_transducer lens fun

.

at-xfs.clj

(defn at [k xform]
  (fn [rf]
    (fn
      ([] (rf))
      ([result] (rf result))
      ([result input]
        (rf result
            (let [f (partial sequence xform)
                  map-entry? (map-entry? input)
                  associative? (associative? input)]
              (if-not (or map-entry? associative?)
                input
                (update input (if map-entry? 1 k) f))))))))

(defn at-in [[k & ks] xform]
  (if ks
    (at k (at-in ks xform))
    (at k xform)))
      

(transduce (at :a (map inc)) conj {:a [1 2 3]})
(transduce (at-in [:a] (map inc)) conj {:a [1 2 3]})
(transduce (at-in [:a :b] (map inc)) conj {:a {:b [1 2 3]}})
(transduce (comp (filter map?)
                 (at :a (filter identity)))
           conj
           [1 2 {:a [nil nil 1]}])

map-xfs.clj

(defn get-xf
  ([k]
    (map (fn [m] (get m k))))
  ([k not-found]
    (map (fn [m] (get m k not-found)))))

(defn assoc-xf [k v & kvs]
  (map (fn [m] (apply assoc m k v kvs))))
 
 (defn update-xf [k f & last-args]
   (map (fn [m] (apply update m k f last-args))))