LLJ is Prolog-like logic language for running on JVM. It allows close integration with Java libraries. LLJ was developed in 2008-2010. The code is fairly commented and covered with various test cases.

• Code is loaded through Java classpath
• Module names correspond to Java classpath entries
• Syntax is based on UTF-8
• Terms can be arbitrary Java objects
• No term/goal expansion
• No DCG's
• No custom operators
• Assert/retract can only be used for ground predicates
• Most ISO-Prolog standard predicates are not implemented

For embedded usage there are no dependencies other than the Java Runtime Environment (>= 1.6). The interactive top-level command-line interface requires jline to be in the classpath.

Compiling requires JDK (>= 1.6) and Maven. Run Maven goal mvn package to build the jar file into target/llj.jar.

Example module (queens benchmark):

:- module(ee:pri:rl:llj:benchmark:queens, [
	queens/2,
	count/2
]).

:- import(llj:list).
:- import(llj:lang).
:- import(llj:findall).

count(N, C):-
	findall(_, queens(N,_), L),
	length(L, C).

queens(N,Qs) :-
	range(1,N,Ns),
	queens(Ns,[],Qs).

queens([],Qs,Qs).
queens(UnplacedQs,SafeQs,Qs) :-
	select(UnplacedQs,UnplacedQs1,Q),
	not_attack(SafeQs,Q),
	queens(UnplacedQs1,[Q|SafeQs],Qs).

not_attack(Xs,X) :-
	not_attack(Xs,X,1).

not_attack([],_,_) :- !.
not_attack([Y|Ys],X,N) :-
	X =\= Y+N, X =\= Y-N,
	N1 is N+1,
	not_attack(Ys,X,N1).

select([X|Xs],Xs,X).
select([Y|Ys],[Y|Zs],X) :- select(Ys,Zs,X).

range(N,N,[N]) :- !.
range(M,N,[M|Ns]) :-
	M < N,
	M1 is M+1,
	range(M1,N,Ns).

Example code of using LLJ from Java:

public class MemberExample {
	
	public static void main(String[] args) throws LLJException {
	
		// Creates new LLJ context
		LLJContext context = new LLJContext("llj:list");
		
		// Creates new LLJ query
		RuntimeQuery query = context.createQuery("member(X, [1,2,3])");
		
		// Executes the query for the first answer
		if (query.execute()) {
			System.out.println("First answer: " + query.getBinding("X"));
		}
		
		// Tries to find more answers
		while (query.hasMore()) {
			System.out.println("Also: " + query.getBinding("X"));
		}
	}
}

The LLJContext class is thread safe while RuntimeQuery must be used only by a single thread at once. Multiple RuntimeQuery classes can be executed at once on the same context at the same time.

Defining predicates through Java code (this is example from llj:io module):

public class Io implements JavaModule {

	@Predicate(name="list_files", arity = 2)
	public AbstractGoal listFiles2(JavaCallGoal goal) throws Exception {
		File file = goal.getObject(0, File.class);
		goal.setVar(1, RuntimeListStruct.fromArray(file.listFiles()));
		
		return goal.G;
	}
}

• WProlog is a Prolog implementation on JVM. While it lacks cut, arithmetics, modules and arithmetics I got some of my implementation ideas from it.
• XProlog enhanced version of WProlog.

• Raivo Laanemets

Some of the test cases were taken from example code of the excellent book Learn Prolog Now!.

LLJ is distributed under LGPL license.