multiprocess program running in Erlang currency programming - concurrency

Question

I am trying to learn Erlang currency programming.
This is an example program got from Erlang.org but no instructions about how to run it.
I run it in this way,
1> counter:start()
<0.33.0>
But, I do not know how to run other functions so that the process (counter:start()) can do the work according to the received message.
How to confirm that two or more processes have really been generated ?
Another question, how to print out received message in a function ?
-module(counter).
-export([start/0,loop/1,increment/1,value/1,stop/1]).
%% First the interface functions.
start() ->
spawn(counter, loop, [0]).
increment(Counter) ->
Counter ! increment.
value(Counter) ->
Counter ! {self(),value},
receive
{Counter,Value} ->
Value
end.
stop(Counter) ->
Counter ! stop.
%% The counter loop.
loop(Val) ->
receive
increment ->
loop(Val + 1);
{From,value} ->
From ! {self(),Val},
loop(Val);
stop -> % No recursive call here
true;
Other -> % All other messages
loop(Val)
end.
Any help will be appreciated.
thanks

Answer 1

Other functions will just use the module you just created, like this:
C = counter:start(),
counter:increment(C),
counter:increment(C),
io:format("Value: ~p~n", [counter:value(C)]).
You can run pman:start() to bring up the (GUI) process manager to see which processes you have.

Answer 2

In addition to what Emil said, you can use the i() command to verify which processes are running. Let's start three counters:
1> counter:start().
<0.33.0>
2> counter:start().
<0.35.0>
3> counter:start().
<0.37.0>
And run i():
...
<0.33.0> counter:loop/1 233 1 0
counter:loop/1 2
<0.35.0> counter:loop/1 233 1 0
counter:loop/1 2
<0.37.0> counter:loop/1 233 1 0
counter:loop/1 2
...
As you can see, the above processes (33, 35 and 37) are happily running and they're executing the counter:loop/1 function. Let's stop process 37:
4> P37 = pid(0,37,0).
<0.37.0>
5> counter:stop(P37).
stop
Checking the new list of processes:
6> i().
You should verify it's gone.