distilled version of enliven segments as an external lib

segments.clj

(ns segments)

(defrecord Lens [fetch putback])

(defprotocol Segment
  (-fetch [seg value])
  (-putback [seg value subvalue]))

(defn fetch [value seg]
  (-fetch seg value))

(defn putback [value seg subvalue]
  (-putback seg value subvalue))

(def base-extend-list [clojure.lang.Keyword 
                       #+clj clojure.lang.Symbol 
                       #+cljs string
                       #+clj String])  

(doseq [t base-extend-list]
  (extend t
    Segment {:-fetch #(get %2 %1)
             :-putback #(assoc %2 %1 %3)}))

(extend Lens
  Segment {:-fetch #((:fetch %1) %1 %2)
           :-putback #((:putback %1) %1 %2))
		   
(defn fetch-in [obj path]
  (reduce fetch obj path))

(defn putback-in [obj value [seg & path]]
  (when (and obj seg)
    (if (empty? path)
      (putback obj seg value)
      (putback obj seg (putback-in (fetch obj seg) value path)))))