Make a more general atom wrapper?

[Frozenlock]

Following what I've written in reagent-project/reagent#76, I decided to explore how to make a more general atom wrapper.

If you have a browser repl active, you can run this code:
https://www.refheap.com/96603

After that, you can implement the classic cursor simply with:

(defn cur [a path]
  (entangle a
            #(get-in % path)
            #(swap! %1 assoc-in path %2)))

But you can also do some more fancy/complex operations, like this one:

(def a (atom {:first-name "Bob" :last-name "Moran" :age 35 :sex "M"}))
=> #<Atom: {:first-name "Bob", :last-name "Moran", :age 35, :sex "M"}>

;; now create a new atom with custom get/update functions
(def b (entangle a
                  (fn [s-a]
                    [(:first-name s-a) (:age s-a)])
                  (fn [s-a new-value]
                    (swap! s-a assoc
                           :first-name (first new-value)
                           :age (last new-value)))))
=> #<Atom(+): ["Bob" 35]>

;; now we modify this new atom
(reset! b ["Mike" 22])
=> ["Mike" 22]

b
=> #<Atom(+): ["Mike" 22]>

;; reflected in the source atom:
a
=> #<Atom: {:first-name "Mike", :last-name "Moran", :age 22, :sex "M"}>

Notice how the same data can be present in two very different structure.

We also have much finer control of what is allowed when we update the source-atom. In this example, only the :first-name and :age fields will be modified. :last-name and :sex can't be seen (let alone modified) from the new atom.

Does anyone think it's worth exploring?

[seancorfield]

Looks interesting. Does it cater for all of the protocol methods "doing the right thing"? Or would that depend on just how elaborate your new getter / updater functions were?

Also, we need to decide whether cursors are part of core or standalone going forward. If the former, some of your earlier tweaks need to get added to core, rather than adding more generic stuff to the standalone library.

With this approach I can definitely see the benefit of a Clojure version!

[Frozenlock]

I implemented something a little more complete in another project.
I was afraid this was stretching cursors a little too far. (In my mind cursors are get-in assoc-in with a path.)