core.clj

(defmacro fork [x & bodies]
  (let [n (gensym)]
    `(let [~n ~x]
       (pvalues
        ~@(map (fn [[a & b]]
                 (cons a (cons n b)))
               bodies)))))

(defn join [results fn]
  (fn results))

;;=> (-> {:a "foo" :b "bar"}
;;     (fork
;;       (-> (assoc :b 8))
;;       (-> (assoc :y 6))
;;       (-> (assoc :x 4)))
;;     (join (partial apply merge)))
;;
;; {:x 4, :y 6, :a "foo", :b "bar"}