defpartial.clj

(letfn [(variadic? [arglist]
          (boolean (some #{'&} arglist)))

        (arglistc [arglist]
          (if (variadic? arglist)
            Double/POSITIVE_INFINITY
            (count arglist)))

        (partial-arglists [arglists argc]
          (drop-while
           (fn [arglist]
             (<= (arglistc arglist) argc))
           arglists))]

  (defmacro defpartial [sym f & args]
    (let [v (cond
              (var? f)
              fn
              (symbol? f)
              (resolve f)
              :else
              (throw (IllegalArgumentException. "f must be a symbol or a var")))
          arglists (:arglists (meta v))
          argc (count args)
          partial-arglists (partial-arglists arglists argc)
          partial-arglists (if (variadic? (first partial-arglists))
                             (let [arglist (first partial-arglists)
                                   [positional-args rest-args] (split-with (complement #{'&}) arglist)
                                   positional-args (drop argc positional-args)]
                               (list (vec (concat positional-args rest-args))))
                             partial-arglists)
          fn-specs (for [arglist partial-arglists]
                     `(~arglist
                       ~(if (variadic? arglist)
                          `(apply ~v ~@args ~@(remove #{'&} arglist))
                          `(~v ~@args ~@arglist))))]
      `(defn sym ~@fn-specs))))