programming-clojure/src/examples/error_kit.clj at 39fe42f23452ed58ba9644fe6351027b25c1ef69 · stuarthalloway/programming-clojure · GitHub

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; example inspired by http://gigamonkeys.com/book/beyond-exception-handling-conditions-and-restarts.html
(ns examples.error-kit
 (:use [clojure.contrib.error-kit]))

(deferror malformed-log-entry [] [msg]
  {:msg msg
   :unhandled (throw-msg IllegalArgumentException)})

; imaginary log message format:
; 2008-10-05 12:14:00 WARN Some warning message here...
(defn parse-log-entry [entry]
  (or
    (next (re-matches #"(\d+-\d+-\d+) (\d+:\d+:\d+) (\w+) (.*)" entry))
    (raise malformed-log-entry entry)))

(def bad-log
 ["2008-10-05 12:14:00 WARN Some warning message here..."
  "<<this is not a log message>>"
  "2008-10-05 12:14:00 INFO End of the current log..."])

(def good-log
 ["2008-10-05 12:14:00 WARN Some warning message here..."
  "2008-10-05 12:14:00 INFO End of the current log..."])

; Example 1. Continue calculation with replacement value
(defn parse-or-nil [logseq]
  (with-handler
    (vec (map parse-log-entry logseq))
    (handle malformed-log-entry [msg]
      (continue-with nil))))

; Example 2. Continue calculation with logging & replacement value
(defn parse-or-warn [logseq]
  (with-handler
    (vec (map parse-log-entry logseq))
    (handle malformed-log-entry [msg]
      (continue-with (println "****warning****: invalid log: " msg)))))

; Example 3. Caller can choose from a fixed set of contiue strategies.
(defn parse-or-continue [logseq]
  (let [parse-log-entry
	(fn [entry]
	  (with-handler (parse-log-entry entry)
			(bind-continue skip [msg] nil)
			(bind-continue log [msg] (println "****warning****: invalid log: " msg))))]
    (vec (map parse-log-entry logseq))))