Sundry Clojure snippets

clojure_snippets.clj

;; current time
(System/currentTimeMillis)

;; make a version 4 random UUID as per http://www.ietf.org/rfc/rfc4122.txt
(java.util.UUID/randomUUID)

;; start a new thread and run a function
(.start (Thread. (fn [] (loop []
                          (do
                            (update-mock-data!)
                            (Thread/sleep 1000))
                          (recur)))))

;; stringify data for sending over the network
(pr-str {:hi "there"})

;; read data safely from untrusted sources (like a browser)
(ns blah
  (:require [clojure.tools.reader :as edn]))
(edn/read-string data)

;; read from a file line by line and write each line to another file
(ns foo
  (:require [clojure.java.io :as io]))
(with-open [r (io/reader "/home/username/input.txt")
            w (io/writer "/home/username/output.txt")]
  (doseq [line (line-seq r)]
    (let [s (str line "\n")]
      (.write w s 0 (count s)))))

;; read from a RandomAccess file
(ns foo
  (:import (java.io RandomAccessFile)))
(let [begin (bar)
      end (baz)
      ba (byte-array (- end begin))]
  (with-open [file (RandomAccessFile. (str path filename) "r")]
    (doto file 
      (.seek begin) 
      (.read ba)))
  (prn (String. ba "UTF-8")))

;; using alt!
(def ch (chan))
(go-loop []
  (alt!
    foo ([msg] (prn "From foo: " foo))
    bar ([msg] (prn "From bar: " bar))
    [baz sth] ([msg _] (>! ch msg))))

;; see also, "fan-in" from https://github.com/martintrojer/go-tutorials-core-async/blob/f0e178011184d1d6a1846104c1be2829407d25f0/src/tut104.clj
(defn fan-in [in1 in2]
  (let [ch (chan)]
    (go-loop []
             ;; note alt! result-expr syntax
             (alt! [in1 in2]
                   ([msg _] (>! ch msg)))
             (recur))
    ch))

;(go
;  (<! readystate)
;  (while true
;    (let [[val ch] (alts! [socketinput socketoutput])]
;      (cond
;        (= ch socketinput) (print "received websocket input: " val)
;        (= ch socketoutput) (do
;                              (print "I have something to send: " val)
;                              (.send @socket (pr-str val)))))))