root/trunk/examples/assignvars/fsm3.lgt

% fsm(Transitions, Initial, Final)
%
% fsm(-list, -nonvar, -list)

fsm([red-0-red, red-1-green, red-2-red,     % a simple finite state machine example
     yellow-0-red, yellow-1-green, yellow-2-red,
     green-0-yellow, green-1-yellow, green-2-red],
    red,
    [red]).


:- object(fsm(_Transitions, _Initial, _Final),
    imports(private::assignvars)).

    :- info([
        version is 1.0,
        author is 'Paulo Moura',
        date is 2005/1/8,
        comment is 'A simple implementation of finite-state machines using assignable variables and parametric objects. Adapted from a similar example by Nobukuni Kino.',
        parnames is ['Transitions', 'Initial state', 'Final states']]).

    :- public(recognise/1).
    :- mode(recognise(+list), zero_or_more).
    :- info(recognise/1,
        [comment is 'Recognise a list of events.',
         argnames is ['Events']]).

    recognise(Events) :-
        parameter(2, Initial),
        ::assignable(Current, Initial),
        recognise(Events, Current).

    recognise([], State) :-
        ::State => Current,
        final_state(Current).
    recognise([Event| Events], State) :-
        ::State => Current,
        transition(Event, Current, Next),
        (write(Current-Event-Next), nl
         ;
         write('backtracking...'), nl, fail),
        ::State <= Next,
        recognise(Events, State).

    transition(Event, Current, Next) :-
        parameter(1, Transitions),
        transition(Transitions, Event, Current, Next).

    transition([Current-Event-Next| _], Event, Current, Next).
    transition([_| Transitions], Event, Current, Next):-
        transition(Transitions, Event, Current, Next).

    final_state(State) :-
        parameter(3, Final),
        final_state(Final, State).

    final_state([State| _], State).
    final_state([_| States], State) :-
        final_state(States, State).

:- end_object.