tagged-out.clj

(require '[clojure.string :as string])

(defn make-tagged-out [tag]
  (let [tag (format "[%s] " tag) 
        tag-lines (fn [s]
                    (string/replace s #"(^|\n)" (str "$1" tag)))
        old-out *out*]
    (proxy [java.io.Writer] []
      (flush []
        (. old-out (flush)))
      (append [c]
        (. old-out (append c)))
      (write
        ([^String s]
           (locking old-out
             (.write old-out (tag-lines s))))))))

(defmacro with-out-tag [tag & body]
  `(binding [*out* (make-tagged-out ~tag)]
     ~@body))

(let [f1 (future
           (with-out-tag "a"
             (dotimes [_ 10]
               (Thread/sleep (max (rand 500) (rand 500)))
               (println "Hello from a!"))))

      f2 (future
           (with-out-tag "b"
             (dotimes [_ 10]
               (Thread/sleep (max (rand 500) (rand 500)))
               (println "Hello from b!"))))])

;; [a] Hello from a!
;; [b] Hello from b!
;; [a] Hello from a!
;; [b] Hello from b!
;; [a] Hello from a!
;; [b] Hello from b!
;; [b] Hello from b!
;; [a] Hello from a!
;; [b] Hello from b!
;; [a] Hello from a!
;; [b] Hello from b!
;; [a] Hello from a!
;; [b] Hello from b!
;; [a] Hello from a!
;; [b] Hello from b!
;; [a] Hello from a!
;; [b] Hello from b!
;; [b] Hello from b!
;; [a] Hello from a!
;; [a] Hello from a!