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.
+
+## Lightweight Processes
+
+This idea is inspired on Erlang concurrency model. Since Nash does
+not aspire to do everything that Erlang does (like distributed programming)
+so this is not a copy, we just take some things as inspiration.
+
+Why call this a process ? On the [Erlang docs](http://erlang.org/doc/getting_started/conc_prog.html)
+there is a interesting definition of process:
+
+```
+the term "process" is usually used when the threads of execution share no
+data with each other and the term "thread" when they share data in some way.
+Threads of execution in Erlang share no data,
+that is why they are called processes
+```
+
+In this context the process word is used to mean a concurrent thread of
+execution that does not share any data. The only means of communication
+are through message passing. Since these processes are lightweight
+creating a lot of them will be cheap (at least must cheaper than
+OS processes).
+
+Instead of using channel instances in this model you send messages
+to processes (actor model), it works pretty much like a networking
+model using UDP datagrams.
+
+The idea is to leverage this as a syntactic construction of the language
+to make it as explicit and easy as possible to use.
+
+This idea introduces 4 new concepts, 3 built-in functions and one
+new keyword.
+
+The keyword **spawn** is used to spawn a function as a new process.
+The function **send** is used to send messages to a process.
+The function **receive** is used to receive messages from a process.
+The function **self** returns the pid of the process calling it.
+
+An example of a simple ping/pong:
+
+```
+pid <= spawn fn () {
+    ping, senderpid <= receive()
+    echo $ping
+    send($senderpid, "pong")
+}()
+
+send($pid, "ping", self())
+pong <= receive()
+
+echo $pong
+```
+
+TODO:
+
+* If send is never blocking, what if process queue gets too big ? just go on until memory exhausts ?
+* What happens when you send to a invalid pid ? (or a pid of a process that is not running anymore)
+* Example on how would fan-out/fan-in look with this idea
+
+## Extend rfork
+
+Converging to a no shared state between concurrent functions initiated
+the idea of using the current rfork built-in as a means to express
+concurrency on Nash. This would already be possible today, the idea
+is just to make it even easier, specially the communication between
+different concurrent processes.
+
+This idea enables an even greater amount of isolation between concurrent
+processes since rfork enables different namespaces isolation (besides memory),
+but it has the obvious fallback of not being very lightweight.
+
+Since the idea of nash is to write simple scripts this does not seem
+to be a problem. If it is on the future we can create lightweight concurrent
+processes (green threads) that works orthogonally with rfork.
+
+The prototype for the new rfork would be something like this:
+
+```sh
+chan <= rfork [ns_param1, ns_param2] (chan) {
+        //some code
+}
+```
+
+The code on the rfork block does not have access to the
+lexical outer scope but it receives as a parameter a channel
+instance.
+
+This channel instance can be used by the forked processes and
+by the creator of the process to communicate. We could use built-in functions:
+
+```sh
+chan <= rfork [ns_param1, ns_param2] (chan) {
+        cwrite($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
+```