Current File : //usr/local/share/man/man3/POE::NFA.3pm

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.\" ========================================================================
.\"
.IX Title "POE::NFA 3"
.TH POE::NFA 3 "2022-03-23" "perl v5.26.3" "User Contributed Perl Documentation"
.\" For nroff, turn off justification.  Always turn off hyphenation; it makes
.\" way too many mistakes in technical documents.
.if n .ad l
.nh
.SH "NAME"
POE::NFA \- an event\-driven state machine (nondeterministic finite automaton)
.SH "SYNOPSIS"
.IX Header "SYNOPSIS"
.Vb 3
\&  use POE::Kernel;
\&  use POE::NFA;
\&  use POE::Wheel::ReadLine;
\&
\&  # Spawn an NFA and enter its initial state.
\&  POE::NFA\->spawn(
\&    inline_states => {
\&      initial => {
\&        setup => \e&setup_stuff,
\&      },
\&      state_login => {
\&        on_entry => \e&login_prompt,
\&        on_input => \e&save_login,
\&      },
\&      state_password => {
\&        on_entry => \e&password_prompt,
\&        on_input => \e&check_password,
\&      },
\&      state_cmd => {
\&        on_entry => \e&command_prompt,
\&        on_input => \e&handle_command,
\&      },
\&    },
\&  )\->goto_state(initial => "setup");
\&
\&  POE::Kernel\->run();
\&  exit;
\&
\&  sub setup_stuff {
\&    $_[RUNSTATE]{io} = POE::Wheel::ReadLine\->new(
\&      InputEvent => \*(Aqon_input\*(Aq,
\&    );
\&    $_[MACHINE]\->goto_state(state_login => "on_entry");
\&  }
\&
\&  sub login_prompt { $_[RUNSTATE]{io}\->get(\*(AqLogin: \*(Aq); }
\&
\&  sub save_login {
\&    $_[RUNSTATE]{login} = $_[ARG0];
\&    $_[MACHINE]\->goto_state(state_password => "on_entry");
\&  }
\&
\&  sub password_prompt { $_[RUNSTATE]{io}\->get(\*(AqPassword: \*(Aq); }
\&
\&  sub check_password {
\&    if ($_[RUNSTATE]{login} eq $_[ARG0]) {
\&      $_[MACHINE]\->goto_state(state_cmd => "on_entry");
\&    }
\&    else {
\&      $_[MACHINE]\->goto_state(state_login => "on_entry");
\&    }
\&  }
\&
\&  sub command_prompt { $_[RUNSTATE]{io}\->get(\*(AqCmd: \*(Aq); }
\&
\&  sub handle_command {
\&    $_[RUNSTATE]{io}\->put("  <<$_[ARG0]>>");
\&    if ($_[ARG0] =~ /^(?:quit|stop|exit|halt|bye)$/i) {
\&      $_[RUNSTATE]{io}\->put(\*(AqBye!\*(Aq);
\&      $_[MACHINE]\->stop();
\&    }
\&    else {
\&      $_[MACHINE]\->goto_state(state_cmd => "on_entry");
\&    }
\&  }
.Ve
.SH "DESCRIPTION"
.IX Header "DESCRIPTION"
\&\s-1POE::NFA\s0 implements a different kind of \s-1POE\s0 session: A
non-deterministic finite automaton.  Let's break that down.
.PP
A finite automaton is a state machine with a bounded number of states
and transitions.  Technically, \s-1POE::NFA\s0 objects may modify themselves
at run time, so they aren't really \*(L"finite\*(R".  Run-time modification
isn't currently supported by the \s-1API,\s0 so plausible deniability is
maintained!
.PP
Deterministic state machines are ones where all possible transitions
are known at compile time.  \s-1POE::NFA\s0 is \*(L"non-deterministic\*(R" because
transitions may change based on run-time conditions.
.PP
But more simply, \s-1POE::NFA\s0 is like POE::Session but with banks of event
handlers that may be swapped according to the session's run-time state.
Consider the \s-1SYNOPSIS\s0 example, which has \*(L"on_entry\*(R" and \*(L"on_input\*(R"
handlers that do different things depending on the run-time state.
POE::Wheel::ReadLine throws \*(L"on_input\*(R", but different things happen
depending whether the session is in its \*(L"login\*(R", \*(L"password\*(R" or
\&\*(L"command\*(R" state.
.PP
\&\s-1POE::NFA\s0 borrows heavily from POE::Session, so this document will only
discuss the differences.  Please see POE::Session for things which
are similar.
.SH "PUBLIC METHODS"
.IX Header "PUBLIC METHODS"
This document mainly focuses on the differences from POE::Session.
.SS "get_current_state"
.IX Subsection "get_current_state"
Each machine state has a name.  \fBget_current_state()\fR returns the name
of the machine's current state.  \fBget_current_state()\fR is mainly used to
retrieve the state of some other machine.  It's easier (and faster) to
use \f(CW$_[STATE]\fR in a machine's own event handlers.
.SS "get_runstate"
.IX Subsection "get_runstate"
\&\fBget_runstate()\fR returns the machine's current runstate.  Runstates are
equivalent to POE::Session HEAPs, so this method does pretty much the
same as POE::Session's \fBget_heap()\fR.  It's easier (and faster) to use
\&\f(CW$_[RUNSTATE]\fR in a machine's own event handlers, however.
.SS "spawn \s-1STATE_NAME\s0 => HANDLERS_HASHREF[, ...]"
.IX Subsection "spawn STATE_NAME => HANDLERS_HASHREF[, ...]"
\&\fBspawn()\fR is \s-1POE::NFA\s0's constructor.  The name reflects the idea that
new state machines are spawned like threads or processes rather than
instantiated like objects.
.PP
The machine itself is defined as a list of state names and hashes that
map events to handlers within each state.
.PP
.Vb 10
\&  my %states = (
\&    state_1 => {
\&      event_1 => \e&handler_1,
\&      event_2 => \e&handler_2,
\&    },
\&    state_2 => {
\&      event_1 => \e&handler_3,
\&      event_2 => \e&handler_4,
\&    },
\&  );
.Ve
.PP
A single event may be handled by many states.  The proper handler will
be called depending on the machine's current state.  For example, if
\&\f(CW\*(C`event_1\*(C'\fR is dispatched while the machine is in \f(CW\*(C`state_2\*(C'\fR, then
\&\fBhandler_3()\fR will be called to handle the event.  The state \-> event \->
handler map looks like this:
.PP
.Vb 1
\&  $machine{state_2}{event_1} = \e&handler_3;
.Ve
.PP
Instead of \f(CW\*(C`inline_states\*(C'\fR, \f(CW\*(C`object_states\*(C'\fR or \f(CW\*(C`package_states\*(C'\fR may
be used. These map the events of a state to an object or package method
respectively.
.PP
.Vb 11
\&  object_states => {
\&    state_1 => [
\&      $object_1 => [qw(event_1 event_2)],
\&    ],
\&    state_2 => [
\&      $object_2 => {
\&        event_1 => method_1,
\&        event_2 => method_2,
\&      }
\&    ]
\&  }
.Ve
.PP
In the example above, in the case of \f(CW\*(C`event_1\*(C'\fR coming in while the machine
is in \f(CW\*(C`state_1\*(C'\fR, method \f(CW\*(C`event_1\*(C'\fR will be called on \f(CW$object_1\fR. If the
machine is in \f(CW\*(C`state_2\*(C'\fR, method \f(CW\*(C`method_1\*(C'\fR will be called on \f(CW$object_2\fR.
.PP
\&\f(CW\*(C`package_states\*(C'\fR is very similar, but instead of using an \f(CW$object\fR, you
pass in a \f(CW\*(C`Package::Name\*(C'\fR
.PP
The \f(CW\*(C`runstate\*(C'\fR parameter allows \f(CW\*(C`RUNSTATE\*(C'\fR to be initialized differently
at instantiation time. \f(CW\*(C`RUNSTATE\*(C'\fR, like heaps, are usually anonymous hashrefs,
but \f(CW\*(C`runstate\*(C'\fR may set them to be array references or even objects.
.PP
State transitions are not necessarily executed immediately by default.  Rather,
they are placed in POEs event queue behind any currently pending events.
Enabling the \f(CW\*(C`immediate\*(C'\fR option causes state transitions to occur immediately,
regardless of any queued events.
.SS "goto_state NEW_STATE[, ENTRY_EVENT[, \s-1EVENT_ARGS\s0]]"
.IX Subsection "goto_state NEW_STATE[, ENTRY_EVENT[, EVENT_ARGS]]"
\&\fBgoto_state()\fR puts the machine into a new state.  If an \s-1ENTRY_EVENT\s0 is
specified, then that event will be dispatched after the machine enters
the new state.  \s-1EVENT_ARGS,\s0 if included, will be passed to the entry
event's handler via \f(CW\*(C`ARG0..$#_\*(C'\fR.
.PP
.Vb 2
\&  # Switch to the next state.
\&  $_[MACHINE]\->goto_state( \*(Aqnext_state\*(Aq );
\&
\&  # Switch to the next state, and call a specific entry point.
\&  $_[MACHINE]\->goto_state( \*(Aqnext_state\*(Aq, \*(Aqentry_event\*(Aq );
\&
\&  # Switch to the next state; call an entry point with some values.
\&  $_[MACHINE]\->goto_state( \*(Aqnext_state\*(Aq, \*(Aqentry_event\*(Aq, @parameters );
.Ve
.SS "stop"
.IX Subsection "stop"
\&\fBstop()\fR forces a machine to stop.  The machine will also stop
gracefully if it runs out of things to do, just like POE::Session.
.PP
\&\fBstop()\fR is heavy-handed.  It will force resources to be cleaned up.
However, circular references in the machine's \f(CW\*(C`RUNSTATE\*(C'\fR are not
\&\s-1POE\s0's responsibility and may cause memory leaks.
.PP
.Vb 1
\&  $_[MACHINE]\->stop();
.Ve
.SS "call_state \s-1RETURN_EVENT,\s0 NEW_STATE[, ENTRY_EVENT[, \s-1EVENT_ARGS\s0]]"
.IX Subsection "call_state RETURN_EVENT, NEW_STATE[, ENTRY_EVENT[, EVENT_ARGS]]"
\&\fBcall_state()\fR is similar to \fBgoto_state()\fR, but it pushes the current
state on a stack.  At some later point, a handler can call
\&\fBreturn_state()\fR to pop the call stack and return the machine to its old
state.  At that point, a \f(CW\*(C`RETURN_EVENT\*(C'\fR will be posted to notify the
old state of the return.
.PP
.Vb 1
\&  $machine\->call_state( \*(Aqreturn_here\*(Aq, \*(Aqnew_state\*(Aq, \*(Aqentry_event\*(Aq );
.Ve
.PP
As with \fBgoto_state()\fR, \f(CW\*(C`ENTRY_EVENT\*(C'\fR is the event that will be emitted
once the machine enters its new state.  \f(CW\*(C`ENTRY_ARGS\*(C'\fR are parameters
passed to the \f(CW\*(C`ENTRY_EVENT\*(C'\fR handler via \f(CW\*(C`ARG0..$#_\*(C'\fR.
.SS "return_state [\s-1RETURN_ARGS\s0]"
.IX Subsection "return_state [RETURN_ARGS]"
\&\fBreturn_state()\fR returns to the most recent state in which \fBcall_state()\fR
was invoked.  If the preceding \fBcall_state()\fR included a return event
then its handler will be invoked along with some optional
\&\f(CW\*(C`RETURN_ARGS\*(C'\fR.  The \f(CW\*(C`RETURN_ARGS\*(C'\fR will be passed to the return
handler via \f(CW\*(C`ARG0..$#_\*(C'\fR.
.PP
.Vb 1
\&  $_[MACHINE]\->return_state( \*(Aqsuccess\*(Aq, @success_values );
.Ve
.SS "Methods that match POE::Session"
.IX Subsection "Methods that match POE::Session"
The following methods behave identically to the ones in POE::Session.
.IP "\s-1ID\s0" 2
.IX Item "ID"
.PD 0
.IP "option" 2
.IX Item "option"
.IP "postback" 2
.IX Item "postback"
.IP "callback" 2
.IX Item "callback"
.PD
.SS "About \fBnew()\fP and \fBcreate()\fP"
.IX Subsection "About new() and create()"
\&\s-1POE::NFA\s0's constructor is \fBspawn()\fR, not \fBnew()\fR or \fBcreate()\fR.
.SH "PREDEFINED EVENT FIELDS"
.IX Header "PREDEFINED EVENT FIELDS"
\&\s-1POE::NFA\s0's predefined event fields are the same as POE::Session's with
the following three exceptions.
.SS "\s-1MACHINE\s0"
.IX Subsection "MACHINE"
\&\f(CW\*(C`MACHINE\*(C'\fR is equivalent to Session's \f(CW\*(C`SESSION\*(C'\fR field.  It holds a
reference to the current state machine, and is useful for calling
its methods.
.PP
See POE::Session's \f(CW\*(C`SESSION\*(C'\fR field for more information.
.PP
.Vb 1
\&  $_[MACHINE]\->goto_state( $next_state, $next_state_entry_event );
.Ve
.SS "\s-1RUNSTATE\s0"
.IX Subsection "RUNSTATE"
\&\f(CW\*(C`RUNSTATE\*(C'\fR is equivalent to Session's \f(CW\*(C`HEAP\*(C'\fR field.  It holds an
anonymous hash reference which \s-1POE\s0 is guaranteed not to touch.  Data
stored in \f(CW\*(C`RUNSTATE\*(C'\fR will persist between handler invocations.
.SS "\s-1STATE\s0"
.IX Subsection "STATE"
\&\f(CW\*(C`STATE\*(C'\fR contains the name of the machine's current state.  It is not
equivalent to anything from POE::Session.
.SS "\s-1EVENT\s0"
.IX Subsection "EVENT"
\&\f(CW\*(C`EVENT\*(C'\fR is equivalent to Session's \f(CW\*(C`STATE\*(C'\fR field.  It holds the name
of the event which invoked the current handler.  See POE::Session's
\&\f(CW\*(C`STATE\*(C'\fR field for more information.
.SH "PREDEFINED EVENT NAMES"
.IX Header "PREDEFINED EVENT NAMES"
\&\s-1POE::NFA\s0 defines four events of its own.  These events are used
internally and may not be overridden by application code.
.PP
See POE::Session's \*(L"\s-1PREDEFINED EVENT NAMES\*(R"\s0 section for more
information about other predefined events.
.PP
The events are: \f(CW\*(C`poe_nfa_goto_state\*(C'\fR, \f(CW\*(C`poe_nfa_push_state\*(C'\fR,
\&\f(CW\*(C`poe_nfa_pop_state\*(C'\fR, \f(CW\*(C`poe_nfa_stop\*(C'\fR.
.PP
Yes, all the internal events begin with \*(L"poe_nfa_\*(R".  More may be
forthcoming, but they will always begin the same way.  Therefore
please do not define events beginning with \*(L"poe_nfa_\*(R".
.SH "SEE ALSO"
.IX Header "SEE ALSO"
Many of \s-1POE::NFA\s0's features are taken directly from POE::Session.
Please see POE::Session for more information.
.PP
The \s-1SEE ALSO\s0 section in \s-1POE\s0 contains a table of contents covering
the entire \s-1POE\s0 distribution.
.SH "BUGS"
.IX Header "BUGS"
See POE::Session's documentation.
.PP
\&\s-1POE::NFA\s0 is not as feature-complete as POE::Session.  Your feedback is
appreciated.
.SH "AUTHORS & COPYRIGHTS"
.IX Header "AUTHORS & COPYRIGHTS"
Please see \s-1POE\s0 for more information about authors and contributors.