Datomic transactor function that works like Clojure's `swap!` function.

rpc.clj

   {:db/id                 #db/id[:db.part/db]
    :db/doc                "Works like clojure.core/swap!, basically."
    :db/ident              :swap!
    :db/fn #db/fn {:lang   "clojure"
                   :params [db id k expr args]
                   :code   "(let [f (if-not (string? expr)
                                      (resolve expr)
                                      (eval (read-string expr)))
                                  e (datomic.api/entity db id)
                                  v (apply f (concat [(get e k 0)] args))]
                              [[:db/add id k v]])"}}

@(d/transact conn [[:swap! 123456 :foo/count '+ [42]]])

@(d/transact conn [[:swap! 123456 :foo/count "(fn [x y] (+ x y))" [42]]])

Possibly want eval on line 7 instead of resolve?

Perhaps to be able to do this:

@(d/transact conn [[:swap! 123456 :foo/count '(fn [x y] (+ x y)) [42]]])

I wonder if that would work?