towards a software ecosystem for java prolog interoperabilty

Towards a Software Ecosystemfor Java-Prolog Interoperability

Sergio Castro Kim Mens RELEASeD LAB

Université catholique de Louvain Belgium

Paulo MouraCenter for Research in Advanced Computing Systems

INESC TECPortugal

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LogicObjects

Future work/Conclusions

Content

JPC

Hydra(or how to build reusable hybrid components)

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OOprogramming

Logicprogramming

Appropriate for modellingreal-world concepts

Often enjoy rich softwareecosystems

Declarative reasoning

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Integration difficult to achieve

• The Prolog community is fragmented.

• Existing Java-Prolog libraries also suffer the symptoms of such fragmentation.

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An ecosystem for Java-Prolog development

• Providing a portable general-purpose library for building Java-Prolog systems (JPC).

• Providing a framework for linguistic symbiosis between Java & Prolog (LogicObjects).

• Providing a set of reusable hybrid components.

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Why Java ?

• None of the authors is a big fan of the Java language itself.

• But we recognise the advantages of the huge Java ecosystem.

• And the emerging languages running on the JVM

• (e.g., Scala, Clojure, JRuby, etc...).

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• Facilitates the creation of hybrid Java-Prolog applications and frameworks.

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

• Compatible with some of the most 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/

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

Lessons learned when building libraries without JPC

• Applications are strongly coupled to a concrete Java-Prolog library.

• Conversion concerns tangled with other concerns.

• Complex routines dealing with conversion heuristics (high cyclomatic complexity).

• Ugly ad-hoc implementation of context-dependent conversions.

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JPC features

• A portable abstraction of a Prolog virtual machine.

• A set of utilities for dealing with Java-Prolog inter-language conversions.

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JPC as a portable layer

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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)

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A Prolog query browser based on

JPC

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JPC as a tool for inter-language conversions

• JPC implements a “new” mechanism for passing artefacts between Java and Prolog.

• Mechanism based on the specification of mappings of such artefacts.

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Common integration techniques

• Shared memory approach (e.g. SOUL).

• Passing object references to Prolog (e.g. JPL).

• Serializing objects (e.g. InterProlog).

• Mapping objects to terms (e.g. JPC, LogicObjects).

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Shared memory approach

• Tight integration/control between Java and Prolog.

• Difficult to implement an efficient Prolog embedded in Java.

• Often no access to well-proven Prolog libraries.

• But easy access to libraries in the host language.

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Passing object references

• The original object reference is preserved.

• Garbage collection may be an issue.

• No control on the term representation of an object on the Prolog side.

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Serializing objects

• Object reference is not preserved.

• No configuration required.

• Only works with serializable objects.

• Support for circular relations.

• No control on the term representation of an object on the Prolog side.

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Mapping objects to terms

• Object reference is difficult to preserve.

• Difficult to support circular relations.

• Requires explicit mappings (where they cannot be inferred).

• Fine control on the (context dependent) term representation of an object.

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JPC mapping features

• Helps to modularize context dependent conversions.

• Converters can receive hints on the expected conversion to apply.

• Custom conversions can be added at any moment.

• Catalog of useful Prolog-Java converters.

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JPC architecture

• Abstraction of a Prolog VM.

• Reification of Prolog data types (e.g. Atom, Compound, etc).

• The conversion context.

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Conversion context components

• The converter manager.

• The type solver.

• The instantiation manager.

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The converter manager

• Converts between Java-Prolog artefacts.

• Composition of specialized converters (primitive types converters, exception converters, multi-valued converters, etc).

• Interprets and refines hints from the user.

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public class CollectionConverter<E> extends JpcConverter<Collection<E>, Term> {@Overridepublic Collection<E> fromTerm(Term listTerm, Type type, Jpc context) { ...}...

}

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A pre-defined converter example

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}

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Java type

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}

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Prolog type

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}

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term to convert

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}

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type guiding conversion

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}

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the conversion context

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A default conversion example

Jpc jpc = ... //the conversion contextTerm listTerm = listTerm(new Atom("1"), new Atom("2")); //[‘1’, ‘2’]List<String> list = jpc.fromTerm(listTerm);assertEquals("1", list.get(0));assertEquals("2", list.get(1));

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Prolog list of atoms

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Java list of Strings

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A typed conversion example

Term listTerm = listTerm(new Atom("1"), new Atom("2"));Type type = new TypeToken<ArrayList<Integer>>(){}.getType();List<Integer> list = jpc.fromTerm(listTerm, type); assertEquals(1, list.get(0));assertEquals(2, list.get(1));

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A typed conversion example

Term listTerm = listTerm(new Atom("1"), new Atom("2"));Type type = new TypeToken<ArrayList<Integer>>(){}.getType();List<Integer> list = jpc.fromTerm(listTerm, type); assertEquals(1, list.get(0));assertEquals(2, list.get(1));

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type guiding conversion

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A custom converter

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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The type solver

• Attempts to infer the best Java type of a Prolog term if no hint is available.

• Question: What does this term look like ?

[A-x,B-y,C-z]

• Answer: It may be a map.

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The instantiation manager

• A customizable mechanism for instantiating abstract classes/interfaces if required.

• E.g. a Prolog list term may be mapped to a Java List or a Map.

• This manager knows which instance of List and Map to use (if the given hint does not contain such information).

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LogicObjects

• A portable Java-Prolog linguistic symbiosis framework.

• Currently being migrated to JPC.

• Based on annotations for specifying mappings between Java-Prolog artefacts.

• Support for context dependent mappings.

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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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* Gybels, K.SOUL and Smalltalk - Just Married: Evolution of the Interaction Between a Logic and an Object-Oriented Language Towards Symbiosis. In Proceedings of the Workshop on Declarative Programming in the Context of Object-Oriented Languages. (2003)

*

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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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PackagesClassesObjectsMethods

Method invocationsReturn values

The OOworld

The logicworld

LibrariesModulesTerms

ClausesQueries

Query solutions

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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

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Relevant concepts

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stations

lines

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Relevant concepts

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stations

linesmetro

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Interesting relations

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line1A C

line2

D

F

B

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line1A C

line2

D

F

B

Connected: Directly connected(e.g., A with B).

connected

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line1A C

line2

D

F

B


Nearby: At most one intermediate station, in the same line (e.g., A with C).

nearby

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line1A C

line2

D

F

B



Reachable: Transitively connected (e.g., A with F).

reachable

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Reachable: Transitively connected (e.g., A with F).

Easily expressed with logic facts and rules

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Which is the best language for this problem?

• A logic language would let us express our problem using facts and rules.

• But we sill want access to a modern OO language to develop a user friendly interface.

• Then let’s do it with both through linguistic symbiosis.

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Implementation Strategy

• Develop the program in Prolog.

• Wrap it with an OO layer (Logtalk).

• Program the Java side using LogicObjects.

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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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Adding an object-oriented layer with Logtalk





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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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:- end_object.



:- end_object.

PARAMETRIC OBJECT

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:- end_object.



:- end_object.

PARAMETRIC OBJECT

PARAMETRIC 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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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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Mapping objects from the two worlds

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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, Maja and Gybels, Kris and Jonckers, Viviane. Seamless integration of rule-based knowledge and object-oriented functionality with linguistic symbiosis. In Proceedings of the 2004 ACM symposium on Applied computing, SAC '04, pages 1328{1335, New York, NY, USA, 2004. ACM.

*

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Translating objects to logic terms

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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(l_name)

Translating objects to logic terms

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Java Logtalk

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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();}

line(l_name)::connects( station(s1_name), station(s2_name)).

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

Java Logtalk

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

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

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Dealing with unbound variables in method calls

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first Java method parameter(as term)


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first Java method parameter(as term)

unbound logic variable


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station(s1_name)::reachable(station(s2_name), IntermediateStations)


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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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Returning values from one solution

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

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)

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variables to terms)

(specified in a methodwith @LSolution)

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variables to terms)

(specified in a methodwith @LSolution)

(default solution)

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Explicitly specification of return values

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

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Implicit specification of return values

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Managing multiplicity

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• A logic routine can have many solutions, in Java only one.

• Multiple values can be grouped with @LComposition.

• The kind of container to return depends on the method signature.

• The kind of object in the container is also extracted from the method signature.

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Returning multiple values from queries



Varxn xn, Varyn yn term(xn, yn)

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Varxn xn, Varyn yn term(xn, yn)

(a composed solution)

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@LObject(args = {"name"})public abstract class Station { ... @LComposition @LSolution("S") @LMethod(args = {"S"}) public abstract List<Station> nearby();}

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station(aName)::nearby(S).

LogtalkJava


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container type


LogtalkJava


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each solution type

container type


LogtalkJava


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Returning a property of the result set

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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.

• Integration with plain Prolog (without Logtalk).

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Future work

• Finishing a full two-ways linguistic symbiosis framework.

• Supporting more Prolog engines.

• Adding support to other kinds of integration techniques (e.g. serialization and objects references).

• Continue the development of reusable hybrid components.

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Inspiration from other domains

• Interoperability layer : JDBC.

• Mapping of artefacts using annotations: JAXB.

• Context dependent conversions: GSON.

• Linguistic symbiosis concepts: SOUL.

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Conclusions

• We have provided portable and simple solutions for many issues concerning Java-Prolog interoperability.

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

• We are targeting complex heterogeneous realistic scenarios.

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

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Thanks

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towards a software ecosystem for java prolog interoperabilty

Technology

reusable hybrid components

logtak method messages

vary2 y2 term

varyn yn term

logic method succeeds

mapping java methods

vary1 y1 term

rule based system