Clojureでオプションで場所指定可能なスレッディングマクロを作る ref: http://qiita.com/kurogelee/items/e23a205d6bc9cc26d0f2

file0.txt

(->> (range 10) (map -) (#(nth % 3)))

file2.txt

user=> (o>> 10 (range) reverse (map -) (nth <> 3) (identity {:a [1 <>]}))
{:a [1 -6]}
user=> (o> 10 (range) reverse (nth 3) (map - (range <>)))
(0 -1 -2 -3 -4 -5)

optional_threading_macro.clj

(defn cons-nth [index x coll]
  (concat (take index coll) [x] (drop index coll)))
(defn cons-last [x coll]
  (concat coll [x]))

(defn- find-symbol [form sym]
  (cond (or (sequential? form) (set? form)) (some #(find-symbol % sym) form)
        (map? form) (or (find-symbol (keys form) sym)
                        (find-symbol (vals form) sym))
        :else (= form sym)))

(defmacro o> [x & forms]
  (loop [x x forms forms]
    (if-not forms x
      (let [form (first forms)
            threaded (if (seq? form)
                       (-> (if (find-symbol form '<>)
                             (list 'let ['<> x] form)
                             (cons-nth 1 x form))
                           (with-meta (meta form)))
                       (list form x))]
        (recur threaded (next forms))))))

(defmacro o>> [x & forms]
  (loop [x x forms forms]
    (if-not forms x
      (let [form (first forms)
            threaded (if (seq? form)
                       (-> (if (find-symbol form '<>)
                             (list 'let ['<> x] form)
                             (cons-last x form))
                           (with-meta (meta form)))
                       (list form x))]
        (recur threaded (next forms))))))