Exchanging messages between two processes in Erlang

Question

Hi I'm learning Erlang and in the process I'm trying to do the exercises from the Erlang execises page. In particular I'm doing the problem where I have to create a function that creates two processes that send M messages to the other and then die. I'm having trouble with it, this my code:

-module(roundtrip).
-export([talk/1]).

talk(M) ->
    init(M).

init(M) ->
    P1 = start(M),
    P2 = start(M),
    P2 ! {P1, a_message}.

myProc(M) ->
    if M =:= 0 ->
        io:format("Bye cruel world!~n")
    end,
    receive
        {From, a_message} ->
            From ! {self(), a_message},
            myProc(M-1);
        _ -> 
            io:format("I don't understand~n"),
            myProc(M)

    end.

start(M) ->
    spawn(?MODULE, myProc, [M]).

I get when I call the talk the following errors:

=ERROR REPORT==== 11-Nov-2013::21:21:00 === Error in process <0.46.0> with exit value: {undef,[{roundtrip,proc,"\n",[]}]}

=ERROR REPORT==== 11-Nov-2013::21:21:00 === Error in process <0.47.0> with exit value: {undef,[{roundtrip,proc,"\n",[]}]}

I'm going around it but I can't figure out what's the problem...

Thanks.

ldx ldx · Accepted Answer · 2013-11-11T22:35:16

Since you use spawn/3 you need to export myProc.

Another problem is that your if condition does not cover the other possibility when M =/= 0. I'd do it like this:

-module(roundtrip).

-export([talk/1]).

talk(M) ->
    init(M).

init(M) ->
    P1 = start(M),
    P2 = start(M),
    P2 ! {P1, a_message}.

myProc(M) ->
    case M of
        0 ->
            io:format("Bye cruel world!~n");
        _ ->
            receive
                {From, a_message} ->
                    From ! {self(), a_message},
                    io:format("~p ~p~n", [self(), M]),
                    myProc(M-1);
                _ ->
                    io:format("I don't understand~n"),
                    myProc(M)

            end
    end.

start(M) ->
    spawn(fun() -> myProc(M) end).

Note that I used case instead of if and spawn/1 instead of spawn/3.

Exchanging messages between two processes in Erlang

2 Answers