taplの3章?

core.clj

(ns tapl.core
  (:require [instaparse.core :as insta]))

(def my-parser
  (insta/parser
   "<T> := 'true' | 'false' | '0' | 
           'succ'<space>T | 'pred'<space>T | 'iszero'<space>T | 
           'if'<space>T<space>'then'<space>T<space>'else'<space>T;
    space := #'\\s';"))

(defn succ [n] (inc n))
(defn pred [n] (dec n))

(defn iszero [n]
  (zero? n))

(defn my-eval [exp]
  (try (first ((fn -eval [[head & tail]]
                 (case head
                   "succ"   (let [[h t] (-eval tail)] [(succ   h) t])
                   "pred"   (let [[h t] (-eval tail)] [(pred   h) t])
                   "iszero" (let [[h t] (-eval tail)] [(iszero h) t])
                   "0"      [0     tail]
                   "true"   [true  tail]
                   "false"  [false tail]
                   "if"     (let [[cnd  rest1] (-eval tail)
                                  [then rest2] (-eval (rest rest1))
                                  [else rest3] (-eval (rest rest2))]
                              [(case cnd true then false else :else nil) rest3])
                   :else nil)) exp))
       (catch Exception e nil)))

(defn -main [& args]
  (->> (line-seq (java.io.BufferedReader. *in*))
       (map #(str % ":" (my-eval (my-parser %))))
       (interpose "\n")
       (apply str)
       (println)))