> Danny, could you maybe hack up a possible very simple example of what
> you mean?
<snip>
> I can not simply store a copy of the Karl person in Tinas :friends
> slot,
> because when Karl gets one year older, then I don‘t want to go through
> all persons, looking for Karls, and have their age updated.
Hi André,
Your shared instance constraint does make things a little more
interesting than in my application, but I think John's suggestion
about not storing friend instances directly is a good thing. If you
were to instead assign an id to each created person, and have each
friend list be a list of ids, the entity referred to by the id can
still be immutable for a given dereference. Not sure how much sense
that makes, so here's something crude that I've hacked up:
(def *everyone* (ref {}))
(def *global-id* (ref 0))
(defn next-id [] (dosync (commute *global-id* inc)))
(defstruct person :name :friend-ids :id)
(defn make-person [name] (struct person name [] (next-id)))
(defn make-and-add-person [name]
(let [person (make-person name)]
(dosync (alter *everyone* assoc (:id person) person))))
(defn befriend [person friend]
(let [new-friend-list (conj (:friend-ids person) (:id friend))
new-person (assoc person :friend-ids new-friend-list)]
(dosync
(alter *everyone* assoc (:id person) new-person))))
The 'down side' here is simply that you refer to people by ids, rather
than directly, but the up side is that a given deref of *everyone* is
immutable and consistent.
Cheers,
Danny.
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