[erlang-questions] bad_return_from_state_function on "3.21 Complex State” of "OTP design principles"

[Guilherme Andrade]

It seems to be a typo. I believe the intention is to set up an event
timeout[1] of 30 seconds, but if so then the timeout interval should be
bundled with the call reply  - "{reply,From,ok}" - in order to make it a
part of the list of actions to perform.

Maybe the following will do the trick:

    {keep_state, Data#{remaining := Rest}, [30000, {reply,From,ok}]}

[1]: http://erlang.org/doc/man/gen_statem.html#type-event_timeout

On 13 December 2017 at 08:01, by <by@REDACTED> wrote:

> Hi,
>
> I am trying the example on http://erlang.org/doc/
> design_principles/statem.html#id77558
> "3.21 Complex State”
>
> But I got exception below:
> ====================================
> Erlang/OTP 20 [erts-9.0] [source] [64-bit] [smp:4:4] [ds:4:4:10]
> [async-threads:10] [hipe] [kernel-poll:false] [dtrace]
>
> Eshell V9.0  (abort with ^G)
> 1> c(code_lock3).
> {ok,code_lock3}
> 2> code_lock3:start_link([1,2,3], 4).
> Locked
> {ok,<0.67.0>}
> 3> code_lock3:button(1).
> Locked
> ** exception exit: {bad_return_from_state_function,
>                        {keep_state,
>                            #{code => [1,2,3],remaining => [2,3]},
>                            30000,
>                            [{reply,
>                                 {<0.60.0>,#Ref<0.235851222.
> 49545217.215838>},
>                                 ok}]}}
>      in function  gen_statem:parse_event_result/8 (gen_statem.erl, line
> 1318)
>      in call from gen_statem:loop_event/6 (gen_statem.erl, line 1015)
>      in call from proc_lib:init_p_do_apply/3 (proc_lib.erl, line 247)
> 4>
> =ERROR REPORT==== 13-Dec-2017::15:46:09 ===
> ** State machine code_lock3 terminating
> ** Last event = {{call,{<0.60.0>,#Ref<0.235851222.49545217.215838>}},
>                  {button,1}}
> ** When server state  = {{locked,4},#{}}
> ** Reason for termination = error:{bad_return_from_state_function,
>                                    {keep_state,
>                                     #{code => [1,2,3],remaining => [2,3]},
>                                     30000,
>                                     [{reply,
>                                       {<0.60.0>,
>                                        #Ref<0.235851222.49545217.215838>},
>                                       ok}]}}
> ** Callback mode = [handle_event_function,state_enter]
> ** Stacktrace =
> **  [{gen_statem,parse_event_result,8,[{file,"gen_statem.
> erl"},{line,1318}]},
>      {gen_statem,loop_event,6,[{file,"gen_statem.erl"},{line,1015}]},
>      {proc_lib,init_p_do_apply,3,[{file,"proc_lib.erl"},{line,247}]}]
> ====================================
>
> I think the problem is on this:
>
> 		[Digit|Rest] -> % Incomplete
> 		    {keep_state, Data#{remaining := Rest, 30000},
> 		     [{reply,From,ok}]};
>
> Change code to below got the **exception
>
> 		[Digit|Rest] -> % Incomplete
> 		    {keep_state, Data#{remaining := Rest}, 30000,
> 		     [{reply,From,ok}]};
>
>
> Actually, this is apparently wrong, since the map can not got “30000" in
> this case. I tried move “30000" out of the map, then the above exception
> occurs.
> And, I also tried just remove the “30000”, and it works fine.
>
> 		[Digit|Rest] -> % Incomplete
> 		    {keep_state, Data#{remaining := Rest},
> 		     [{reply,From,ok}]};
>
>
> My question is:
> Is this a typo in the documentation?
> Or
> Am I missing something?
>
> Thanks
> by
>
> _______________________________________________
> erlang-questions mailing list
> erlang-questions@REDACTED
> http://erlang.org/mailman/listinfo/erlang-questions
>
>


-- 
Guilherme
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