[erlang-questions] gen_statem badrecord,trans_opts

[Torben Hoffmann]

Hi Raimo,

I'm looking forward to that particular error being more understandable in a
future release. It is too small to patch for.

The other day I was trying to capture a somewhat elaborate state machine in
a gen_statem and I hadn't read the manual well enough, so I thought that I
could do some nifty checking in the enter clause and decide if another
state was more suited to do the job. But with state_enter you cannot change
state in the enter clause. It is clearly written din the manual, but... you
know... blush.

However, the error message doesn't point you to this fundamental rule:

** Last event = {internal,init_state}
** When server state  = {starting,happy}
** Reason for termination = error:{bad_return_from_state_function,
                                      {next_state,creating,happy}}
** Callback mode = [state_functions,state_enter]

It would be nice if it said something about not changing state when
entering.

Same kind of issue when adding next_event in a state enter:

** Reason for termination = error:{bad_action_from_state_function,
                                      {next_event,internal,jump}}

Maybe the error should be
{dont_add_next_event_in_enter_read_the_manual_dude, ...} or something to
that effect!

And when you misspell next_state you get this:

** Reason for termination = error:{bad_return_from_state_function,
                                   {next_stete,running,happy,
                                    {next_event,internal,jump}}}

It would be nice if it said what it could not like.

I have not looked in the gen_statem code, but I am pretty sure that the
checks would allow more details in the error messages. The information must
be at hand when it errors out.

Cheers,
Torben


On Wed, May 16, 2018 at 8:52 AM Raimo Niskanen <
raimo+erlang-questions@REDACTED> wrote:

> On Tue, May 15, 2018 at 10:59:50PM +0200, Torben Hoffmann wrote:
> > Hi,
> > Would it be possible to add some text to the gen_statem docs that this
> > message usually means you have messed up one of the actions?
> > It wasn't obvious to me before I googled it and then it was a matter of
> > chasing down all my actions.
> > Somehow I find the error messages of gen_statem harder to work with than
> > gen_fsm, which is a pity since gen_statem is so much easier to work with
> in
> > terms of design flexibility and easy to raed code.
> >
> > Cheers,
> > Torben
>
> The message in this case:
>   Context:    child_terminated Reason:
>     {{badrecord,trans_opts},
>      [{gen_statem,loop_event_actions_list,10,
>        [{file,"gen_statem.erl"},{line,1210}]},
>       {proc_lib,init_p_do_apply,3,
>        [{file,"proc_lib.erl"},{line,247}]}]}
>
> I classify this as a bug, but not severe enough to merit a patch on the
> 20.3
> release.  What the code was trying to do but crashed instead was to
> terminate with error:{bad_action_from_state_function,Action} in a regular
> Error Report.
>
> The intention is that gen_statem should pinpoint bad actions fairly
> precisely.  That crash is fixed in 20.0-rc1 and later so it will terminate
> as intended.
>
> I agree that the OP:s crash is not a good error message for end users, but
> it was a good one for me since it was precise for me to fix the crash.
>
> Do you have any suggestions about improving correct error messages from
> gen_statem, or can you exemplify some that you find hard to understand?
> In comparison with gen_fsm, if you like.
>
> Best Regards
> / Raimo
>
>
> >
> >
> > On Tue, May 15, 2018 at 8:23 AM getonga <getonga@REDACTED> wrote:
> >
> > > Thanks, I recheck my source code, and I found the error reason. Thanks
> for
> > > your time.
> > > _______________________________________________
> > > erlang-questions mailing list
> > > erlang-questions@REDACTED
> > > http://erlang.org/mailman/listinfo/erlang-questions
> > >
> > --
> > https://www.linkedin.com/in/torbenhoffmann/
>
> > _______________________________________________
> > erlang-questions mailing list
> > erlang-questions@REDACTED
> > http://erlang.org/mailman/listinfo/erlang-questions
>
>
> --
>
> / Raimo Niskanen, Erlang/OTP, Ericsson AB
> _______________________________________________
> erlang-questions mailing list
> erlang-questions@REDACTED
> http://erlang.org/mailman/listinfo/erlang-questions
>
-- 
https://www.linkedin.com/in/torbenhoffmann/
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://erlang.org/pipermail/erlang-questions/attachments/20180516/f27d547d/attachment.htm>