A solution to the classic "Find the index-of the first occurrence of substring in a string" problem, written in Clojure.

index_of.clj

(ns ward.index-of)

(defn index-of
  "Returns the first index where needle is found in haystack.
  Returns -1 if the needle is  not found in haystack."
  [needle haystack]  
  (loop [n needle h haystack index 0]
    (let [n-length (count n)
          h-length (count h)]
      (cond 
        (> n-length h-length) -1
        (= n (subs h 0 n-length)) index
        :else (recur n (subs h 1) (inc index))))))