Created
June 10, 2013 23:08
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taplの3章?
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| (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))) |
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