A function to update multiple parts of a structure.given a function that gives the locations of those parts.

update_at.clj

(defn update-at
  "Takes a structure, a function that returns a list of paths (lists of keys) and a
  function to apply at those paths within the structure.

  For any paths that don't already exist in m, then f will be called with nil
  and that path in the structure will be created with the result."
  [m path-generation-fn f]
  (reduce (fn [s v] (update-in s v f))
          m
          (path-generation-fn m)))

Perhaps this should just take a list of paths and leave the calling function to call the path generating function.