[rules-users] Drools as Lexer / Parser (sequential data processing)
Wolfgang Laun
wolfgang.laun at gmail.com
Fri Sep 4 02:16:28 EDT 2009
The rules for a lexer are bound to be all of the same standard pattern:
rule FirstDigit
salience 0
where
$i : Input( $ch : character matches "\d" )
$l : Lexer( state == State.IDLE, $buf : buffer )
then
$buf.setLength( 0 ); $buf.append( $ch );
modify( $l ){ setState( State.NUMBER ) }
modify( $i ){ next() }
end
Rather boring.
If you prefer, you can write a single rule exercising the FSM mechanism
(match input, execute action, change state) that is driven by a set of
(static and immutable) facts defining the individual
state/symbol/action/transition quadruples.
rule Lex
salience 0
when
$k : CharClass( $pattern : pattern ) // such as
"\d" or "\w"
$i : Input( $ch : character matches $pattern )
$l : Lexer( $state : state ) //
also containing a StringBuilder buffer
$t : StateTransition( state == $state, charClass == $k, $action :
action, $succ : succ )
then
$action.execute( $i );
modify( $l ){ setState( $succ ) }
modify( $i ){ next() }
end
An additional rule catches errors. - An advantage of this approach is that
any lexer can now be configured by a set of fact definitions, e.g., this one
for a sequence of identifiers, numbers and quoted strings:
CharClass letter = new CharClass( "[A-Za-z_]" );
CharClass digit = new CharClass( "\\d" );
CharClass escape = new CharClass( "\\\\" );
CharClass space = new CharClass( "\\s" );
CharClass quote = new CharClass( "\"" );
CharClass empty = new CharClass( "\u0000" );
ins( letter, digit, escape, space, quote, empty );
Action<State,TokenType> save = new SaveAction( this );
Action<State,TokenType> skip = new SkipAction( this );
Action<State,TokenType> halt = new HaltAction( this );
Action<State,TokenType> emit = new EmitAction( this );
StateTransition<State,TokenType> t1 = new
StateTransition<State,TokenType>( State.S0, letter, save, State.IDENT );
StateTransition<State,TokenType> t2 = new
StateTransition<State,TokenType>( State.S0, digit, save, State.NUMBER );
StateTransition<State,TokenType> t3 = new
StateTransition<State,TokenType>( State.S0, quote, skip, State.STRING );
StateTransition<State,TokenType> t4 = new
StateTransition<State,TokenType>( State.S0, space, skip );
StateTransition<State,TokenType> t5 = new
StateTransition<State,TokenType>( State.S0, empty, halt );
ins( t1, t2, t3, t4, t5 );
StateTransition<State,TokenType> u1 = new
StateTransition<State,TokenType>( State.IDENT, letter, save );
StateTransition<State,TokenType> u2 = new
StateTransition<State,TokenType>( State.IDENT, digit, save );
StateTransition<State,TokenType> u3 = new
StateTransition<State,TokenType>( State.IDENT, space, emit, State.S0 );
StateTransition<State,TokenType> u4 = new
StateTransition<State,TokenType>( State.IDENT, empty, halt );
ins( u1, u2, u3, u4 );
StateTransition<State,TokenType> v1 = new
StateTransition<State,TokenType>( State.NUMBER, digit, save );
StateTransition<State,TokenType> v2 = new
StateTransition<State,TokenType>( State.NUMBER, space, emit, State.S0 );
StateTransition<State,TokenType> v3 = new
StateTransition<State,TokenType>( State.NUMBER, empty, halt );
ins( v1, v2, v3 );
StateTransition<State,TokenType> w1 = new
StateTransition<State,TokenType>( State.STRING, letter, save );
StateTransition<State,TokenType> w2 = new
StateTransition<State,TokenType>( State.STRING, digit, save );
StateTransition<State,TokenType> w3 = new
StateTransition<State,TokenType>( State.STRING, space, save );
StateTransition<State,TokenType> w4 = new
StateTransition<State,TokenType>( State.STRING, quote, emit, State.S0 );
StateTransition<State,TokenType> w5 = new
StateTransition<State,TokenType>( State.STRING, escape, save, State.ESCAPE
);
StateTransition<State,TokenType> w6 = new
StateTransition<State,TokenType>( State.STRING, empty, halt );
ins( w1, w2, w3, w4, w5, w6 );
StateTransition<State,TokenType> x1 = new
StateTransition<State,TokenType>( State.ESCAPE, letter, save, State.STRING
);
StateTransition<State,TokenType> x2 = new
StateTransition<State,TokenType>( State.ESCAPE, digit, save, State.STRING
);
StateTransition<State,TokenType> x3 = new
StateTransition<State,TokenType>( State.ESCAPE, space, save, State.STRING
);
StateTransition<State,TokenType> x4 = new
StateTransition<State,TokenType>( State.ESCAPE, quote, emit, State.STRING
);
StateTransition<State,TokenType> x5 = new
StateTransition<State,TokenType>( State.ESCAPE, escape, save, State.STRING
);
StateTransition<State,TokenType> x6 = new
StateTransition<State,TokenType>( State.ESCAPE, empty, halt );
ins( x1, x2, x3, x4, x5, x6 );
-W
On Wed, Sep 2, 2009 at 11:54 PM, André Thieme <
address.good.until.2009.dec.14 at justmail.de> wrote:
> Hello group, I recently had the idea:
> "A rule system (like Drools) is ideal for making programs with complex
> rules simpler. Writing a lexer or parser can be non-trivial. So, is it
> possible and also meaningful to express such a task with rules?"
>
> Anyone here who maybe tried that already?
> The two big questions for me are:
> 1) how easy is it to express a lexer with rules?
> 2) how bad (good?) will it perform?
>
> If you happen to have a good idea of how to do it, could you please give
> me an example for a simple lexer?
> Let's say it will get natural language (a string, such as this email) as
> input and should return a sequence (say, ArrayList) of Tokens, which may
> look like this:
>
> public class Token {
> public String value;
> public String category;
>
> Token(String value, String category) {
> this.value = value;
> this.category = category;
> }
> }
>
> We could have three categories:
> "word", "numeric" and "whitespace".
>
> An input String could be:
> "We can see 500 cars"
> And it should produce an ArrayList with the contents:
> [
> Token("We", "word"),
> Token(" ", "whitespace"),
> Token("can", "word"),
> Token(" ", "whitespace"),
> Token("see", "word"),
> Token(" ", "whitespace"),
> Token("500", "numeric"),
> Token(" ", "whitespace"),
> Token("cars", "word")
> ]
>
> At the moment I have difficulties to see if/how this could be achieved.
> If you find this easy, please post a solution.
> I am aware that JavaCC is really good for such tasks and will also
> perform extremly well.
>
>
> Greetings,
> André
> _______________________________________________
> rules-users mailing list
> rules-users at lists.jboss.org
> https://lists.jboss.org/mailman/listinfo/rules-users
>
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