util.clj

(defmacro _->
  ([x] x)
  ([x form] (if (seq? form)
              (if (seq-utils/includes? form '_)
                (with-meta `(~@(replace {'_ x} form)) (meta form))
                (with-meta `(~(first form) ~x ~@(next form)) (meta form)))
              (list form x)))
  ([x form & more] `(_-> (_-> ~x ~form) ~@more)))

;-> and _-> are almost the same behavior until _ is specified.
(_-> {} (assoc :a 1) seq)
;=> ([:a 1])

; Insert expr in arbitrary place by marking _.
(_-> "abcdef" (map int _) (apply hash-map _) (assoc _ 99 :d))
;=> {97 98, 99 :d, 101 102}