[erlang-questions] gen_statem badrecord,trans_opts

[Raimo Niskanen]

On Wed, May 16, 2018 at 10:39:36AM +0200, Torben Hoffmann wrote:
> 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.

That one was changed to
  error:{bad_state_enter_return_from_state_function,
	 {next_state,creating,happy}}
in commit ea1963553ffb06eb1bda636f328718f91136ed9c on 'master'

I hope that is not too subtle - I wanted to stick to the format.
At least it indicates state_enter and state function.


> 
> 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!
> 

That one was changed to
  error:{bad_state_enter_action_from_state_function,
	 {next_event,internal,jump}}
in the same commit.


> 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.

In this case you return a tuple with a tag that is not recognized, and it
would take some artificial intelligence to figure out that it most likely
should be next_state that the user attempted.

The tuple does not match any of the allowed patterns, simply, so it is
a bad return value.  Difficult to do anything smarter.


Best regards
/ Raimo


> 
> 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.
> > > --
> > > https://www.linkedin.com/in/torbenhoffmann/
> >
> > --
> >
> > / Raimo Niskanen, Erlang/OTP, Ericsson AB
> -- 
> https://www.linkedin.com/in/torbenhoffmann/


-- 

/ Raimo Niskanen, Erlang/OTP, Ericsson AB