LogicObjects

A Portable and Extensible Approach for Linguistic Symbiosis between an Object-Oriented and a Logic

Programming Language

Sergio Castro [email protected]

RELEASeD LABUniversité catholique de Louvain

Belgium

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LogicObjects

Monday 26 August 13

mailto:[email protected]

mailto:[email protected]

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OOprogramming

Logicprogramming

Useful for modellingnon-declarative concepts

Often enjoy rich softwareecosystems

Declarative reasoning &Knowledge representation

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OOprogramming

Logicprogramming




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OOprogramming

Logicprogramming




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OOprogramming

Logicprogramming




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Considerable amount of integration tools and techniques

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Considerable amount of integration tools and techniques

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SOUL

TyRuBaJPL

InterProlog

PDT Connector

TuPrologLogicJava Jiprolog

Kernel-Prolog CIAO Prolog

Jasper

Jekejeke PrologKiev

PrologBeans

PrologCafeJavaLogMINERVA

jProlog

ProvaK-PrologCommonRules Styla

Jinni Prolog

Yajxb

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Research scope

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“Bidirectional linguistic integration between Prolog and a statically typed, class based OO language (Java)”

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But... why Java ?

• A huge Java ecosystem.

• Emerging languages running on the JVM (e.g. Scala, Clojure, etc.).

• A large user base.6

We recognize the advantages of:

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Outline

• JPC: a portable Java-Prolog interoperability layer.

• LogicObjects: a linguistic integration framework.

• Research status.

• Future work.

• Conclusions.

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Prolog engines

Bridge libraries

JPC drivers

JPC library

Java-Prolog applications

Java-Prolog frameworks

(layer coupling denoted by the direction of the arrows)

High level architectural overview

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• Attempts to facilitate the creation of hybrid Java-Prolog applications and frameworks.

• Inspired by JPL, InterProlog and Google’s Gson libraries.

• Compatible with some popular open source Prolog engines (XSB,YAP, SWI) and more coming soon.

• Available at the Maven central snapshot repository and GitHub1 (currently) under the LGPL license.

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1https://github.com/sergio-castro/

JPC: Java-Prolog connectivity

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http://www.declarativa.com/interprolog/


JPC features

• A portable abstraction of a Prolog virtual machine.

• A set of utilities for dealing with common Java-Prolog interoperability problems.

• High level support for :

•Mapping between Java objects and Prolog terms.

•Dealing with object references and object serialization in Prolog.

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Converter manager

Instantiation managerType solver

JPC Context

A conversion context as a first order entity

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Converter1 Converter2 Converter3

Dispatcher

Converter manager

The converter manager

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Dispatcher

(chain of responsibility pattern)

Converter manager


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Dispatcher

(chain of responsibility pattern)

Converter manager


primitive types converters,exception converters, multi-valued converters, etc.

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Modularising mappings between Java objects and Prolog terms

public class StationConverter extends JpcConverter<Station, Compound> { public static final String STATION_FUNCTOR = "station"; @Override public Compound toTerm(Station station, Jpc context) { return new Compound(STATION_FUNCTOR, asList(new Atom(station.getName()))); } @Override public Station fromTerm(Compound term, Jpc context) { String stationName = ((Atom)term.arg(1)).getName(); return new StationJpc(stationName); }}

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Java type

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Prolog type

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Configuring a JPC context with a custom converter

Jpc jpcContext = JpcBuilder.create().registerConverter(new StationConverter()).build();

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Default builder (includes pre-defined converters)

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Default builder (includes pre-defined converters)

Extending JPC with new conversions

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Querying Prolog with JPC

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public class StationManager {public static final Jpc jpcContext = ... //context includes StationConverter...public Station mainStation() { String stationVarName = "Station"; Term goal = new Compound("main_station", asList(new Variable(stationVarName))); Query query = getPrologEngine().query(goal, jpcContext); return query.<Station>selectObject(stationVarName).oneSolutionOrThrow();}

}

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}

main_station(Station)

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}

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}binding of Station as a Java object

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Dealing with object references@Testpublic void testJRef() {

Object o = new Object();Term jRef = RefManager.jRefTerm(o);assertTrue(o == jpc.fromTerm(jRef));o = null;System.gc();try {

jpc.fromTerm(jRef); fail();

} catch(RuntimeException e) {}}

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Dealing with serialized objects

@Testpublic void testSerializedTerm() { String s = "hello"; Term term = SerializedObject.serializedObjectTerm(s); String s2 = jpc.fromTerm(term); assertFalse(s == s2); assertEquals(s, s2);}

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A Prolog query browser based on

JPC

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JPC as a library for implementing Java-Prolog frameworks

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Prolog engines

Bridge libraries

JPC drivers

JPC library




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JPC as a library for implementing Java-Prolog frameworks

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Prolog engines

Bridge libraries

JPC drivers

JPC library




e.g. LogicObjects

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LogicObjects

• A portable Java-Prolog linguistic symbiosis framework.

• Implemented on top of the JPC library.

• Provides obliviousness concerning integration (in common scenarios).

• Based on annotations for customising the integration and minimising programming effort.

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Symbiosis

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“The intimate living together of two dissimilar organisms in a mutually beneficial relationship.” (Merriam-Webster dictionary)

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Linguistic symbiosis

• Objects from different worlds must understand each other.

• Invoking routines from another language as if they were defined in their own language.

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• Easier to achieve if the languages belong to the same paradigm.

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A paradigm leak

“The event of concepts leaking from one programming paradigm to another”

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* Brichau, J. et al.Towards linguistic symbiosis of an object-oriented and a logic programming language.

In Proceedings of the Workshop on Multiparadigm Programming with Object-Oriented Languages. (2002)

*

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The inhabitants of our two worlds

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The OOworld

The logicworld

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The inhabitants of our two worlds

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ClassesObjects

MessagesMethods

Return valuesPackages The OO

worldThe logic

world

FactsRulesTerms

QueriesQuery solutions

Modules

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Reducing the gap with Logtalk

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The OOworld

The logicworld

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Reducing the gap with Logtalk

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The OOworld

The logicworld

LogtalkAn object-oriented layer

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Case study

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The London underground

from:

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Relevant concepts

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Relevant concepts

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stations

linesmetro

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A rule based system using Prolog

connected(station(bond_street), station(oxford_circus), line(central)). connected(station(oxford_circus), station(tottenham_court_road), line(central)). ...

nearby(S1,S2) :- connected(S1,S2,_). nearby(S1,S2) :- connected(S1,S3,L), connected(S3,S2,L).

reachable(S1,S2,[]) :- connected(S1,S2,_). reachable(S1,S2,[S3|Ss]) :- connected(S1,S3,_), reachable(S3,S2,Ss).

line(Name) :- setof(L, S1^S2^connected(S1,S2,L), Ls), list::member(line(Name), Ls).

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FACTS

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RULES

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Adding an object-oriented layer with Logtalk





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:- object(metro).

:- end_object.

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:- object(metro).

:- end_object.

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:- object(metro).

:- end_object.

:- public([connected/3, nearby/2, reachable/3, line/1]). ACCESS MODIFIERS

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Logtalk parametric objects

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:- object(line(_Name)).

:- public([name/1, connects/2]). name(Name) :- parameter(1, Name). ...

:- end_object.

:- object(station(_Name)).

:- public([name/1, connected/2, nearby/1, reachable/2]). name(Name) :- parameter(1, Name). ...

:- end_object.

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PARAMETRIC OBJECT


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:- end_object.



:- end_object.

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PARAMETRIC OBJECT

PARAMETRIC OBJECT


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:- end_object.



:- end_object.

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Invoking a Logtak method

Messages in Logtalk are expressed with the :: operator.

For example:

line(central)::connects(Station1, Station2)

Answers all the stations connected by the line ‘central’

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Invoking a Logtak method

Messages in Logtalk are expressed with the :: operator.

For example:

line(central)::connects(Station1, Station2)

Answers all the stations connected by the line ‘central’

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Meta-level artefacts from the two worlds

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The Java world

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public abstract class Line { String name; public Line(String name) {this.name = name;} public abstract boolean connects(Station s1, Station s2); public abstract int segments();}

public abstract class Station { ...}

public abstract class Metro { ...}

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The Java world

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public abstract class Line { String name; public Line(String name) {this.name = name;} public abstract boolean connects(Station s1, Station s2); public abstract int segments();}

public abstract class Station { ...}

public abstract class Metro { ...}

@LObject(args={“name”})

@LMethod(name={“connects”}, args={“_”, “_”})

@LObject(args={“name”})

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Linguistic symbiosis challenges

• Translating objects to logic terms (and back).

•Mapping OO methods to logic queries.

•Dealing with unbound variables.

• Returning values from queries.

•Managing multiplicity.

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* Some of them presented a bit differently in:D'Hondt, M. et al.Seamless integration of rule-based knowledge and object-oriented functionality with linguistic symbiosis. In Proceedings of the 2004 ACM symposium on Applied computing.(2004)

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public abstract class Metro {...}

@LObject(name = "my_metro")public abstract class Metro {...}

@LObject(args = {"name"})public abstract class Line { private String name; ...}

metro

my_metro

line(lineName)

Translating objects to logic terms

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Java Prolog

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metro

my_metro

line(lineName)


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Java Prolog

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metro

my_metro

line(lineName)


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Java Prolog

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metro

my_metro

line(lineName)


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Java Prolog

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line(lName)::connects( station(s1Name), station(s2Name)).

line(lName)::connects(_, _).

Mapping Java methods to Logtalk methods

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@LObject(args = {"name"})public abstract class Line { private String name;

public abstract boolean connects(Station s1, Station s2); @LMethod(name = "connects", args = {"_", "_"}) public abstract int segments();}

Java Prolog

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Java Prolog

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Java Prolog

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Java Prolog

anonymous variables

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Interpreting a query result as a Java object

The logic solutions

Varx1 x1, Vary1 y1Varx2 x2, Vary2 y2

Varxn xn, Varyn yn

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The logic solutions


Varxn xn, Varyn yn(set of frames binding logic

variables to terms)

frame 1frame 2

frame n

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The logic solutions



variables to terms)

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The logic solutions The method return value


aJavaObject


variables to terms)

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The logic solutions



variables to terms)

Returning values from one solution

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The logic solutions The method return value


aJavaObject


variables to terms)


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The logic solutions The method return valueterm(Varx1, Vary1)



variables to terms)

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variables to terms)

(specified in a methodwith @LSolution)

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Varx1 x1, Vary1 y1 term(x1, y1)Varx2 x2, Vary2 y2 term(x2, y2)

Varxn xn, Varyn yn term(xn, yn)(set of frames binding logic

variables to terms)

(specified in a methodwith @LSolution)

(default solution)

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Explicit specification of return values

@LObject(args = {"name"})public abstract class Station {

@LSolution("IStations") @LMethod(name = "reachable", args = {"$1", "IStations"}) public abstract List<Station> intermediateStations(Station station); ...

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Explicit specification of return values


@LSolution("IStations") @LMethod(name = "reachable", args = {"$1", "IStations"}) public abstract List<Station> intermediateStations(Station station); ...

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• Preprocessing macros and Java exp.

• Convert arguments to terms.

• Convert method to a predicate.

• Convert receiver object to a term.

• Build a query.

• Determine solution heuristic.

• Convert solution terms back to objects.

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• $1 => station (method parameter).

• (station(a), IStations)

• intermediateStations => reachable/2.

• station(b)

• station(b)::reachable(station(a), IStations)

• Binding of IStations (first solution).

• [station(x), station(y)] => List<Station>

Processing Steps Outcome


@LSolution("IStations") @LMethod(name = "reachable", args = {"$1", "IStations"}) public abstract List<Station> intermediateStations(Station station); ...}

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first Java method parameter





• Build a query.



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• station(b)







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first Java method parameter





• Build a query.



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• station(b)







unbound Prolog variable

(as term)

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• Build a query.



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• station(b)







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• Build a query.



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• station(b)







(instance of)

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station(b)::reachable(station(a), IStations)





• Build a query.



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• station(b)







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• Build a query.



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• station(b)







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• Build a query.



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• station(b)







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Returning multiple values from queries



Varxn xn, Varyn yn term(xn, yn)

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Returning multiple values from queries



Varxn xn, Varyn yn term(xn, yn)

a composed solution (specified in a methodwith @LComposition)

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The logic solutions The method return value(a property of the result set)


Varxn xn, Varyn yn

Returning a property of the result set

}

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The logic solutions The method return value(a property of the result set)


Varxn xn, Varyn yn

is there a solution ?


how many solutions ?}

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should return if logic method succeeds or not

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should return if logic method succeeds or not

should return the number of solutions

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Instantiating a symbiotic object in Java

Line line = newLogicObject(Line.class, “central”);System.out.println("Number of segments of line " + line + ": " + line.segments());

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Other features

• Java expressions embedded in logic terms (symbiosis terms).

•Dependency management support.

• Integration with plain Prolog (without Logtalk).

•Not constrained to a specific execution environment (e.g. an Eclipse plugin).

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Inspiration from other domains

• Common interoperability layer : JDBC.

•Mapping artefacts using annotations: JAXB.

• Context dependent conversions: GSON.

• Linguistic symbiosis concepts: SOUL.

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1http://www.oracle.com/technetwork/java/javase/jdbc/2https://jaxb.java.net/3http://code.google.com/p/google-gson/4http://soft.vub.ac.be/SOUL/

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2

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implemented

pending

if time allows it

• Integration direction• OO to Prolog• Prolog to OO

• Linguistic symbiosis• Automation• Transparency• Customisation

• Preserving causality• Inter-language reflection

• Code querying• Code transformation• Code generation• Symbiotic inter-language reflection

• Portability

Research status

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Future work

• Finishing a full bidirectional linguistic symbiosis framework.

• Exploring the feasibility of our approach for symbiotic inter-language reflection.

• Adding support for more Prolog engines (e.g. an embedded one).

• Continue the development of reusable hybrid components.

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Conclusions

•We are actively exploring how far we can get in automation/transparency regarding Java-Prolog linguistic integration.

•We are attempting to provide reusable hybrid components and frameworks that may be helpful to the community.

• And we are trying to do this in a portable way.

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Technology

LogicObjects