reducers.clj

(ns dropwarlock.reducers)

(defprotocol Reducee
  (reduce-with-instructions [this inst f]))

(extend-protocol Reducee
  clojure.lang.PersistentVector
  (reduce-with-instructions [this [action acc] f]
    (case action
      :cont (if (empty? this)
              [:done acc]
              (recur (rest this) (f (first this) acc) f))
      :halt [:halted acc])))

(defn map-reducee
  [f coll]
  (let [[_ acc] (reduce-with-instructions coll
                                          [:cont []]
                                          (fn [x a] [:cont (conj a (f x))]))]
    acc))

(defn take-reducee
  [n coll]
  (let [[_ [acc _]] (reduce-with-instructions coll
                                              [:cont [[] n]]
                                              (fn [x [a cnt]]
                                                [(if (= 1 cnt) :halt :cont) [(conj a x) (dec cnt)]]))]
    acc))

(take-reducee 2 [1 2 3])
(map-reducee inc [1 2 3])