Access to the previous value is an interesting idea. I'm thinking of putting together a lib where all updates are collection into transactions (which would run *inside* of Clojure transactions, but would have their own semantics also). Something like this:
(update-model (update x 3) (update y (+ (val y) 7))) Where update-model is a macro that makes sure everything gets recomputed before commit. It would be pretty straightforward to keep the previous value around. On Mar 3, 10:22 am, Raffael Cavallaro <raffaelcavall...@gmail.com> wrote: > On Mar 3, 8:04 am, Rich Hickey <richhic...@gmail.com> wrote: > > > If B and C depend on A in such > > a way that their values must be coordinated in order to be valid and > > consistent, then maybe they shouldn't independently depend on A, > > instead a transaction should fire on changes to A, which in turn > > updates B and C. Then any downstream consumers of B and C could never > > see inconsistent values, all without a notion of 'current step'. > > Certainly you can and should hide the notion of current step for > *most* purposes. Using transactions is certainly a good way to do it; > but you must expose it in some way or you won't be able to express the > full range of dataflow models. > > Typically: > > ;; pseudocode > > (defmodel my-model > (a :independent) > (b (fn [a] (+ (* a 12) 3))) > (c (fn [a] (+ a 7))) > (d (fn [c b] (* c d 3.2)))) > > Here the step is implicit, as each reference to another cell in the > functions above really means "the value of that cell at the *same* > step/time/data pulse." However: > > 1. failing to deal with this issue in the *implementation* of a > dataflow > library leads to the wrong semantics; the absence of this notion or > any other means of dealing with it, (such as a transaction as you > suggest) > commonly leads to bugs in cells-alikes, including at least one clojure > implementation. > > 2. there are certain types of models where one needs to be able to > refer to previous pulses/steps. These include reflexive update > functions, (where the value of a cell depends on its own value at some > previous step/time/pulse), and time lags. With these types of > dependencies, there must be some way to explicitly refer to step or > pulse: > > ;pseudocode with time lag > > ((defmodel my-model > (a :independent) > (b (fn [a] (previous a 3))) ;; i.e., how many steps back > (c (fn [a] (+ a 7))) > (d (fn [c d] (* c d 3.2)))) > > You can certainly hide the most common case (i.e., everything is at > the same > step/pulse), but it has to be there internally or the semantics will > be > wrong, and it has to be exposed in some way or you won't be able to > express many interesting types of models in a simple, direct way. --~--~---------~--~----~------------~-------~--~----~ 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 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 -~----------~----~----~----~------~----~------~--~---
