Thanks for your replies - Meikel and Alex, I considered using refs, but perhaps wrongly, came to the conclusion that they were for coordinating multiple changes atomically within an STM ? I figured that I only had one shared piece of state - current- state (input and output are both effectively thread-local) and that an atom would be far more efficient for my use case.
I didn't notice that compare-and-set! had been exposed - so, yes, I could use this, but it would probably only be to roll my own swap! variant that allowed me to return a tuple of [new-state output].... So, am I just being awkward :-) - should I really be using refs/STM's to achieve this ? or am I using a pattern peculiar to myself because of some other wierd design decision that I have taken ? or have others come across the same requirement and been similarly perplexed ? thx Jules On May 17, 5:12 pm, ataggart <alex.tagg...@gmail.com> wrote: > If you are not concerned about coordination, then why not just grab > the value beforehand and use compare-and-set: > > (let [old @my-atom > new (my-fn arg)] > (compare-and-set! my-atom old new) > (do-stuff old new)) > > On the other hand, if you are concerned about coordination between > multiple threads changing the state, then don't use atoms; use refs. > > On May 17, 8:49 am, Jules <jules.gosn...@gmail.com> wrote: > > > > > Guys, > > > I have a hybrid Java/Clojure project. > > > I'm finding myself moving more and more of the concurrent code from > > Java to Clojure - because it is so much simpler to code in Clojure > > :-)... > > > ... but I am also finding that I have a recurring problem for which > > Clojure only seems to have a partial solution. > > > The problem occurs in stateful objects when you receive an input and > > need to both modify your state and generate and deliver some output. > > > I store my internal state in an atom and use swap! to update it, > > passing in the input and a pure function which accepts the object's > > current state and input and returns the object's new state. > > > The problem is that there is no clear mechanism for returning the > > object's output from the same pure function. > > > At the moment, I am going with a solution whereby my object's state is > > a variable length tuple e.g. [current-state & latest-output]. This > > allows me to return both new state and output from the same swap! I > > just have to agree with all other functions reading the state that > > only e.g. current-state is relevant and latest-output can be ignored > > by all except the caller of swap!, which actually processes/delivers > > it, but this feels clumsy. > > > Am I missing something here or is there room for another member of the > > swap! family which returns a tuple with a similar structure to mine, > > but only uses the head of this tuple to reset the state of the > > enclosing atom ? > > > Thanks for your time, > > > Jules > > > -- > > You received this message because you are subscribed to the Google > > Groups "Clojure" group. > > To post to this group, send email to clojure@googlegroups.com > > Note that posts from new members are moderated - please be patient with > > your first post. > > To unsubscribe from this group, send email to > > clojure+unsubscr...@googlegroups.com > > For more options, visit this group > > athttp://groups.google.com/group/clojure?hl=en > > -- > You received this message because you are subscribed to the Google > Groups "Clojure" group. > To post to this group, send email to clojure@googlegroups.com > Note that posts from new members are moderated - please be patient with your > first post. > To unsubscribe from this group, send email to > clojure+unsubscr...@googlegroups.com > For more options, visit this group > athttp://groups.google.com/group/clojure?hl=en -- You received this message because you are subscribed to the Google Groups "Clojure" group. To post to this group, send email to clojure@googlegroups.com Note that posts from new members are moderated - please be patient with your first post. To unsubscribe from this group, send email to clojure+unsubscr...@googlegroups.com For more options, visit this group at http://groups.google.com/group/clojure?hl=en
