[erlang-questions] Question concerning the use of -export

[Gaspar Chilingarov]

David Cabana wrote:
> I have a question regarding the -export module attribute; it arose as  
> a result of playing with this code,
> taken from the section 3.2 of the Erlang Getting Started document  
> available at
> http://www.erlang.org/doc/getting_started/part_frame.html
> 
> Here is the code:
> %---------------------------------------------------
> -module(tut15).
> 
> -export([start/0, ping/2, pong/0]).
> 
> ping(0, Pong_PID) ->
>      Pong_PID ! finished,
>      io:format("ping finished~n", []);
> 
> ping(N, Pong_PID) ->
>      Pong_PID ! {ping, self()},
>      receive
>          pong ->
>              io:format("Ping received pong~n", [])
>      end,
>      ping(N - 1, Pong_PID).
> 
> pong() ->
>      receive
>          finished ->
>              io:format("Pong finished~n", []);
>          {ping, Ping_PID} ->
>              io:format("Pong received ping~n", []),
>              Ping_PID ! pong,
>              pong()
>      end.
> 
> start() ->
>      Pong_PID = spawn(tut15, pong, []),
>      spawn(tut15, ping, [3, Pong_PID]).
>      %---------------------------------------------------
> 
> My question is this: why do ping/2 and pong/0 have to be exported?   
> The code is invoked from a command line by tut15:start(),
> so I can see that start/0 has to be exported. I tried not exporting  
> pong/0, and got a compiler warning:
> ./tut15.erl:17: Warning: function pong/0 is unused
> 
> I disregarded the warning and executed the code, producing a runtime  
> error:
> Error in process <0.141.0> with exit value: {undef,[{tut15,pong,[]}]}
> 
> It seems to me that pong/0 is not  being called from outside the  
> tut15 module, so I should not have to export it. What am I missing here?

I had the same questions, when I first started programming erlang -- the 
annswer is that spawn'ed functions should be exported.

or you should do

spawn(fun pong() end ) or spawn(fun pong/0)

which will not require exporting of function ping



-- 
Gaspar Chilingarov

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