mapmap.clj

(defn mapmap
  "Return a new map, mapping keys to (keyfn key) and mapping values to
(valfn val)"
  ([valfn map]
     (mapmap identity valfn map))
  ([keyfn valfn map]
     (persistent! (reduce 
		   (fn [c [k v]] (assoc! c (keyfn k) (valfn v)))
		   (transient {})
		   map))))

Inspired by http://tech.puredanger.com/2010/09/24/meet-my-little-friend-mapmap/:

Performance using persistant data only:

user> (time ((mapmap identity identity bigmap) :noexist))
"Elapsed time: 513.812028 msecs"

Transients can really speed-up:
user> (time ((mapmap identity identity bigmap) :noexist))
"Elapsed time: 301.332019 msecs"