computation in bg thread

(defmacro bg
  "Runs the body in a background thread.

  Creates two vars, one named bg<N> and the other bg<N>'.

  The first will eventually contain the result of the computation,
  or any exception that is thrown.

  The second is a map that prints with status & runtime information.

  Returns the name of the first var."
  [& body]
  (let [base (str "bg" (swap! bg-id-counter inc))
        sym (symbol base)
        sym' (symbol (str base \'))]
    `(do (println "Starting background task" '~sym)
         (def ~sym)
         (def ~sym')
         (run-and-report (var ~sym) (var ~sym') '~sym (fn [] ~@body))
         '~sym)))