Add update-existing-in

[eraserhd]

Like update-in, except do not update missing keys.

[weavejester]

Thanks for the PR. It's a good change, but I think we should go with something closer to the definition of clojure.core/update-in, so:

(defn update-existing-in [m ks f & args]
  (let [up (fn up [m ks f args]
             (let [[k & ks] ks]
               (if-let [kv (find m k)]
                 (if ks
                   (assoc m k (up (val kv) ks f args))
                   (assoc m k (apply f (val kv) args)))
                 m)))]
     (up m ks f args)))

[eraserhd]

@weavejester commit amended and force-pushed. Your code is unchanged, except for docs and metadata.

I was wondering this while looking at clojure.core/update-in's source, but I guess Clojure is smart enough to not try to not make a thunk when there are no variables closed over, and that's why f and args are passed to up?

[weavejester]

There doesn't seem to be much of a measurable difference in performance between passing f and args as arguments, and passing them via the lexical scope.

I think I like the look of passing them as arguments slightly better, and when in doubt I'm happy to defer to the core implementation, as at least then we can be sure we're not making things any slower.

[eraserhd]

So, I think this is ready then?

[weavejester]

Yep, looks fine. Thanks for your work on this.