[rules-dev] Duck Typing in Java and Drools 5.2.0 (Re: Traits, Duck Typing and Dynamic Semantic Learning)

[Wolfgang Laun]

On 6 July 2011 16:37, Mark Proctor <mproctor at codehaus.org> wrote:

> http://blog.athico.com/2011/07/traits-duck-typing-and-dynamic-semantic.html
>

Duck typing is available in Java, and therefore it can be used in Drools
even today, although without the syntactic sugar described by Mark.
Nevertheless, a purely Java-based implementation helps to understand issues
and develop a feeling about the usefulness of the proposed feature.

Here are the Java classes a user might write, using class TripleSet as
provided by the support for duck typing:

public class Person extends TripleSet {
    String name;
    int age;
    Gender gender; // enum Gender { MALE, FEMALE }
}

public interface EatingHabits {
    String getLikes();
    void setLikes( String value );
    String getDislikes();
    void setDislikes( String value );
}

public interface FussyEater {
}

Class SessionManager which wraps a StatefulKnowledgeSession provides a
little "glue" for performing "dons" in various forms. It should also be
available as a global.

public class SessionManager {
    public SessionManager( StatefulKnowledgeSession kSession ){...}
    public FactHandle insert( Object object ){...}
    public Object dons( TripleSet tripleSet, Class<?>... interfaces
){...}
    public FactHandle insert( TripleSet tripleSet, Class... interfaces
){...}
    public void insertLogical( RuleContext rContext, TripleSet tripleSet,
Class... interfaces ){...}
}

We can insert a Person and don it with EatingHabits:

    Person p1 = new Person( "John", 12, Gender.MALE );
    EatingHabits ehp1 = (EatingHabits)sessionMgr.dons( p1,
EatingHabits.class );
    ehp1.setLikes( "cheese" );
    ehp1.setDislikes( "spinach" );
    sessionMgr.insert( p1 );
    sessionMgr.insert( ehp1 );

And here is some DRL code, followed by the resulting output:

function String isa( Object object, java.lang.Class clazz ){
    return object.toString() +
           (clazz.isInstance( object ) ? " implements " : " doesn't
implement " ) +
           clazz.getSimpleName();
}

rule showPerson
salience 100
when
    $p: Person()
then
    System.out.println( "Person name: " + $p.toString() );
    System.out.println( isa( $p, EatingHabits.class ) );
    System.out.println( isa( $p, FussyEater.class ) );
end

rule showLikes
when
    $l: EatingHabits( likes == "cheese" )
    $p: Person( this == $l )
then
    System.out.println( $p.getName() + " likes cheese" );
end

rule showDislikes
when
    $d: EatingHabits( dislikes != null )
    $p: Person( this == $d )
then
    System.out.println( $p.getName() + " dislikes " + $d.getDislikes() );
    sessionMgr.insertLogical( kcontext, $p, FussyEater.class );
end

rule gotFussyEater
when
    $d: FussyEater()
    $p: Person( this == $d )
then
    System.out.println( $p.getName() + " is a fussy eater" );
end

rule notFussyEater
when
    $p: Person()
    not FussyEater( this == $p )
then
    System.out.println( $p.getName() + " is not a fussy eater" );
end

rule removeDislikes
salience -100
no-loop  true
when
    $d: EatingHabits()
then
    modify( $d ){ setDislikes( null ) }
end

=======================================================
Person name: Person<John,12,MALE>
Person<John,12,MALE> doesn't implement EatingHabits
Person<John,12,MALE> doesn't implement FussyEater
John dislikes spinach
John is a fussy eater
John likes cheese
John is not a fussy eater
John likes cheese
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