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Plain FSM - Support library for wrigin textbook Erlang FSMs

Copyright (c) 2014-23 Ulf Wiger

Authors: ulf@wiger.net.

A behaviour/support library for writing plain Erlang FSMs.

This library implements an OTP behaviour for writing plain Erlang FSMs, alleviating a long-standing gripe of mine that the OTP behaviours, for all their power, force programmers into a coding style that is very much different from that taught in the Basic Erlang Course (or the book, or online tutorials, ...) -- the type of programming that made us want to use Erlang in the first place.

Only in my old age have I begun to understand fully what a sacrifice this is. See e.g. my presentation Death by Accidental Complexity (QCon SF 2010) for a more detailed discussion of the issues involved. (Slides also available in the doc/ directory of this repos)

The requirements that drove us away from plain Erlang programming in the first place were:

• The need to support system messages to control upgrade, state inspection, shutdown, etc. The plain_fsm library solves this in a reasonable way, I think.
• The need for debugging support. The debugging support in e.g. gen_server is, I believe, rendered obsolete by the new powerful trace support (and dbg) in later versions of OTP.
• In the case of gen_server, reducing the need to reinvent the wheel, a valid point, but more so for e.g. the client side of gen_server:call(). In a protocol state machine, the only thing that really needs reusing is the handling of system messages.

However, the behaviours provided by OTP for FSM programming, gen_server and gen_fsm (gen_server is perhaps a more common choice than gen_fsm), both have the distinct drawback that you cannot normally start with a classic Erlang design and then migrate to a behaviour without significant code rewrite. In addition, the two behaviours are semantically different from the classic Erlang design

Using plain_fsm

First, write your state machine without worrying about OTP system messages. Once you're happy with it, figure out where you really want to handle system messages. Normally, it will suffice to do it in a fairly stable state. A good rule of thumb is that the top-level state machine should handle system messages, while the transient (sub-) states shouldn't

In the states where you want to handle system messages, you have three choices:

(A) Insert the system messages in the receive clause:

idle(S) ->
   Parent = plain_fsm:info(parent),
   receive
      {system, From, Req} ->
         plain_fsm:handle_system_msg(
             Req, From, S, fun(S1) -> idle(S1) end);
      {'EXIT', Parent, Reason} ->
         plain_fsm:parent_EXIT(Reason, S);
      ... %% your original code here
   end.

This has the advantage that everyone can understand what's going on. The part that plain_fsm.erl helps you with is the set of functions system_code_change(), system_continue(), system_shutdown(), format_status(), which are required callbacks when you handle system messages directly.

(B) Handle system messages and unknown messages together:

idle(S) ->
   Parent = plain_fsm:info(parent),
   receive
      ... %% your original code here
      Msg ->
         plain_fsm:handle_msg(Msg, State, fun(S1) -> idle(S1) end)
   end.

This is quite convenient if the receive statement already has a 'catch-all' clause, discarding unknown messages. plain_fsm:handle_msg/3 will handle system messages properly and ignore any other message.

(C) Write a pseudo wrapper function around your receive clause:

idle(S) ->
   plain_fsm:extended_receive(
      receive
         ... %% your original code
      end).

The function plain_fsm:extended_receive/1 is replaced in a parse_transform into something that looks very much like the previous program (A). The code, as it reads, requires the reader to know that the transformation takes place, otherwise the semantics would be confusing (you cannot solve the problem using a real function that way.) On the plus side, this is a fairly small violation of both the original code and Erlang's semantics.

Note that for this to work, you must include "plain_fsm.hrl"in your module.

Example

In the module fsm_example.erl (included in the plain_fsm package), we choose to handle system messages in the idle state. The example code is runnable, and supports suspend, resume, status inspection, and code change.

Imagine that the code initially looked like this:

idle(S) ->
    receive
     a ->
         io:format("going to state a~n", []),
         a(S);
     b ->
         io:format("going to state b~n", []),
         b(S)
    after 10000 ->
         io:format("timeout in idle~n", []),
         idle(S)
    end).

The change required to handle system messages is as follows:

idle(S) -><a docgen-rel="seemfa" docgen-href="#extended_receive/1" href="https://github.com/uwiger/plain_fsm/blob/master/doc/README.md#extended_receive-1">plain_fsm:extended_receive</a>(
      receive
          a ->
              io:format("going to state a~n", []),
              a(S);
          b ->
              io:format("going to state b~n", []),
              b(S)
      after 10000 ->
              io:format("timeout in idle~n", []),
              idle(S)
      end).

In addition, we change the start function from, in this case:

spawn_link() ->
    spawn_link(fun() ->
                       process_flag(trap_exit, true),
                       idle(mystate)
               end).

Is changed into:

spawn_link() -><a docgen-rel="seemfa" docgen-href="#spawn_link/2" href="https://github.com/uwiger/plain_fsm/blob/master/doc/README.md#spawn_link-2">plain_fsm:spawn_link</a>(?MODULE, fun() ->
                                          process_flag(trap_exit,true),
                                          idle(mystate)
                                  end).

See also spawn/2 and spawn_opt/3 for information on other possible start functions.

To be fully compliant, you also need to supply a code_change/3 function. See behaviour_info/1 for details.

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