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<  Erlang  ~  (newbie) including 'unused' local functions
rlysens
Posted: Tue Oct 02, 2007 8:13 pm Reply with quote
Joined: 02 Oct 2007 Posts: 1 Location: Belgium
Hi all,

Below is a variant of the clock example from the 'Programming Erlang' book. It's using the MFA version of spawn.
To get it to work I had to export tick/2. If I don't, I get this:

2> clock:start(1000,io:format("Tick~n") end).
** 1: syntax error before: 'end' **
2> clock:start(1000,fun() -> io:format("Tick~n") end).
true

=ERROR REPORT==== 2-Oct-2007::22:02:58 ===
Error in process <0.40.0> with exit value: {undef,[{clock,tick, [1000,#Fun<erl_eval.20.62269157>]}]}

Exporting tick/2 is probably not the right solution. Can anybody tell me what is the correct approach?

Thanks,

Ruben.


Code:

-module(clock).
-export([start/2, tick/2, stop/0]).

stop() -> clock ! stop.

tick(Time, Fun) ->
    receive
   stop ->
       void
    after Time ->
       Fun(),
       tick(Time, Fun)
    end.

start(Time, Fun) ->
    Pid = spawn(clock, tick, [Time, Fun]),
    register(clock, Pid).
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hao
Posted: Fri Oct 12, 2007 8:59 am Reply with quote
User Joined: 20 Aug 2007 Posts: 18 Location: Uppsala, Sweden
Hi, rlysens,

I have tried your code on the Erlang shell. Here is an alternative solution I have figured out.

Code:

-module(clock).
-export([start/2, stop/0]).

stop() -> clock ! stop.

tick(Time, Fun) ->
    receive
        stop ->
            void
    after Time ->
            Fun(),
            tick(Time, Fun)
    end.

start(Time, Fun) ->
    F = fun() -> tick(Time, Fun) end,
    Pid = spawn(F),
    register(clock, Pid). 


I hope this will be helpful. It works well without exporting tick/2 function. I think function variable/object in Erlang is quite useful and/or powerful.
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francesco
Posted: Mon Oct 15, 2007 2:26 pm Reply with quote
User Joined: 07 Jul 2006 Posts: 249 Location: London
Hi rlysens,

You should export tick/2, as all functions in the spawn(M,F,A) BIF have to be exported, as it is "called" by a function outside the module. That is not the case with the spawn(fun()-> ... end), as local functions are encapsulated in the fun. So what you did is right.

Regards,
Francesco
--
http://www.erlang-consulting.com
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