I've now read those pages. I knew about E for its capability model but not at all for its concurrency model, which is indeed very interesting (more interesting,

I've now read those pages. I knew about E for its capability model but not at all for its concurrency model, which is indeed very interesting (more interesting, to me) and close to home. There's a presentation mentioned there that I found slightly clearer than the page itself; the link is dead but it can be read here: http://web.archive.org/web/20070626045558/http://www.drjava.de/e-presentation/html-english/img0.html http://web.archive.org/web/20070626045558/http://www.drjava.... This model deserves to better known, especially since all this stuff is in play again nowadays. Are there other implementations of it? (Node.js might be a natural platform, given its event loop and the popularity of promises there.) I also wonder what the biggest differences are between it and Erlang. In your pipelining example (t3 := (x <- a()) <- c(y <- b())), what happens if the messages arrive out of order? (Or does the system prevent that and if so how?) I suppose if an expression can't be evaluated because a promise hasn't resolved yet, it could just go back to the end of the queue and eventually everything will bubble up? Also: that Xanadu? Wow, no idea. Edit: I forgot another thing I wanted to include in this omnibus comment. The paper "On the Development of Reactive Systems" that's mentioned at your first link and which can be found at [1], starts off with a very interesting and exciting distinction between "transformational" systems (which take inputs and transform them into outputs) and "reactive" systems that "are repeatedly prompted by the outside world and [whose] role is to continuously respond to external inputs. A reactive system, in general, does not compute or perform a function, but is supposed to maintain a certain ongoing relationship, so to speak, with its environment". I think this is extraordinarily lucid and that the authors are right to talk about concurrency—and probably complexity in general—in the latter sort of system as being a different challenge than in the former. Unfortunately, the paper quickly sinks into a pit of software process goo and never returns. A little Googling gives me the impression that important technical work did come out of it, though, and if anyone knows what the high points are I would like to see them. [1] http://www.wisdom.weizmann.ac.il/~dharel/SCANNED.PAPERS/ReactiveSystems.pdf http://www.wisdom.weizmann.ac.il/~dharel/SCANNED.PAPERS/Reac...