Problem with code !

[!!!@!!!]

Hi! I'm having problem with this code:

-module(sample).

-export([start/0, loop/0]).

start() ->
Pid = spawn(sample, loop, []),
Pid ! {self(), do_sum_of, 1, 2},
receive
{Pid, Msg} ->
io:format("~w~n", [Msg])
end,
Pid ! stop.

loop() ->
receive
{From, sum, First, Second} ->
From ! {self(), First + Second},
loop();
{From, do_sum_of, First, Second} ->
self() ! {From, sum, First, Second};
stop -> true
end.

When I type "sample:start()." the shell hangs, it doesn't return
anything.
Can anybody explain what the problem is and why does this happen?
Can erlang processes send messages to themselves without crashing?

[Ale]

2010/9/6 !!!@!!! <elto...@gmail.com>

Hi! I'm having problem with this code:

-module(sample).

-export([start/0, loop/0]).

start() ->
   Pid = spawn(sample, loop, []),
   Pid ! {self(), do_sum_of, 1, 2},
   receive
       {Pid, Msg} ->

 
 you are waiting on a message with this pattern {Pid, Msg}

 but the message sent has another pattern.
 it has {From, sum, First, Second} pattern
 this is why it hangs, it waits for a message that never comes.
 

           io:format("~w~n", [Msg])
   end,
   Pid ! stop.

loop() ->
   receive
       {From, sum, First, Second} ->
           From ! {self(), First + Second},
           loop();
       {From, do_sum_of, First, Second} ->
           self() ! {From, sum, First, Second};
       stop -> true
end.

When I type "sample:start()." the shell hangs, it doesn't return
anything.
Can anybody explain what the problem is and why does this happen?
Can erlang processes send messages to themselves without crashing?

 

--
Ale.

[Yoshi]

On Sep 6, 1:55 pm, "!!!@!!!" <eltoni...@gmail.com> wrote:
> Hi! I'm having problem with this code:
>
> -module(sample).
>
> -export([start/0, loop/0]).
>
> start() ->
>     Pid = spawn(sample, loop, []),
>     Pid ! {self(), do_sum_of, 1, 2},
>     receive
>         {Pid, Msg} ->
>             io:format("~w~n", [Msg])
>     end,
>     Pid ! stop.
>
> loop() ->
>     receive
>         {From, sum, First, Second} ->
>             From ! {self(), First + Second},
>             loop();
>         {From, do_sum_of, First, Second} ->
>             self() ! {From, sum, First, Second};

Hello, here you are sending message to loop() process, but it seems
not receiving this message.
inserting 'loop();' would fix your problem ? :)

[!!!@!!!]

If you see carefully, first I am sending to the loop() process the
{self(), do_sum_of, 1, 2} message.
This message matches with the second pattern in the receive of the
loop().
The second pattern then sends to itself (the loop() process) the
{From, sum, First, Second} message which is captures by the first
pattern of the loop() process.
The first pattern then sends to start() ( From = start() process pid )
the {self(), First+Secong} message which should match the {Pid, Msg}
pattern in which Pid should be = self() and Msg should be = First
+Second.
I don't see where the problem is???

Note: This is my first time and the third day of learning Erlang and I
have skiped many things!

On Sep 6, 11:11 pm, Ale <peralta.alejan...@gmail.com> wrote:
> 2010/9/6 !!!@!!! <eltoni...@gmail.com>

[!!!@!!!]

I don't get it?
Where should I insert the loop(); ???

[Ale]

2010/9/6 !!!@!!! <elto...@gmail.com>

I don't get it?
Where should I insert the loop(); ???

Sorry for misguiding you the first time I didn't fully understand your program. Yoshi is right.

I'll give you a hint, instead of straight answer, hoping you think about it :-)

> > loop() ->
> >     receive
> >         {From, sum, First, Second} ->
> >             From ! {self(), First + Second},
> >             loop();
> >         {From, do_sum_of, First, Second} ->

                 %% here you send a message to the process that's running the loop function
                 %% using self() ! Msg, but then the function and the process, terminate.
                 %% so where should you put the tail-call to loop?

> >             self() ! {From, sum, First, Second};

Cheers,

--
Ale.

[!!!@!!!]

This is my new code, that prints "3" and "stop":

-module(sample).
-export([start/0, loop/0]).
start() ->
Pid = spawn(sample, loop, []),
Pid ! {self(), do_sum_of, 1, 2},
receive
{Pid, Msg} ->

io:format("~p~n", [Msg])
end,
Pid ! stop.

loop() ->
receive
{From, sum, First, Second} ->
From ! {self(), First + Second},
loop();
{From, do_sum_of, First, Second} ->

self() ! {From, sum, First, Second},
loop();
stop -> true
end.

Thanks for the help Yoshi and Ale!!!