
(defn recite1 [verse]
  `("On the "
    ~((:sts data) (dec verse))
    " day of Christmas my true love gave to me: "
    ~@(->> (range (dec verse) -1 -1)
           (map (:verses data))
           (interpose-last ", " ", and "))
    "."))

;; recite verses from start to end
(defn recite [start end]
  (->> (range start (inc end))
       (map recite1)
       (interpose "\n\n")
       (apply concat)
       (apply str)))

(recite 1 12)