[erlang-questions] Communicating processes

[Peter Lund]

Fredrik Hoback skrev:
> Hi, I'm having some trouble with inter-process communication. First of 
> all I have a process which receives a message it looks something like 
> this:
>
> %%%%%%%%%%%%%%%%%%%%%%%%
> receive_msg() ->
> receive
> {Msg} ->
> io:format("Recieved Message");
> _->
> io:format("Nothing to report")
> end.
> %%%%%%%%%%%%%%%%%%%%%%%%
> Note: this process is spawned and registered as proxy.
>
> To continue: I have another module which I start from within the shell 
> it looks similar to this:
>
> %%%%%%%%%%%%%%%%%%%%%%%%
> send_message()->
> Msg = "Hello World", % My message
> send(Msg), % here I send my message somewhere unknown and the reply is 
> sent to the process proxy above
> %Here I want to receive the message that is received in proxy
> %%%%%%%%%%%%%%%%%%%%%%%%
>
> So how to I accomplish this? Is there any other way then using " ! "? 
> What I would like to do is to call the process proxy from within 
> send_message and the return answer should be Msg, is this possible? 
> And how should it look?

I am not sure what you are looking for, but anyhow if you would like to
implement a function "call" to retrieve something from another process:

%% process A
...
   NameOfB = call(BPid, what_is_your_name)
...

call(Pid, Msg) ->
   Ref =  make_ref(),
   Pid ! {call_request, Ref, self(), Msg},
   receive
      {call_response, Ref, Resp} -> Resp
   after 5000 -> timeout
   end.


%% process B
loop() ->
    receive {call_request, Ref, From, what_is_your_name} ->
       From ! {call_response, Ref, "Barbados"}
   end,
   loop().


The make_ref() returns a unique refernce assuring the caller that it is
the response to the correct request. Observer that the call here is
allowed to timeout after 5000 millisecs.