Clojure Programming Chapter2 P.73〜P.76 logger

logger-01.clj

(defn print-logger
  [writer]
  #(binding [*out* writer]
     (println %)))

(def *out*-logger (print-logger *out*))
;= #'user/*out*-logger
(*out*-logger "hello")
; hello
;= nil

logger-02.clj

(def writer (java.io.StringWriter.))
;= #'user/writer
(def retained-logger (print-logger writer))
;= #'user/retained-logger
(retained-logger "hello")
;= nil
(str writer)
;= "hello\n"

logger-03.clj

(require 'clojure.java.io)

(defn file-logger
  [file]
  #(with-open [f (clojure.java.io/writer file :append true)]
     ((print-logger f) %)))

(def log->file (file-logger "message.log"))
;= #'user/log->file
(log->file "hello")
;= nil

; $ more message.log
; hello

logger-04.clj

(defn multi-logger
  [& logger-fns]
  #(doseq [f logger-fns]
     (f %)))

(def log (multi-logger
          (print-logger *out*)
          (file-logger "message.log")))
;= #'user/log
(log "hello again")
; hello again
;= nil

; $ more message.log
; hello
; hello again

logger-05.clj

(defn timestamed-logger
  [logger]
  #(logger (format "[%1$tY-%1$tm-%1$te %1$tH:%1$tM:%1$tS] %2$s" (java.util.Date.) %)))

(def log-timestamped (timestamed-logger
                      (multi-logger
                       (print-logger *out*)
                       (file-logger "message.log"))))

(log-timestamped "goodbye, now")
;[2012-07-25 10:42:59] goodbye, now
;nil

; $ more message.log
;hello
;hello again
;[2012-07-25 10:42:59] goodbye, now