plain_fsm - for beginners and purists

Ulf Wiger <>
Wed Feb 11 17:51:21 CET 2004


On Wed, 11 Feb 2004 00:11:10 -0500, Shawn Pearce <> 
wrote:

> I have to say, I think plain_fsm is a nice advance over gen_fsm, and
> would love to see some form of it in OTP proper.

Thank you. (:


> I only have two comments:
>
> The parse transform wrapper doesn't make sense when you intially read
> it.  In fact, it looks like it couldn't possibly do what it is actually
> doing, as it seems like a non tail-recursive function being setup,
> which is horrible in a persistant server (yes, I know its actually
> making correct code, I'm saying that reading it initially has you
> reading it wrong):
>
> 	idle(S) ->
> 		plain_fsm:exented_receive(
> 			receive
> 			do_nothing -> idle(S)
> 			end).

I know. This is a problem. I thought that it was a reasonable
violation, but I agree that if you don't know that the code
is transformed into something else, it is confusing.


> I'd suggest a different parse transform, or just a plain macro:
>
> 	idle(S) ->
> 		receive
> 		'SYSTEM' -> 'SYSTEM';
> 		do_nothing -> idle(S)
> 		end.

This construct suffers from the same problem as my wrapper
function. The code above would terminate if it received the
message 'SYSTEM' (unless you know that there is a transformation,
of course.)


> 	idle(S) ->
> 		receive
> 		?PLAIN_FSM_SYSTEM
> 		do_nothing -> idle(S)
> 		end.

I've been down this path a couple of times, but a problem that's
difficult to get around is that the ?PLAIN_FSM_SYSTEM macro needs
to pass the state (the variable S in the above example), and there
is just no elegant way to do that, I think.



> I find the second the lazyman's way out, while the first with the parse
> transform is much more elegant, and keeps to basic Erlang syntax.

My conclusion is that there is no way to do this while preserving
both syntax and semantics - assuming you want to actually simplify
things compared to what's already there in OTP.

Joe recently suggested a -compile({token_transform, Module}) directive,
so that he could use "!!" without forcing everyone to hack the parser.
Using a token transform, I could introduce a new keyword (local to the
behaviour) that would signal the change in semantics:

    idle(S) ->
       extended_receive
          ...
       end.

(One could also imagine a directive to replace the parser...,
and why not the linter too, while we're at it. ;)


> It also lets you order the system/parent exit events with regards to the
> other events in the receive statement.

I'd have to think about whether or not that's a good thing. (:


> My other comment has to do with exit failure.  If a plain_fsm crashes
> its parent will receive the {'EXIT', P, R} message (or crash itself).
> But who writes out the error_report through error_logger?  I find this
> very valuable that every gen_server and gen_fsm process will write
> its crash report to the error log, without the developer needing to
> ensure this happens.

You change the spawn function of your FSM to one of :
  - plain_fsm:spawn(Mod, Fun)
  - plain_fsm:spawn_link(Mod, Fun)
  - plain_fsm:spawn_opt(Mod, Fun)

These will perform some local magic and then call the corresponding
functions in proc_lib.erl. Thus, you will get crash reports if the
process dies.

(BTW, the plain_fsm docs failed to mention that you need to include
"plain_fsm.hrl" in order to get the parse_transform to happen.)

/Uffe
-- 
Ulf Wiger, Senior System Architect
EAB/UPD/S

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