395-2010-winter-hw1.clj

(ns hw1 (:refer-clojure :exclude [eval]))

(defrecord StringLit [s])
(defrecord Concat [l r])
(defrecord RestAfter [l r])

(defn parse [sexp]
  (cond (symbol? sexp) (StringLit. (str sexp))
        (sequential? sexp)
        (case (second sexp)
              '& (Concat. (parse (first sexp)) (parse (nth sexp 2)))
              '% (RestAfter. (parse (first sexp)) (parse (nth sexp 2)))
              :else (throw (IllegalArgumentException. (str "bad list: " sexp))))
        :else (throw (IllegalArgumentException. (str "bad arg: " sexp)))))

(defprotocol Interpreter
  (eval [this]))

(extend-protocol Interpreter
  StringLit
  (eval [s] (:s s))
  Concat
  (eval [c] (str (eval (:l c)) (eval (:r c))))
  RestAfter
  (eval [ra]
        (let [l (eval (:l ra))
              r (eval (:r ra))
              i (.indexOf l r)]
          (if (< i 0)
            (throw (Exception. (str r " not found in " l)))
            (.substring l (+ i 1))))))

(-> '[a & [b & c] % b] parse eval)