rpn.clj

(ns rpn.core
  [:use [clojure [string :only [split]]]])

(defn parse-int [s]
  (Integer/parseInt (re-find #"\A-?\d+" s)))

(defn parse [expression]
  (let 
    [operators {"+" + "-" - "*" * "/" quot}
     parse-token 
     (fn [token]
       (if (contains? operators token)
         (get operators token)
         (parse-int token)))]
    (map parse-token 
         (split expression #"\s"))))

(defn process-symbol [stack symbol]
  (if (number? symbol)
    (conj stack symbol)
    (conj (pop (pop stack))
          (apply symbol (take-last 2 stack)))))

(defn evaluate [expression]
  (peek 
    (reduce 
      process-symbol
      []
      (parse expression))))