LogicObjects

A Portable and Extensible Approach for Linguistic
Symbiosis between an Object-Oriented and a Logic
Programming Language
Sergio Castro
Sergio.Castro@uclouvain.be
RELEASeD LAB
Université catholique de Louvain
Belgium
1
LogicObjects
Monday 26 August 13

2
OO
programming
Logic
programming
Useful for modelling
non-declarative concepts
Often enjoy rich software
ecosystems
Declarative reasoning &
Knowledge representation
Monday 26 August 13

Considerable amount of
integration tools and techniques
4
Monday 26 August 13

Considerable amount of
integration tools and techniques
4
SOUL
TyRuBa
JPL
InterProlog
PDT Connector
TuProlog
LogicJava Jiprolog
Kernel-Prolog
CIAO Prolog
Jasper
Jekejeke Prolog
Kiev
PrologBeans
PrologCafeJavaLog
MINERVA
jProlog
ProvaK-Prolog
CommonRules Styla
Jinni Prolog
Yajxb
Monday 26 August 13

Research scope
5
“Bidirectional linguistic integration between Prolog and a
statically typed, class based OO language (Java)”
Monday 26 August 13

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:
Monday 26 August 13

Outline
• JPC: a portable Java-Prolog interoperability layer.
• LogicObjects: a linguistic integration framework.
• Research status.
• Future work.
• Conclusions.
7
Monday 26 August 13

8
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
Monday 26 August 13

• 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.
9
1https://github.com/sergio-castro/
JPC: Java-Prolog connectivity
Monday 26 August 13

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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Monday 26 August 13

11
Converter
manager
Instantiation
manager
Type solver
JPC Context
A conversion context as a ﬁrst
order entity
Monday 26 August 13

12
Converter1 Converter2 Converter3
Dispatcher
Converter manager
The converter manager
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Dispatcher
(chain of responsibility pattern)
Converter manager
Monday 26 August 13

12
Dispatcher
(chain of responsibility pattern)
Converter manager
primitive types converters,
exception converters,
multi-valued converters,
etc.
Monday 26 August 13

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);
}
}
13
Monday 26 August 13


}

}
}
13
Java type
Monday 26 August 13


}

}
}
13
Prolog type
Monday 26 August 13

Conﬁguring a JPC context with a
custom converter
Jpc jpcContext = JpcBuilder.create()
.registerConverter(new StationConverter()).build();
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Monday 26 August 13

custom converter
14
Default builder
(includes pre-deﬁned
converters)
Monday 26 August 13

custom converter
14
Default builder
(includes pre-deﬁned
converters)
Extending JPC with
new conversions
Monday 26 August 13

Querying Prolog with JPC
15
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();
}
}
Monday 26 August 13

15
...
}
}
main_station(Station)
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15
...
}
}
binding of Station as a Java object
Monday 26 August 13

Dealing with object references
@Test
public 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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Monday 26 August 13

Dealing with serialized objects
@Test
public 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
Monday 26 August 13

JPC as a library for implementing
Java-Prolog frameworks
20
Prolog engines
Bridge libraries
JPC drivers
JPC library
Java-Prolog
applications
Java-Prolog
frameworks
Monday 26 August 13

JPC as a library for implementing
Java-Prolog frameworks
20
Prolog engines
Bridge libraries
JPC drivers
JPC library
Java-Prolog
applications
Java-Prolog
frameworks
e.g. LogicObjects
Monday 26 August 13

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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Monday 26 August 13

Symbiosis
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“The intimate living together of two dissimilar organisms in a
mutually beneficial relationship.” (Merriam-Webster dictionary)
Monday 26 August 13

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.
Monday 26 August 13

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)
*
Monday 26 August 13

The inhabitants of our two
worlds
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The OO
world
The logic
world
Monday 26 August 13

The inhabitants of our two
worlds
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Classes
Objects
Messages
Methods
Return values
Packages The OO
world
The logic
world
Facts
Rules
Terms
Queries
Query solutions
Modules
Monday 26 August 13

Reducing the gap with Logtalk
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The OO
world
The logic
world
Monday 26 August 13

Reducing the gap with Logtalk
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The OO
world
The logic
world
Logtalk
An object-oriented layer
Monday 26 August 13

Case study
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The London underground
from:
Monday 26 August 13

Relevant concepts
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Monday 26 August 13

Relevant concepts
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stations
linesmetro
Monday 26 August 13

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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Monday 26 August 13

...
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FACTS
Monday 26 August 13

...
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RULES
Monday 26 August 13

Adding an object-oriented layer
with Logtalk
...
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:- object(metro).
:- end_object.
Monday 26 August 13

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
Monday 26 August 13

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]).
...
:- end_object.
Monday 26 August 13

PARAMETRIC OBJECT
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...
:- end_object.
...
:- end_object.
Monday 26 August 13

PARAMETRIC OBJECT
PARAMETRIC OBJECT
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...
:- end_object.
...
:- end_object.
Monday 26 August 13

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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Monday 26 August 13

Meta-level artefacts from the two
worlds
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Monday 26 August 13

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 {
...
}
Monday 26 August 13

The Java world
34
String name;
public Line(String name) {this.name = name;}

public abstract boolean connects(Station s1, Station s2);

public abstract int segments();
}
...
}
public abstract class Metro {
...
}
@LObject(args={“name”})
@LMethod(name={“connects”}, args={“_”, “_”})
@LObject(args={“name”})
Monday 26 August 13

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.
35
* 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)
*
Monday 26 August 13

public abstract class Metro {...}
@LObject(name = "my_metro")
public abstract class Metro {...}
@LObject(args = {"name"})
private String name;
...
}
metro
my_metro
line(lineName)
Translating objects to logic terms
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Java Prolog
Monday 26 August 13

line(lName)::connects(
station(s1Name), station(s2Name)).
line(lName)::connects(_, _).
Mapping Java methods to Logtalk
methods
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public abstract
boolean connects(Station s1, Station s2);

@LMethod(name = "connects", args = {"_", "_"})
public abstract
int segments();
}
Java Prolog
Monday 26 August 13

line(lName)::connects(
station(s1Name), station(s2Name)).
line(lName)::connects(_, _).
Mapping Java methods to Logtalk
methods
37
public abstract

public abstract
int segments();
}
Java Prolog
anonymous variables
Monday 26 August 13

38
Interpreting a query result as a
Java object
The logic solutions
Varx1 x1, Vary1 y1
Varx2 x2, Vary2 y2
Varxn xn, Varyn yn
Monday 26 August 13

38
Java object
The logic solutions
Varx1 x1, Vary1 y1
Varx2 x2, Vary2 y2
Varxn xn, Varyn yn
(set of frames binding logic
variables to terms)
frame 1
frame 2
frame n
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39
Java object
The logic solutions
Varx1 x1, Vary1 y1
Varx2 x2, Vary2 y2
Varxn xn, Varyn yn
variables to terms)
Monday 26 August 13

39
Java object
The logic solutions The method return value
Varx1 x1, Vary1 y1
Varx2 x2, Vary2 y2
aJavaObject
Varxn xn, Varyn yn
variables to terms)
Monday 26 August 13

40
The logic solutions
Varx1 x1, Vary1 y1
Varx2 x2, Vary2 y2
Varxn xn, Varyn yn
variables to terms)
Returning values from one
solution
Monday 26 August 13

40
Varx1 x1, Vary1 y1
Varx2 x2, Vary2 y2
aJavaObject
Varxn xn, Varyn yn
variables to terms)
solution
Monday 26 August 13

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solution
term(Varx1, Vary1)
Varx1 x1, Vary1 y1
Varx2 x2, Vary2 y2
Varxn xn, Varyn yn
variables to terms)
Monday 26 August 13

41
solution
term(Varx1, Vary1)
Varx1 x1, Vary1 y1
Varx2 x2, Vary2 y2
Varxn xn, Varyn yn
variables to terms)
(speciﬁed in a method
with @LSolution)
Monday 26 August 13

41
solution
term(Varx1, Vary1)
Varx1 x1, Vary1 y1 term(x1, y1)
Varxn xn, Varyn yn term(xn, yn)
variables to terms)
with @LSolution)
(default solution)
Monday 26 August 13

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Explicit speciﬁcation of return
values
@LSolution("IStations")
@LMethod(name = "reachable", args = {"$1", "IStations"})
public abstract List<Station> intermediateStations(Station station);
...
Monday 26 August 13

• 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
...
}
Monday 26 August 13

ﬁrst Java method parameter
• Build a query.
43
• station(b)
...
}
Monday 26 August 13

ﬁrst Java method parameter
• Build a query.
43
• station(b)
...
}
unbound
Prolog variable
(as term)
Monday 26 August 13

• Build a query.
43
• station(b)
...
}
(instance of)
Monday 26 August 13

station(b)::reachable(station(a), IStations)
• Build a query.
43
• station(b)
...
}
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44
Returning multiple values from
queries
term(Varx1, Vary1)
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44
Returning multiple values from
queries
term(Varx1, Vary1)
a composed solution
with @LComposition)
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45
(a property of the result set)
Varx1 x1, Vary1 y1
Varx2 x2, Vary2 y2
Varxn xn, Varyn yn
Returning a property of the result
set
}
Monday 26 August 13

45
(a property of the result set)
Varx1 x1, Vary1 y1
Varx2 x2, Vary2 y2
Varxn xn, Varyn yn
is there a solution ?
set
how many solutions ?
}
Monday 26 August 13

set
46

public abstract

public abstract
int segments();
}
Monday 26 August 13

set
46

public abstract

public abstract
int segments();
}
should return if logic method succeeds or not
Monday 26 August 13

set
46

public abstract

public abstract
int segments();
}
should return if logic method succeeds or not
should return the number of solutions
Monday 26 August 13

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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Monday 26 August 13

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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Monday 26 August 13

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/
1
2
3
4
Monday 26 August 13

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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Monday 26 August 13

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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Monday 26 August 13

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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Monday 26 August 13

LogicObjects

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