cheap merge-with when you only need lookup

gistfile1.clj

(defn cheap-merge-with
  "Merges two maps-as-functions" [f a b]
  (memoize (fn this 
             ([k] (this k nil))
             ([k default]
               (let [av (a k this)
                     bv (b k this)]
                 (if (identical? this av)
                   (if (identical? this bv)
                     default
                     bv)
                   (if (identical? this bv)
                     av
                     (f av bv))))))))

(defn fold-vec-maps-into-map [coll]
  "Provided a reducer, concatenate into a vector.
   Note: same as (into [] coll), but parallel."
  (let [merge-fn (partial cheap-merge-with into)]
    (r/fold (r/monoid merge-fn hash-map) merge-fn coll)))