Here's the source code for update-in:
(defn update-in
([m [k & ks] f]
(if ks
(assoc m k (update-in (get m k) ks f))
(assoc m k (f (get m k)))))
([m [k & ks] f a]
(if ks
(assoc m k (update-in (get m k) ks f a))
(assoc m k (f (get m k) a))))
([m [k & ks] f a b]
(if ks
(assoc m k (update-in (get m k) ks f a b))
(assoc m k (f (get m k) a b))))
([m [k & ks] f a b c]
(if ks
(assoc m k (update-in (get m k) ks f a b c))
(assoc m k (f (get m k) a b c))))
([m [k & ks] f a b c & args]
(if ks
(assoc m k (apply update-in (get m k) ks f a b c args))
(assoc m k (apply f (get m k) a b c args)))))
As far as I know (and I don't now much), this always gives the same result:
(defn my-update-in2
([m [k & ks ] f & args]
(if ks
(assoc m k (apply update-in (get m k) ks f args))
(assoc m k (apply f (get m k) args)))))
My question: why isn't update-in (and many other Clojure functions) implemented this way? I would guess there are performance issues, ie. not using apply is faster.