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##============================================================================== | ||
## Copyright 2010 Erlang Solutions Ltd. | ||
## | ||
## Licensed under the Apache License, Version 2.0 (the "License"); | ||
## you may not use this file except in compliance with the License. | ||
## You may obtain a copy of the License at | ||
## | ||
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##============================================================================== | ||
.PHONY: rel all clean | ||
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all: compile | ||
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compile: | ||
./rebar compile | ||
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clean: | ||
./rebar clean | ||
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test: | ||
./rebar eunit | ||
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doc: | ||
./rebar doc | ||
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Plain_fsm, copyright 2010 Erlang Solutions | ||
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This product contains code developed at Erlang Solutions. | ||
(http://www.erlang-solutions.com/) | ||
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The original was implemented by Ulf Wiger while at Ericsson AB, | ||
and released under the EPL license as part of http://jungerl.sf.net/ |
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A behaviour/support library for writing plain Erlang FSMs. | ||
========================================================== | ||
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This module 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. | ||
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The requirements that drove us away from plain Erlang programming | ||
in the first place were: | ||
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- 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. | ||
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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 | ||
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Using plain_fsm | ||
--------------- | ||
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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 | ||
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In the states where you want to handle system messages, you have | ||
three choices: | ||
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### (A) Insert the system messages in the receive clause:# | ||
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idle(S) -> | ||
Parent = plain_fsm:info(parent), | ||
receive | ||
{system, From, Req} -> | ||
plain_fsm:handle_system_msg( | ||
From, Req, S, fun(S1) -> idle(S1) end); | ||
{'EXIT', Parent, Reason} -> | ||
plain_fsm:parent_EXIT(Reason, S); | ||
... %% your original code here | ||
end. | ||
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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()`, `code>system_shutdown()`, | ||
`format_status()`, which are required callbacks when you handle system | ||
messages directly. | ||
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### (B) Handle system messages and unknown messages together: # | ||
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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. | ||
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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. | ||
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### (C) Write a pseudo wrapper function around your receive clause: | ||
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idle(S) -> | ||
plain_fsm:extended_receive( | ||
receive | ||
... %% your original code | ||
end). | ||
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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. | ||
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*Note that for this to work, you must include "plain_fsm.hrl" | ||
in your module.* | ||
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#### Example # | ||
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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. | ||
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Imagine that the code initially looked like this: | ||
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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). | ||
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The change required to handle system messages is as follows: | ||
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idle(S) -> | ||
plain_fsm:extended_receive( | ||
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). | ||
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In addition, we change the start function from, in this case: | ||
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spawn_link() -> | ||
spawn_link(fun() -> | ||
process_flag(trap_exit, true), | ||
idle(mystate) | ||
end). | ||
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Is changed into: | ||
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spawn_link() -> | ||
plain_fsm:spawn_link(?MODULE, fun() -> | ||
process_flag(trap_exit,true), | ||
idle(mystate) | ||
end). | ||
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See also spawn/2 and spawn_opt/3 for information on other possible | ||
start functions. | ||
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To be fully compliant, you also need to supply a `code_change/3` function. | ||
See behaviour_info/1 for details. |
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