WIP: Concurrency design

proposal/2-concurrency.md

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+# Proposal: Concurrency on Nash
+
+There has been some discussion on how to provide concurrency to nash.
+There is a [discussion here](https://github.com/NeowayLabs/nash/issues/224) 
+on how concurrency could be added as a set of built-in functions.
+
+As we progressed discussing it seemed desirable to have a concurrency
+that enforced no sharing between concurrent functions. It eliminates
+races and forces all communication to happen explicitly, and the
+performance overhead would not be a problem to a high level language
+as nash.
+
+Converging to a no shared state between concurrent functions initiated
+the idea of using the current rfork built-in as a means to express
+concurrency on Nash. This would already be possible today, the idea
+is just to make it even easier, specially the communication between
+different concurrent processes.
+
+This idea enables an even greater amount of isolation between concurrent
+processes since rfork enables different namespaces isolation (besides memory),
+but it has the obvious fallback of not being very lightweight.
+
+Since the idea of nash is to write simple scripts this does not seem
+to be a problem. If it is on the future we can create lightweight concurrent
+processes (green threads) that works orthogonally with rfork.
+
+The prototype for the new rfork would be something like this:
+
+```sh
+chan <= rfork [ns_param1, ns_param2] (chan) {
+        //some code
+}
+```
+
+The code on the rfork block does not have access to the
+lexical outer scope but it receives as a parameter a channel
+instance.
+
+This channel instance can be used by the forked processes and
+by the creator of the process to communicate. We could use built-in functions:
+
+```sh
+chan <= rfork [ns_param1, ns_param2] (chan) {
+        cwrite($chan, "hi")
+}
+
+a <= cread($chan)
+```
+
+Or some syntactic extension:
+
+```sh
+chan <= rfork [ns_param1, ns_param2] (chan) {
+        $chan <- "hi"
+}
+
+a <= <-$chan
+```
+
+Since this channel is meant only to be used to communicate with
+the created process, it will be closed when the process exit:
+
+```sh
+chan <= rfork [ns_param1, ns_param2] (chan) {
+}
+
+# returns empty string when channel is closed
+<-$chan
+```
+
+Fan out and fan in should be pretty trivial:
+
+```sh
+chan1 <= rfork [ns_param1, ns_param2] (chan) {
+}
+
+chan2 <= rfork [ns_param1, ns_param2] (chan) {
+}
+
+# waiting for both to finish
+<-$chan1
+<-$chan2
+```