positional partial

ppartial.clj

;;; positional partial for giggles
;;; use #(foo %1 %2) instead

(defn- placeholder-indices
  ([template]
     (placeholder-indices '_ template))
  ([placeholder-sym template]
     (->> template
          (map vector (range))
          (filter (comp (partial = placeholder-sym) last))
          (map first))))

(defn- merge-args [template values]
  (let [slots-values (map vector (placeholder-indices template) values)]
    (reduce (fn [args [slot val]]
              (assoc args slot val))
            (vec template)
            slots-values)))

(defn ppartial* [f template]
  (fn [& args]
    (apply f (merge-args template args))))

(defmacro ppartial [f & args]
  `(ppartial* ~f '~(vec args)))

(comment
  ((ppartial - _ 3) 10) ;=> 7
  ((ppartial assoc _ :y _) {} 2) ;=> {:y 2}
  )