with-socket.clj

; A with-socket macro for a network application I began writing
; in Clojure. Since the application never got finished but I
; sometimes refer to this macro I put it here for posterity.

(defmacro with-socket
  "The way to interact with a socket. Given a socket name and
   ip + port, will connect to the server and run stmts in an
   environment where the socket is connected. Always closes
   the socket, even if exceptions happen.
   The returned map consists of a :readln entry and a :writeln
   entry, both of which work as the docstrings for readln and
   writeln indicate." 
  [sock [ip port] & stmts]

  (let [socket-obj (gensym 'socket)]
    `(let [~socket-obj (Socket.)]
       (try
         (.setSoTimeout ~socket-obj 2000)
         (.connect ~socket-obj (InetSocketAddress. ~ip ~port))

         (let [~sock {:readln  (partial readln  (BufferedReader. (InputStreamReader. (.getInputStream  ~socket-obj))))
                      :writeln (partial writeln (PrintWriter.                        (.getOutputStream ~socket-obj)))}]
           ~@stmts)

         (finally (.close ~socket-obj))))))


(defn readln
  "Reads a line from the BufferedReader, stripping \\r\\n. If
   read is semi-blocking, this function returns nil when the
   read times out."
  [reader]

  (try (.readLine reader)
    (catch SocketTimeoutException e nil)))


(defn writeln
  "Writes a line to the PrintWriter, appending \\n."
  [writer msg]

  (.println writer msg)
  (.flush writer))