Thanks for the quick responses.
I'll try to answer Andy's question: "How do you know, in advance, that
it doesn't need to handle such concurrent changes?" ... and at the
same time I will try to provide this example to Stuart, hoping I can
see how using a map inside an atom might work:
Let's say my users log into a web page and each user has a queue that
does stuff for them. Since the users id is unique and each user can
only be logged in from one session, when I use the user id as a key
within a hash-map then I know *well-enough* there will not be any
concurrent changes to that key value pair, particularly since enqueing
means each change is actually an just addition to the stack. -- So I
am picking queue's to make a point... -- Queue's are chosen over lists
as they are both constant time and fast. Being atomic is great, but
wouldn't making a copy of a queue and re-assembling it defeat the
purpose of using it?
So let's try this:
> (def user-queues* (atom (hash-map)))
#'project/user-queues*
> (swap! user-queues* assoc "user1" clojure.lang.PersistentQueue/EMPTY)
{"user1" #<PersistentQueue clojure.lang.PersistentQueue@0>}
> (@user-queues* "user1")
#<PersistentQueue clojure.lang.PersistentQueue@0>
I would like to add an item to the users queue, but it seems when
using an atom I can only swap in and out the value as opposed to
modifying the value in-place.
So let's start with just the basic atom'd queue:
> (def q (atom clojure.lang.PersistentQueue/EMPTY))
#'project/q
> (swap! q conj (seconds))
#<PersistentQueue clojure.lang.PersistentQueue@d3232253>
> (apply list (swap! q conj (seconds)))
(1315961557 1315961570)
awesome.
Now I want to store each users queue in the user-queues* hash-map. How
would I do that while maintaining a real queue? My initial attempts
always lead to reading the full queue into a list, then to conj and
item on that list, then I have to remake a queue to then be stored
back into the map via swap....so it's at that point I might as well
not be using a queue - right?
Certainly hash-maps with queue's would be a reasonable idea - right?.
Thanks.
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