What is wrong with my message passing example?

Question

I am attempting to send a message from one process that I spawned to another for an assignment, I feel like I am very close here, but I think my syntax is just a bit off:

-module(assignment6).
-export([start/1, process1/2, process2/0, send_message/2]).

process1(N, Pid) ->

    Message = "This is the original Message",

    if 
        N == 1 ->
            timer:sleep(3000),
            send_message(Pid, Message);

        N > 1 ->
            timer:sleep(3000),
            send_message(Pid, Message),
            process1(N-1, Pid);

        true ->
            io:fwrite("Negative/0, Int/Floating-Point Numbers not allowed")
    end.

process2() ->
    recieve
        Message ->
            io:fwrite(Message),
            io:fwrite("~n");
    end.

send_message(Pid, Message) ->
    Pid ! {Message}.

start(N) ->
    Pid = spawn(assignment6, process2, []),
    spawn(assignment6, process1, [N, Pid]).

The goal of this program is that the Message, will be printed out N times when the function is started, but be delayed enough so that I can hot-swap the wording of the message mid-run. I just can't quite get the Message to process2 for printout.

Answer 1

Four small things:

• It's spelled receive, not recieve
• Remove the semicolon in process2. The last clause in a receive expression does not have a terminating semicolon. You can see this in the if expression in process1: the first two clauses end with a semicolon, but the third one does not.

• In process2, print the message like this:

  io:fwrite("~p~n", [Message])
  

  Since Message is a tuple, not a string, passing it as the first argument to io:fwrite causes a badarg error. Let's ask io:fwrite to format it for us instead.

• process2 should probably call itself after printing the message. Otherwise, it will receive one message and then exit.

---

So now you can run the code, and while it's running you can load a new version of the module with a different message (so called "hot code swapping"). Will that change the message being printed? Why / why not?

It won't. process1 does a local call to itself, which means that it stays in the old version of the module. Do an external call instead (explicitly specifying the module: assignment6:process1(N-1, Pid)), and it will switch to the new version.