map_transform.clj

;; in response to https://gist.github.com/733674

(def m
  {"Lisa Rose" {"Lady in the Water" 2.5 "Snakes on a Plane" 3.5} 
   "Gene Seymour" {"Lady in the Water" 3.0 "Snakes on a Plane" 3.5}})

;; one way
(reduce
 (fn [m [critic movie rating]]
   (assoc-in m [movie critic] rating))
 {}
 (for [[critic ratings] m
       [movie rating] ratings]
   [critic movie rating]))

;; another way
(reduce
 (fn [m [critic ratings]]
   (reduce
    (fn [m [movie rating]]
      (assoc-in m [movie critic] rating))
    m ratings))
 {} m)

;; yet another way
(apply
 merge-with merge
 (for [[critic ratings] m
       [movie rating] ratings]
   {movie {critic rating}}))